Don’t Miss Our Newsletter: IoT Technology Focus

In tomorrow’s IoT Technology Focus newsletter you’ll get news and trends about the products and technologies needed to build IoT implementations and devices. image002

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Embedded Boards. This content looks at embedded board-level computers. The focus here is on modules—Arduino, Raspberry Pi, COM Express, and other small-form-factor —that ease prototyping efforts and let you smoothly scale up production volumes.

Analog & Power. This newsletter content zeros in on the latest developments in analog and power technologies including DC-DC converters, AD-DC converters, power supplies, op-amps, batteries, and more.

Microcontroller Watch. This newsletter keeps you up-to-date on latest microcontroller news. In this section, we examine the microcontrollers along with their associated tools and support products.

Don’t Miss Our Newsletter: Microcontroller Watch

Circuit Cellar’s Microcontroller Watch newsletter is coming to your inbox tomorrow. This newsletter keeps you up-to-date on latest microcontroller news. We examine the microcontrollers along with their associated tools and support products.20150220-rh850-d1x

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IoT Technology Focus. The Internet-of-Things (IoT) phenomenon is rich with opportunity. This newsletter tackles news and trends about the products and technologies needed to build IoT implementations and devices.

Embedded Boards. Embedded boards are critical building blocks around which system developers can build all manor of intelligent systems. The focus here is on both standard and non-standard embedded computer boards.

Analog & Power. This newsletter content zeros in on the latest developments in analog and power technologies including DC-DC converters, AD-DC converters, power supplies, op-amps, batteries and more.

Don’t Miss Our Newsletter: Analog & Power

Circuit Cellar’s Analog & Power themed newsletter is coming to your inbox tomorrow. In tomorrow’s newsletter you’ll get news about the products and technologies trends in the analog, mixed-signal and power markets.MAX77756_EVKit_image

This newsletter content zeros in on the latest developments in analog and power technologies including DC-DC converters, AD-DC converters, power supplies, op-amps, batteries and more.

 

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Microcontroller Watch. This newsletter keeps you up-to-date on latest microcontroller news. In this section, we examine the microcontrollers along with their associated tools and support products.

IoT Technology Focus. The Internet-of-Things (IoT) phenomenon is rich with opportunity. This newsletter tackles news and trends about the products and technologies needed to build IoT implementations and devices.

Embedded Boards. Embedded boards are critical building blocks around which system developers can build all manor of intelligent systems. The focus here is on both standard and non-standard embedded computer boards.

Don’t Miss Our Bonus Newsletter: FPGA Technologies

As you know, Circuit Cellar’s newsletter covers four key themes each month. But August is a special month with a 5th Tuesday! As result, tomorrow coming to your inbox with be a special bonus newsletter theme: FPGA Technologies. In tomorrow’s newsletter you’ll get news about the products and technologies trends in the FPGA market. FPGAs have sv_gs_diagramevolved to become complete system chips. Today’s FPGAs pack in levels of processing, I/O and memory on one chip that once required several ICs or boards.

Also: We’ve added Drawings for Free Stuff to our weekly newsletters. Make sure you’ve subscribed to the newsletter so you can participate.

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Analog & Power. This newsletter content zeros in on the latest developments in analog and power technologies including DC-DC converters, AD-DC converters, power supplies, op-amps, batteries, and more.

Microcontroller Watch. This newsletter keeps you up-to-date on latest microcontroller news. In this section, we examine the microcontrollers along with their associated tools and support products.

IoT Technology Focus. The Internet-of-Things (IoT) phenomenon is rich with opportunity. This newsletter tackles news and trends about the products and technologies needed to build IoT implementations and devices.

Embedded Boards. Embedded boards are critical building blocks around which system developers can build all manor of intelligent systems. The focus here is on both standard and non-standard embedded computer boards.

Don’t Miss Our Newsletter: Embedded Boards

Circuit Cellar’s Embedded Boards themed newsletter is coming to your inbox tomorrow. In tomorrow’s newsletter you’ll get news about the products and technologies trends in the board-level embedded computer market. Embedded boards are a critical building block around which system developers can build all manor of intelligent systems. PR_EPM-43_HI

The focus here is on both standard and non-standard embedded computer boards that ease prototyping efforts and let you smoothly scale up to production volumes.

 

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Analog & Power. This newsletter content zeros in on the latest developments in analog and power technologies including DC-DC converters, AD-DC converters, power supplies, op-amps, batteries, and more.

Microcontroller Watch. This newsletter keeps you up-to-date on latest microcontroller news. In this section, we examine the microcontrollers along with their associated tools and support products.

IoT Technology Focus. The Internet-of-Things (IoT) phenomenon is rich with opportunity. This newsletter tackles news and trends about the products and technologies needed to build IoT implementations and devices.

…and…

August has a 5th Tuesday. So look for a bonus Newsletter this  month!

Don’t Miss Our Newsletter: IoT Technology Focus

In tommorrow’s IoT Technology Focus newsletter you’ll get news and trends about the products and technologies needed to build IoT implementations and devices.LoRa-NNNCo-PR-graphic-press

Bonus: We’ve added Drawings for Free Stuff to our weekly newsletters. Make sure you’ve subscribed to the newsletter so you can participate.

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Embedded Boards. This content looks at embedded board-level computers. The focus here is on modules—Arduino, Raspberry Pi, COM Express, and other small-form-factor —that ease prototyping efforts and let you smoothly scale up production volumes.

Analog & Power. This newsletter content zeros in on the latest developments in analog and power technologies including DC-DC converters, AD-DC converters, power supplies, op-amps, batteries, and more.

