Op-Amp Versus Comparator (EE Tip #128)

Practically every lecture course or textbook on electronics describes how to use an operational amplifier as a comparator. Here we look at the possibility in more detail, and see how it can often be a very poor idea.

The idea behind the comparator configuration is simple. An op-amp has a very high open-loop DC gain which means that even a tiny differential input voltage will drive the output to one extreme or the other. If the voltage at the non-inverting (“+”) input is greater than that at the inverting (“–“) input the output goes high; otherwise the output goes low. In other words the two voltages are compared and the output is a binary indication of which of the two is the greater.

Figure 1: SPICE simulation results: an LT1028 op-amp pressed into service as a comparator versus a real comparator type LT1720.

Figure 1: SPICE simulation results: an LT1028 op-amp pressed into service as a comparator versus a real comparator type LT1720.

So the op-amp looks like the perfect device to use as a comparator. But why then do there exist special-purpose comparator devices? Looked at from the outside, op-amps and comparators appear indistinguishable. Besides power connections, they both have “+” and “–” inputs and a single output. Taking a look at the internal circuit diagram, again the two devices appear broadly very similar (although a comparator device with an open-collector or open-drain output does look more obviously different from an op-amp). The big difference, which is not apparent without looking at the circuit more closely, is that the output stages of operational amplifiers are designed for linear operation, with the general aim of amplifying the input signal with as little distortion as possible (assuming that some negative feedback is provided), but in the case of a comparator the output circuit is designed to operate in saturation, that is, to switch between the upper and lower output voltage limits without the provision of external feedback. Comparators often also offer a ground connection in addition to the usual power connections, and provide digital logic levels at their outputs while accepting symmetrical analogue input signals.

What do these differences mean in practice? Comparators can react very quickly to changes in their input voltages with short propagation delays and output rise- and fall-times all specified by the manufacturer.

In contrast, because op-amps are not expected to be used in this mode, manufacturers tend not to give explicit specifications for propagation delay and rise- and fall-times (although they do normally specify slew rate), and these characteristics can be considerably poorer for op-amps than for comparators. To take an extreme example, a low-power op-amp might have a propagation delay measured in milliseconds, whereas a comparator might react in nanoseconds: a million times faster.

There is a further problem with op-amps. Many devices exhibit significantly increased power consumption when the output is in saturation, the resulting power dissipation on occasion being enough to destroy the device. Also, many op-amps (those not advertised as having “rail-to-rail outputs”) are not capable of driving their outputs close to the supply rails, for example having a maximum output voltage of 3 V with a 5-V supply. There can also be restrictions on the inputs. Some op-amps are equipped with antiparallel diodes across their input terminals, which prevent differential input voltages of more than about 0.6 V, whereas comparators’ inputs are often allowed to vary over the whole supply range.

Of course, there are many noncritical applications where an op-amp will work perfectly acceptably as a comparator, but it is not a practice to be recommended. The skeptic should lash up a quick test with a comparator and an op-amp side-by-side, each fed with a squarewave signal with rapid edges. Some of the potential pitfalls are shown up more easily in simulation, such as the possibility of an op-amp being so slow that it entirely misses a narrow pulse. It is hard to guarantee circuit performance, current consumption, and even the survival of the device.

The illustrations show a SPICE simulation of a relatively nimble op-amp (an LT1028 with a minimum slew rate of 11 V/µs) and a type LT1720 comparator. It is clear that the comparator responds sooner and with a much shorter rise-time. Its output swings all the way to +5 V rather than the 3 V managed by the op-amp. The situation is similar when the output swings low: the op-amp is much slower and only reaches an output voltage of –3 V rather than –5 V. The original squarewave is hardly recognizable at the op-amp’s output. Although the LT1028 cannot achieve its maximum specified gain with a –5-V supply, it is still a factor of at least 20 faster than an LM324 (with a slew rate of 0.5 V/µs); what the latter would make of our squarewave would not be a pretty sight. The op-amp fails to cope at all with shorter pulses, which are then effectively “swallowed,” while the comparator continues to handle them without difficulty.

Worthwhile further reading on this subject is Texas Instruments application note SLOA067 by Bruce Carter entitled “Op Amps and Comparators—Don’t Confuse Them!.”— Michael Holzl, Elektor January 2011

CircuitCellar.com is an Elektor International Media publication.

New 8-bit PIC Microcontrollers: Intelligent Analog & Core Independent Peripherals

Microchip Technology, Inc. announced Monday from EE Live! and the Embedded Systems Conference in San Jose the PIC16(L)F170X and PIC16(L)F171X family of 8-bit microcontrollers (MCUs), which combine a rich set of intelligent analog and core independent peripherals, along with cost-effective pricing and eXtreme Low Power (XLP) technology. Available in 14-, 20-, 28-, and 40/44-pin packages, the 11-member PIC16F170X/171X family of microcontrollers integrates two op-amps to drive analog control loops, sensor amplification and basic signal conditioning, while reducing system cost and board space.

PIC16F170X/171X MCUs reduce design complexity and system BOM cost with integrated op-amps, zero cross detect, and peripheral pin select.

PIC16F170X/171X MCUs reduce design complexity and system BOM cost with integrated op-amps, zero cross detect, and peripheral pin select.

These new devices also offer built-in Zero Cross Detect (ZCD) to simplify TRIAC control and minimize the EMI caused by switching transients. Additionally, these are the first PIC16 MCUs with Peripheral Pin Select, a pin-mapping feature that gives designers the flexibility to designate the pinout of many peripheral functions.

The PIC16F170X/171X are general-purpose microcontrollers that are ideal for a broad range of applications, such as consumer (home appliances, power tools, electric razors), portable medical (blood-pressure meters, blood-glucose meters, pedometers), LED lighting, battery charging, power supplies and motor control.

The new microcontrollers feature up to 28 KB of self-read/write flash program memory, up to 2 KB of RAM, a 10-bit ADC, a 5-/8-bit DAC, Capture-Compare PWM modules, stand-alone 10-bit PWM modules and high-speed comparators (60 ns typical response