Microcontroller Watch. This newsletter keeps you up-to-date on latest microcontroller news. In this section, we examine the microcontrollers along with their associated tools and support products.

…and…

August has a 5th Tuesday. So look for a bonus Newsletter this  month!

Don’t Miss Our Newsletter: Microcontroller Watch

In tommorrow’s Microcontroller Watch we’ll feature key updates on the latest microcontroller technology  — the latest MCU design wins — new MCU product announcements — MCU industry events –and more.35352057604_77bb4aab93_m

Plus: we’ve added Drawings for Free Stuff to our weekly newsletters. Make sure you’ve subscribed to the newsletter so you can participate.

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IoT Technology Focus. The Internet-of-Things (IoT) phenomenon is rich with opportunity. This newsletter tackles news and trends about the products and technologies needed to build IoT implementations and devices.

Embedded Boards. This content looks at embedded board-level computers. The focus here is on modules—Arduino, Raspberry Pi, COM Express, and other small-form-factor —that ease prototyping efforts and let you smoothly scale up production volumes.

Analog & Power. This newsletter content zeros in on the latest developments in analog and power technologies including DC-DC converters, AD-DC converters, power supplies, op-amps, batteries, and more.

…and…

August has a 5th Tuesday. So look for a bonus Newsletter this  month!

Find and Eliminate Ground Loops

Everything had been fine with my home entertainment center—comprising a TV, surround-sound amplifier, an AM/FM tuner, a ROKU, and a CD/DVD/BlueRay player—until I connected my desktop PC, which stores many of my music and video files on one of its hard drives. With the PC connected, the speakers put out a low level, annoying, 60-Hz hum—a clear indication of a ground loop. All my audio and video (AV) devices are fairly new, quality, brand-name products equipped with two-prong power cords, so even though the PC has a three-prong plug, there should not be multiple signal returns causing the ground loop. This article describes an approach to eliminating ground loops in analog AV systems.

GROUND LOOPS

By definition, ground loops bring about unwanted currents flowing through two or more signal return paths. Thus induction coils are formed, usually of one turn only. These loops pick up interference signals from the environment. Because every conductor has a finite impedance, a voltage potential—Vi = Ig(R1 + R2)—develops between the two connected signal return points. This voltage is the source of the interference: a hum, hiss noise that high-frequency signals pick up (e.g., a local AM station), and so forth. A simplified example is illustrated in Figure 1.

FIGURE 1: Cause of the ground loop interference.

FIGURE 1: Cause of the ground loop interference.

An audio signal source VS in Figure 1—an audio card inside the PC, for example—is connected to an amplifier via a shielded cable. The shield is grounded at both ends to the chassis of both devices. Three-prong power plugs connect the chassis of both AV components to the house power distribution ground wire. Let’s consider the amplifier ground to be the reference point. (It doesn’t matter which point in the loop we pick.) The loop, comprising the cable shield and the power distribution ground wire, picks up all kinds of signals causing loop current Ig to flow and as a result interference voltage Vi to be generated.

Vi is added to the signal from the audio card. The Ig current induced into the loop comes from many potential sources. It can be induced in the ground wire by the current flowing in the 120-VAC hot and its return neutral wires, acting like a transformer. There can be leakages, induction by magnetic fields, capacitive coupling, or an electromagnetic interference (EMI) induction into the loop. Once Vi is added to the signal it is generally impossible to filter it out.

Much of electrical equipment requires the third power prong for safety. This is connected to the chassis and at the electrical distribution panel to the neutral (white wire) and the local ground—usually a metal stake buried in the earth. The earth ground is there to dissipate lightning strikes but has no effect on the ground loops we are discussing.
The ground wire’s primary purpose is safety plus transient and lightning diversion to ground. Under normal circumstances no current should flow through this wire. Should an internal fault in an appliance connect either the neutral (white) or the hot (black or red) wire to the chassis, the green wire shunts the chassis to the ground. Ground fault interrupters (GFI) compare the current through the hot wire to the return through the neutral. If not identical, the GFI disconnects.

Manufacturers of audio equipment know that grounding sensitive equipment at different places along the ground wire results in multiple returns causing ground loops. These facilitate the interference noise to enter the system. From the perspective of electrical safety, the small currents induced in the ground loop can be ignored. Unfortunately, they are large enough to play havoc with sensitive electronics. The simplest solution to the dilemma is to avoid creating ground loops by not grounding the AV equipment. Thus the two-prong plugs have been used on such equipment. To satisfy the safety requirements, the equipment is designed with double insulation, meaning that even in case of an internal fault, a person cannot come to contact with a live metallic part by touching anywhere on the surface of the equipment.

My PC, like most desktops, has a three-prong plug. Figure 2 shows the arrangement. The PC is grounded through its power cord. Unfortunately, the cable TV (CATV) introduces a second ground connection through its coax connector. I measured the resistance between the coax shield as it entered the house and the house power distribution ground wire. The resistance was 340 mΩ, indicating a hard connection between the coax shield and the house ground, the cause of the ground loop. I was unable to establish where that connection was made, but it wasn’t through the earth.

FIGURE 2: Ground loop in my entertainment system

FIGURE 2: Ground loop in my entertainment system

There can be multiple ground loops around a computer system if you have hard-wired peripherals with three-prong plugs, such as some printers, scanners and so forth. Digital circuits are much less sensitive to ground loops than the analog ones, but it is a good idea to minimize potential loops by connecting all your peripherals, other than wireless, into a single power bar.

Ground loops may also be created when long shielded cables are used to interface the PC and the home theatre box. Two shielded cables needed for stereo represent two signal returns creating a ground loop of their own. And then there are video cables. Another loop. Fortunately, connectors on the back of the PC and AV equipment are very close to each other, which means a minimal potential difference between them at low frequencies. Stereo cables keep the loop small. To minimize all the loops’ areas for interference pick-up, I have bundled the interface cables very close to each other with plastic wire ties. In severe situations re-routing the cables or the use of a metal conduit or wireless interfaces may be needed to kill the interference.

FIXES

Having disconnected the CATV cable from the TV, the hum went away. As well, temporarily replacing the PC with a laptop, which is not grounded, also fixed the problem. So how else can we fix those offending multiple returns?

The obvious answer is to break the loop. I strongly suggest you don’t disconnect the PC from the ground by using a two-prong plug adapter or just cutting the ground prong off. It will render your system unsafe. What you need is a ground isolator. Jensen Transformers, for example, sell isolators such as VRD-IFF or PC-2XR to break the ground connection, but you can build one for a small fraction of the purchase price. Figure 3 and Figure 4 show you how.

FIGURE 3: Ground isolator for CATV coax

FIGURE 3: Ground isolator for CATV coax

To break the ground loop caused by the CATV, you can make a little gizmo shown in Figure 3. J1 and J2 are widely available cable TV female connectors. C1 and C2 capacitors placed between them should be about 0.01 µF each. The assembly does not require a printed circuit board. You might place it in a tiny box or just solder everything together, wrap it with electrical tape, and put it somewhere out of the way. Remember that the capacitors’ working voltage must be at least double the power distribution voltage. That is 250 V in North America and more than 500 V elsewhere in the world.

FIGURE 4: Ground isolator for three-prong powered appliances

FIGURE 4: Ground isolator for three-prong powered appliances

Figure 4 shows how to break ground for appliances, such as a PC, with three-prong plugs. You can build this circuit into a computer or another appliance, but I find it better to build it as an independent break-out box. The diodes provide open loop for signals up to about 1.3 VPP. A hum is usually of a substantially lower amplitude. C1, 0.01 µF, provides bypass for high-frequency EMI to ground. The loop would be closed for voltages higher than 1.3 VPP, such as the ones due to isolation fault of the hot wire to the chassis. For 120 VAC distribution, D1, D2, and C1 should be rated for 250 V at a minimum. In a circuit branch with a 15-A breaker or fuse, the diodes need to be rated for a minimum of 20 A so that the breaker opens up before the diodes blow. If the appliance takes only a fraction of the rated fuse current, say 2 A, you could use 5-A diodes and include an optional fuse rated for 2 A. For countries with 230-VAC power, the components must be rated accordingly.

You can also break the ground loop by using a power isolation transformer between the power line and the PC, or quality signal transformers on the signal lines. The downside of this is that good isolation and signal transformers are costly and not widely available. Equipment powered from wall warts—and especially those with optically coupled inputs and outputs, common today—is inherently ground loop impervious.

TRIAL & ERROR

This article describes an approach to eliminating ground loops in analog AV systems. While you need to understand how ground loops occur, finding them and eliminating their effects may turn out to be a matter of frustrating trial and error.

George Novacek is a professional engineer with a degree in Cybernetics and Closed-Loop Control. Now retired, he was most recently president of a multinational manufacturer for embedded control systems for aerospace applications. George wrote 26 feature articles for Circuit Cellar between 1999 and 2004. Contact him at gnovacek@nexicom.net with “Circuit Cellar” in the subject line.

This article appears in Circuit Cellar 301 August 2015.

Don’t Miss CC’s Newsletter: IoT Technology Watch

The Internet-of-Things (IoT) phenomenon is rich with opportunity. Circuit Cellar’s IoT Technology Focus themed newsletter is coming to your inbox tomorrow. The newsletter will update you on the latest news and trends including IoT gateways, IoT device security, IoT wireless connectivity and IoT cloud implementations.

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Embedded Boards. This content looks at embedded board-level computers. The focus here is on modules (e.g., Arduino, Raspberry Pi, COM Express, and other small-form-factor modules) that ease prototyping efforts and let you smoothly scale up production volumes.

Analog & Power. This newsletter content zeros in on the latest developments in analog and power technologies including DC-DC converters, AD-DC converters, power supplies, op-amps, batteries, and more.

Microcontroller Watch. This newsletter keeps you up-to-date on latest microcontroller news. In this section, we examine the microcontrollers along with their associated tools and support products.

Circuit Cellar Newsletters to Focus on Microcontrollers, IoT and More

Circuit Cellar’s ongoing mission is to provide important information to help you make smart choices with your engineering projects—from prototype to production. As part of that effort, we’re now offering themed newsletter content each week that focuses on critical areas of system development.

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Our new enhanced weekly CC Newsletter will switch its theme each week, covering these four areas every month:

Microcontroller Watch. This newsletter keeps you up-to-date on latest microcontroller news. In this section, we examine the microcontrollers along with their associated tools and support products.

IoT Technology Focus. The Internet-of-Things (IoT) phenomenon is rich with opportunity. This newsletter tackles news and trends about the products and technologies needed to build IoT implementations and devices.

Embedded Boards. This content looks at embedded board-level computers. The focus here is on modules (e.g., Arduino, Raspberry Pi, COM Express, and other small-form-factor modules) that ease prototyping efforts and let you smoothly scale up production volumes.

Analog & Power. This newsletter content zeros in on the latest developments in analog and power technologies including DC-DC converters, AD-DC converters, power supplies, op-amps, batteries, and more.

Analog Tips & Tricks

Are you looking for ways to improve your analog and RF circuitry? Engineer Ed Nisley provides a few tips for getting started. He shows you how easy it is to take your PCB wiring skills to the next level. Who knows, your digital projects just might improve too.

Circuit Cellar has always attracted readers who enjoy building gizmos, both at work and for their own use. My December 2004 column, “Building Boxes,” prompted enough comments and suggestions regarding additional techniques that I decided a follow-up was in order.

Although these tricks are designed to improve your analog and RF circuitry, even your digital projects will benefit, because digital is just analog with the gain cranked way up. You’re sure to find at least one technique that will make your next project work better.

I wire most of my projects on PCBs built in my basement shop, using a process that produces both circuit documentation and reasonably high-quality hardware without too much effort. I’ve come up with some tricks that should help you get good results too.

I use CadSoft’s EAGLE schematic capture and board layout software, which runs on Windows, Linux, and Mac OS X (www.cadsoftusa.com). The free version can handle most of the circuits in this column, and the Standard version is reasonably priced. EAGLE is perfectly stable on my SuSE Linux 9.2 desktop system. The board layout program can produce output files in nearly any format, including the Gerber files used in board production shops. I save the output for each layer as a Postscript file, and then import the files into the GNU Image Manipulation Program (GIMP) image-editing program at 600 dpi.

The top image is the top copper layer from an EAGLE board design. The bare board shows several flaws, but the one on the bottom came out fine. The ruler scales are 0.050″ vertically and 1 mm horizontally. The board has extremely small features!

The top image is the top copper layer from an EAGLE board design. The bare board shows several flaws, but the one on the bottom came out fine. The ruler scales are 0.050″ vertically and 1 mm horizontally. The board has extremely small features!

The top image in Photo 1 shows the copper plane pattern for the charge pump LED power supply I described in my April 2005 column. I panelize them with the GIMP to produce a single image with multiple patterns in a rectangular grid. Because all this happens digitally, there’s no loss of resolution and no smudges. I then print the image through an HP LaserJet 1200 on a sheet of toner-transfer film from either Pulsar (www.pulsar.gs) or Techniks (www.techniks.com). It turns out that toner contains a thermoplastic that both adheres to bare copper and resists the etching chemical solution.

Because most of my boards are extremely small, they don’t fill a complete sheet of the toner-transfer film even after I panelize them. I print a sheet of paper, tape a square of film that’s approximately 1″ larger than the patterns atop them, and then run the paper through the printer again. The adhesive on cheaper tapes tends to melt at laser printer temperatures, so use good tape and monitor your results. Put a single strip on the leading edge of the toner-transfer film to allow the paper and film to shift slightly as they pass through the fuser rollers.

This article first appeared in Circuit Cellar 181. You can read the entire article here.

Ed Nisley is an electrical engineer, author, and long-time Circuit Cellar columnist living in Poughkeepsie, NY. His column “Above the Ground Plane” appears in Circuit Cellar every other month. You can contact him at ed.nisley@pobox. com. Write “Circuit Cellar” in the subject line to avoid spam filters.

Analog Filter Essentials

Analog frequency-selective filters are useful for noise reduction, antialiasing before digitizing a signal, frequency response correction, and more. In this article, Circuit Cellar columnist Robert Lacoste explains the differences between filters and how to design them with computer-aided tools.
 

The following article by Robert Lacoste appears in Circuit Cellar 307, 2016.

 
 
Welcome back to the Darker Side. I spoke about operational amplifiers (op-amps) in my last few columns. Op-amps shine in plenty of applications—in particular, to build active filters. This month, I’ll focus on filters—more precisely, analog frequency-selective filters, which are used in audio devices, as well as for noise reduction, antialiasing before digitizing a signal, separation of frequency-multiplexed signals, frequency response correction, and so on.

So analog filters must be in the bag of tricks of any designer. Unfortunately, filter design, or even their use, is often perceived as a difficult task close to black magic. This is, well, unfortunate. Filters are definitively useful, simple, and even fun. I bet a textbook about filters full of math would bore you, right? Well, relax. My goal for this article is more pragmatic. I will try to help you to specify a filter, understand the main filter variants, and efficiently use some great computer-aided design tools. I promise, no Laplace transforms or poles or zeros, just electronics.

FILTER SPECIFICATIONS

Let’s start with some vocabulary. By definition, a filter is a circuit that attenuates some signals more than others, depending on their frequency. Figure 1 depicts the most classic filter types. A low-pass filter lets the low frequencies pass through, but attenuates high-frequency signals. It is perfect for removing high-frequency noise on a signal coming from a sensor.

FIGURE 1: Four classic types of frequency filters. Each one attenuates a specific frequency range.

FIGURE 1: Four classic types of frequency filters. Each one attenuates a specific frequency range. Click image to enlarge.

Conversely, a high-pass filter attenuates the low frequencies, and could in particular remove any DC component of a signal. Band-pass filters are a combination of both, and they attenuate all frequencies below or above a given range. For example, any radio frequency receiver is a band-pass filter, providing attenuation of all signals except for frequencies close to its preset frequency. Lastly, a band stop filter, often called a notch filter, does the opposite, and it attenuates a selected range of frequencies. For example, a 50- or 60-Hz notch filter is included in virtually every weight scale to remove EMC perturbations from the surrounding power lines.

Want to specify a filter? Figure 2 illustrates this on a low-pass filter. The first parameter is the filter cut-off frequency, of course. By definition, this is the frequency at which the filter attenuates the power of the signal by 50%. This means that the losses of the filter will be 3 dB at that frequency. Aren’t you fluent with decibels? A decibel is one tenth of a Bel, and a Bel is the base-10 logarithm of the ratio of two powers. Take your calculator and enter 10 × log(0.5), you will get –3.01, which everybody rounds to –3 dB.

FIGURE 2 A filter (low-pass in this case) is specified by its cutoff frequency f3dB, its ripple in the pass-band, and its rejection in the stopband.

FIGURE 2: A filter (low-pass in this case) is specified by its cutoff frequency f3dB, its ripple in the pass-band, and its rejection in the stopband. Click image to enlarge.

But perhaps an attenuation of 3 dB is already too much for your application. The maximum tolerated variation of signal power in the pass-band (here from DC to fPB) is called the ripple of the filter. Lastly, you will very probably want to specify that the filter must provide a given minimum attenuation, called rejection, above some frequency fSB. Of course, these specifications must be established with care. If you decide that you need a filter with 0.01 dB of ripple up to 10 kHz and 100 dB of rejection from 11 kHz upward, you will probably need plenty of time and cash for the design.

RC FILTERS

I propose to start with the most basic designs: RC filters. The basic low-pass filter is built with one series resistor and one capacitor to ground (see Figure 3). The capacitor impedance gets lower when the frequency increases, and the signal power is attenuated. This filter is called a first-order filter, and it provides an attenuation of 6 dB per octave or 20 dB per decade. (. Simply because 23.33 = 10, and 3.33 × 6 = 20.) That means that, above its cut-off frequency, its attenuation is increased by 6 dB each time the frequency is doubled, or by 20 dB each time it is multiplied by 10. I did the simulation for you with Labcenter Electronics Proteus. Figure 3 shows the result. You can do the same with any Spice-based simulator like the free LT-Spice. The attenuation of this RC filter is –20 dB at 100 kHz, and 20 dB more, meaning –40 dB, at 10 × 100 kHz = 1 MHz as expected.

FIGURE 3 A first-order RC filter (top) provides an attenuation of 20 dB/decade (green curve), whereas a second-order filter provides 40 dB/decade (red).

FIGURE 3: A first-order RC filter (top) provides an attenuation of 20 dB/decade (green curve), whereas a second-order filter provides 40 dB/decade (red). Click image to enlarge.

Such a RC filter can be designed for any cutoff frequency. Just select the proper values for R and C. You might wonder how to calculate the values of the R and C. For a single RC cell, it is really easy. The cutoff frequency is 1/(2pRC).

If you want to increase the steepness of the attenuation, you can chain several RCs. For example, I simulated a second-order RC filter, with two RC cells in series (see Figure 3). As expected, the attenuation is now 12 dB (i.e., 2 × 6) per octave, or 40 dB (i.e., 2 × 20) per decade. Nothing magic. The 3-dB cutoff frequency is pushed downward as compared to a single RC cell, simply because at the 3-dB cutoff of each cell the attenuation is now 6 dB. However, you can see in the graph that even if the falloff in high frequencies is two times better, the drop around the cutoff frequency isn’t improved: it is still “soft.” That’s a limitation of cascaded RC cells. I will present you with a better solution.

Maybe a low-pass filter isn’t what you need. If you prefer a high-pass filter, then just exchange capacitors and resistors. A series capacitor and a resistor to ground would make it. Do you want a band-pass? Just put a low-pass cell in series with a high-pass cell with the appropriate cutoff frequencies. For example, a 10-to-50-kHz band-pass can be built with a 10-kHz high pass and a 50-kHz low pass. And for a notch filter? Do the same with the two filters in parallel. With the same example, a 10-to-50-kHz band stop may be implemented with a 10-kHz low pass and 50-kHz high pass in parallel. Easy.

LC FILTERS

For a given filter performance, is it possible to build a passive analog filter with fewer parts than a multicell RC filter and with improved performance? Yes, you can use LC filters. Here the filter is made with capacitors and inductors, as illustrated in Figure 4. How steep can be their attenuation profile? Very easy. It is the same in the case of RC filters. Count the number of capacitors, add the number of inductors, and you’ll get the order or the filter. Then multiply by 6 dB to get the attenuation per octave, or by 20 dB for an attenuation per decade! For example, the first filter simulated in Figure 4 has one inductor and one capacitor. Two parts, so it is a second-order filter, with the same 40 dB/octave attenuation as the dual RC example in Figure 3. The bottom example has three capacitors and two inductors; therefore, its attenuation is 100 dB (i.e., 5 × 5) per decade or 30 dB (i.e., 5 × 6) per octave. I promise. It’s simple!

FIGURE 4: This simulation shows the frequency response of three LC filters, respectively, of order two, three, and five from top to bottom. Their outputs are open-ended, which imply some overshoot.

FIGURE 4: This simulation shows the frequency response of three LC filters, respectively, of order two, three, and five from top to bottom. Their outputs are open-ended, which imply some overshoot. Click image to enlarge.

Well, nearly. Let’s now see the small details. If you refer back to Figure 4, you’ll see that such LC filters have a weird response around their cutoff frequencies. There is an overshoot, which means they have a positive gain at some frequencies. Of course, such passive filters can’t “create energy.” This positive gain is due to the fact that their output is open-circuited so no energy actually flows anywhere. Don’t be confused. This is not an artifact of the simulation. This would be exactly the same on an actual circuit. The amplitude of the overshoot is directly linked to the so-called quality factor of the L and C parts, and in particular their series resistance. If the capacitor and inductors are ideal, then the overshoot will be infinite at the frequency where the L and C oscillate. That’s why I added a small 47-Ω series resistor on the simulations. If you change the value of this series resistor, then the shape of the gain curve changes. I illustrated it in Figure 5 (top graph) which shows a series resistor ranging from 5 to 100 Ω.

FIGURE 5: The top simulation shows the frequency response of an open-ended LC filter with varying serial resistor value. The bottom simulation shows that the overshoot disappears when the filter is connected to a matched load. X varies from 5 to 100 Ω.

FIGURE 5: The top simulation shows the frequency response of an open-ended LC filter with varying serial resistor value. The bottom simulation shows that the overshoot disappears when the filter is connected to a matched load. X varies from 5 to 100 Ω. Click image to enlarge.

How do you avoid such oscillations? Simply connect the filter’s output to a proper load. If you are a regular reader of my columns, you won’t be surprised: this load must provide an impedance matching with the source impedance. Look at the second example in Figure 5. I added a load resistor R3 of the same value as the source resistor R2 (denoted X). I then asked the simulator to show the resulting gain versus frequency graph with different values for these resistors X, ranging from 5 to 100 Ω again. The shapes are varying, but there are no overshoots. Moreover a precise resistor value provides a very clean and flat response, linked of course with the values of the L and C parts. This value, here 50 ohm, is the characteristic impedance of the LC filter.

So LC filters must be calculated to get the required frequency response but also taking into account the impedance of the load. For second order filters, using just one inductor and one capacitor, the calculation are straightforward. The cutoff frequency is f3dB = 1/[2p√(LC)], and the characteristic impedance is Z = √(L/C). If you know the required cutoff frequency and designed impedance, then you can easily calculate L and C from these two formulas.

The calculation is not so straightforward for higher-order filters, especially as the design choices are numerous. More on that below. Our ancestors used the abacus; now we can use web-based design tools. (Refer to the Resources section of this article for some links to free LC filter calculators.) There is even a great design tool from Coilcraft that allows you to directly order the samples of the required inductors with a mouse click. Easy, I promised.

FROM LC TO ACTIVE FILTERS

Using inductors often isn’t pleasant. They can be heavy and large, and they’re always significantly more expensive than capacitors and resistors. Moreover, inductors are often quite far from ideal components. They can have a high series resistance as well as parasitic capacitance, nasty electromagnetic compatibility behavior, and a couple of other issues. How can you keep the performance of an LC filter without using inductors? With an active filter, usually built around our dear friend the op-amp.

There are basically three ways to build an active filter. The first is to simply add an amplifier to the RC filters I’ve already talked about. For example, you can add a voltage follower after an RC cell in order to reduce its output impedance or to provide some gain. You can also wire an op-amp as a differentiator or integrator, which are first-order filters.

The second solution is to build a switched-capacitor filter circuit. (I devoted my Circuit Cellar 277 column to the subject.) So let’s talk about the third option, which is based on so-called gyrators. What is that? A gyrator is a circuit that mimics the behavior of an inductor, using an op-amp and only resistors and capacitors. You will find plenty of literature on the subject. Of course, this is explained in the bible, Paul Horowitz and Winfield Hill’s The Art of Electronics, but Rod Elliott provides a clear presentation on the subject in “Active Filters Using Gyrators – Characteristics, and Examples,” (Elliott Sound Products, 2014).

Look at Figure 6 where I have illustrated the basic concept. The top part of the schematic is a classic second-order LC high-pass filter with matched source and load impedances. I used a 390-nF capacitor and a 1-mH inductor, resulting in a cutoff frequency of 8 kHz and a characteristic impedance of 50 Ω—here roughly matched with source and load 56-Ω resistors.

FIGURE 6 This simulation shows the transformation of an LC high-pass filter (top) into a gyrator-based circuit (middle), which is really close to the common Sallen Key filter (bottom).

FIGURE 6: This simulation shows the transformation of an LC high-pass filter (top) into a gyrator-based circuit (middle), which is really close to the common Sallen Key filter (bottom). Click image to enlarge.

The response curve shows noting surprising with a 40 dB/decade (i.e., 2 × 20) attenuation in the stopband. Its gain is –6 dB in the passband, as the voltage is divided by two due to the source and load resistors. (The power is divided by 22 = 4, giving –6 dB.) Now look at the middle section of the schematic in Figure 6. The circuit is exactly the same, but I replaced the inductor with an op-amp, a capacitor, and two resistors. That’s a gyrator. If you look now at the resulting graph, you will see that its frequency response is exactly the same as the LC version, at least up to 1 MHz where the characteristics of the op-amp start to be limiting.

Now another magic trick. Compare the gyrator-based schematic with the schematic at the bottom of Figure 6. If you move the parts and the wires around, you will see that they are exactly identical, except the output is now directly connected to the op-amp output. Do you recognize the new schematic? It is a Sallen-Key second-order active high-pass filter. I modified the part values to a more reasonable range, but you can see that the output frequency response is still the same. More precisely, it doesn’t suffer from the 6-dB losses as the signal is taken directly at the output of the op-amp. So Sallen-Key filters, gyrator-based filters, and LC filters are more than cousins.

FILTER RESPONSES

If you want to design a single-cell filter, either a first-order RC filter, a second-order LC, or an active filter, then you will not have a lot of design choices. You can select the desired filter type, cutoff frequency, and impedance, but nothing more. However, for higher-order filters, the choices are wider. The filter is made of several cells, and you can tune each cell separately. Therefore, you will have a better attenuation curve thanks to the higher order (remember, 6 dB per octave multiplied by the order of the filter), as well as more control on the shape of the filter.

Nothing prevents you from designing your own filter, tweaking each cell however you want. However, mathematicians have already calculated several “optimal” filters for certain applications. Do you want to have a response curve as flat as possible in the passband? Stephen Butterworth calculated it for you in 1930. It’s now called the Butterworth filter, of course. Do you prefer to attenuate as quickly as possible the stop-band even if it implies a higher level of ripple in the pass-band? Use a Chebyshev filter, derived from the Chebyshev polynomials. More precisely, this is a family of filters based on the acceptable ripple (e.g., 0.5 dB). The so-called elliptic filters are close.

The last common variant, the Bessel filter, is a little more complex. A Bessel filter is not a great option both in terms of flatness and attenuation; however, it has a key advantage in the time domain. Its so-called group delay is nearly flat. That brings us a little too far here, but these characteristics preserve the shape of the filtered signals in the time domain. I will tackle that subject in another article.

Of course, each variant has drawbacks. For the same filter complexity, a higher ripple in the passband must be accepted to get a higher attenuation in the stop-band. Similarly, a better phase flatness implies a worse frequency response. Life is difficult, but you are the designer, so you have the control. Figure 7 shows the characteristic responses of each filter variant.[1] For more information, I strongly encourage you to have a look at the “Analog Filters” chapter in Hank Zumbahlen’s Linear Circuit Design Handbook (Analog Devices, 2008).

FIGURE 7: These plots show the typical frequency and time (step and impluse) response of the three most common filter variants. (Source: Linear Circuit Design Handbook, Analog Devices)

FIGURE 7: These plots show the typical frequency and time (step and impluse) response of the three most common filter variants. Click image to enlarge. (Source: Linear Circuit Design Handbook, Analog Devices)

DESIGNER TOOLS

So you have plenty of options when designing a filter. Fortunately, there are great computer-based design tools made for the design engineer. Some are expensive, but plenty are free. In particular, several op-amp suppliers offer filter design tools for their products. I like Analog Devices’s Analog Filter Wizard (www.analog.com/designtools/en/filterwizard/). It’s powerful and doesn’t require a PC installation. Other solutions include Texas Instruments’s Webench Filter Designer, Microchip Technology’s FilterLab, Linear Technology’s FilterCAD, and some others.

FIGURE 8 With a tool like the Analog Filter Wizard (Analog Devices), life is easy

FIGURE 8: With a tool like the Analog Filter Wizard (Analog Devices), life is easy. Click image to enlarge.

As an example, Figure 8 shows a typical session with Analog Devices’s Analog Filter Design. Basically, you start by selecting the filter type (here a low-pass), the required gain in the pass-band, the cutoff frequency, and the attenuation you want at a given stop-band frequency. A slider enables you to browse through several designs—namely, Chebyshev, Butterworth, and others. The next window enables you select the desired tolerance for the capacitors and resistors and actually draw the filter’s full schematic (of course using an op-amp from the supplier who offered the tool). Lastly, the resulting frequency, phase, and time plots are generated, taking into account the tolerance of the parts. Other options enable you to calculate the power consumption of the design or its noise figure. Of course, the beauty of such a tool is that you can try tens of designs in minutes and select the most adequate for your specifications and budget.

WRAPPING UP

Here we are. As always, I have only scratched the subject’s surface. Anyway, I hope you grasped the key concepts. Go through the content listed in the Resources section of this article, and don’t forget to practice on your own. Maybe you should stop reading this magazine now (don’t forget to come back to the issue later), download one of the filter design tools, and play with the settings. It would be the best way to really understand the difference between a fourth-order Butterworth filter and a third-order Chebyshev filter. Have fun and don’t be afraid of filters.

Robert Lacoste lives in France, near Paris. He has 25 years of experience in embedded systems, analog designs, and wireless telecommunications. A prize winner in more than 15 international design contests, in 2003 he started his consulting company, ALCIOM, to share his passion for innovative mixed-signal designs. His book (Robert Lacoste’s The Darker Side) was published by Elsevier/Newnes in 2009.

Video Decoder with MIPI-CSI2 Output Interface Supports Next-Generation SoCs

Intersil Corp. recently introduced the TW9992 analog video decoder, which features an integrated MIPI-CSI2 output interface that provides compatibility with the newest SoC processors. The decoder’s MIPI-CSI2 interface simplifies design by making it easier to interface with SoCs, while also lowering the system’s EMI profile. The TW9992 decoder takes both single-ended and differential composite video inputs from a vehicle’s backup safety camera, and is the latest addition to Intersil’s video decoder product family for automotive applications.TW9992-intersil

Designed with built-in diagnostics and superior video quality, the TW9992 addresses the biggest challenges faced by automotive video systems. For example, the decoder’s Automatic Contrast Adjustment (ACA) image enhancement feature overcomes a major challenge for backup camera systems by adapting to rapidly changing lighting conditions. ACA is able to automatically boost up or reduce the brightness/contrast of an image for greater visibility and safety.

In addition, vehicle backup cameras typically employ differential twisted pair cables that require designers to use an operational amplifier (op amp) in front of the video decoder to convert the differential signal to single-ended. The TW9992 decoder eliminates the need for an external op amp by supporting direct differential CVBS inputs, thus reducing system cost and board space. The built-in short-to-battery and short-to-ground detection capability on each differential input channel further enhances video performance and automotive system reliability.

Features and specifications:

  • NTSC/PAL 10-bit ADC analog video decoder with 4H adaptive comb filter
  • MIPI-CSI2 output interface
  • Software selectable analog input control allows for combinations of single-ended or differential CVBS
  • Advanced image enhancement features: automatic contrast adjustment, and programmable hue, brightness, saturation, contrast and sharpness
  • Output voltage: 1.8 to 3.3 V with 3.3 V tolerance
  • Low-power consumption: 100-mW typical
  • Integrated short-to-battery and short-to-ground detection tests
  • AEC-Q100 qualified

The automotive-grade TW9992 analog video decoder is available in a 32-pin wettable flank QFN package. It costs $3 in 1,000-piece quantities.

Source: Intersil Corp.

Online Classroom for Analog Design

Texas Instruments recently launched TI Precision Labs, which is a comprehensive online classroom for analog engineers to take on-demand courses. The free, modular curriculum includes more than 30 training experiences and lab videos covering analog amplifier design considerations with online coursework.TI OnlineClassroomAnalog

TI Precision Labs incorporates a variety of tools to bring the online training experience to life. A $199 TI Precision Labs Op Amp Evaluation Module (TI-PLABS-AMP-EVM) enables engineers to complete each demonstrated learning activity along with the trainer. The curriculum also provides access to free design tools, such as TI Designs reference designs and TI’s TINA-TI SPICE model simulator.

Engineers can evaluate circuits and small-signal AC performance created during the trainings with National Instruments’s VirtualBench all-in-one instrument and TI’s Bode Analyzer Software for VirtualBench, as well as standard engineering bench equipment.

Key features and benefits of TI Precision Labs:

  • Experiential learning applies theory to real-world, hands-on examples with lab demonstration videos.
  • A customized learning environment provides recommended training tracks on topics such as noise, bandwidth and input/output swing, while enabling engineers to pick and choose courses based on individual needs and interests.
  • Accelerated learning for recent graduates eases the transition from undergraduate theoretical-based learning to real-world designing.
  • Robust learning materials include a downloadable presentation workbook and lab manual, as well as TI’s Analog Engineer’s Pocket Reference, which puts commonly used board- and system-level formulas at your fingertips.
  • Expert support: A TI Precision Labs support forum is available on the TI E2E Community to answer questions resulting from the training.

The TI Precision Labs training curriculum is free to anyone with a myTI account. In addition to free training, other benefits of myTI registration include the ability to purchase TI integrated circuits (ICs), evaluation modules, development kits and software; request product samples; get technical assistance through the TI E2E Community; create, simulate and optimize systems in the WEBENCH Design Center; and more.
TI Precision Labs curriculum is housed in the new TI Training Center, which connects engineers with the technical training they need to find solutions to their design challenges anytime, anywhere.

In addition to the on-demand courses, in-person, hands-on trainings covering a variety of precision amplifier topics, such as noise, offset, input bias, slew rate and bandwidth, are scheduled for May in Schaumburg, IL and Pewaukee, WI. Both live trainings require registration and cost $99 to attend. More in-person training dates in the United States will be added.

Source: Texas Instruments

New Microcontrollers Feature Advanced Analog & Digital Integration

Microchip Technology recently announced a new family of 8-bit PIC microcontrollers (MCUs) with the PIC16(L)F1769 family, which is the first to offer up to two independent closed-loop channels. This is achieved with the addition of the Programmable Ramp Generator (PRG), which automates slope and ramp compensation, increases stability and efficiencies in hybrid power conversion applications. The PRG provides real-time responses to a system change, without CPU interaction for multiple independent power channels. This allows customers the ability to reduce latency and component counts while improving system efficiency.Microchip PIC16(L)F1769

The PIC16(L)F1769 family includes intelligent analog and digital peripherals, including tristate op-amps, 10-bit ADCs, 5- and 10-bit DACs, 10- and 16-bit PWMs, and high-speed comparators, along with two 100-mA, high-current I/Os. The combination of these integrated peripherals help support the demands of multiple independent closed-loop power channels and system management, while providing an 8-bit platform that simplifies design, enables higher efficiency and increase performance while helping eliminate many discrete components in power-conversion systems.

In addition to power-conversion peripherals, these PIC MCUs have a unique hardware-based LED dimming control function enabled by the interconnections of the Data Signal Modulator (DSM), op amp and 16-bit PWM. The combination of these peripherals creates a LED-dimming engine synchronizing switching control eliminating LED current overshoot and decay. The synchronization of the output switching helps smooth dimming, minimizes color shifting, increases LED life and reduces heat. This family also includes Core Independent Peripherals (CIPs), such as the Configurable Logic Cell (CLC), Complementary Output Generator (COG), and Zero Cross Detect (ZCD). These CIPs take 8-bit PIC MCU performance to a new level, as they are designed to handle tasks with no code or supervision from the CPU to maintain operation, after initial configuration. As a result, they simplify the implementation of complex control systems and give designers the flexibility to innovate. The CLC peripheral allows designers to create custom logic and interconnections specific to their application, reducing interrupt latency, saving code space and adding functionality. The COG peripheral is a powerful waveform generator that can generate complementary waveforms with fine control of key parameters, such as phase, dead-band, blanking, emergency shut-down states, and error-recovery strategies. It provides a cost-effective solution, saving both board space and component cost. The ZCD senses when high voltage AC signal crosses through ground, ideal for TRIAC control functions.

These new 8-bit PIC MCUs provide the capability for multiple independent, closed loop power channels and system management making these products appealing to various power supply, battery management, LED lighting, exterior/interior automotive lighting and general-purpose applications. Along with all these features, the family offers EUSART, I2C/SPI and eXtreme Low Power (XLP) Technology, which are all offered in small form-factor packages, ranging from 14- to 20-pin packages.

The PIC16(L)F1769 family is supported by Microchip’s standard suite of world-class development tools, including the MPLAB ICD 3 (part # DV164035, $199.95) and PICkit 3 (part # PG164130, $47.95) and MPLAB Code Configurator, which is a plug-in for Microchip’s freeMPLAB X IDE provides a graphical method to configure 8-bit systems and peripheral features, and gets you from concept to prototype in minutes by automatically generating efficient and easily modified C code for your application.

The PIC(L)F1764, PIC(L)F1765, PIC16(L)F1768, and PIC(L)F1769 are available now for sampling in 14- and 20-pins in PDIP, SOIC, SSOP, TSSOP, and QFN packages. Pricing for the family starts at $0.87 each, in 10,000-unit quantities.

Source: Microchip Technology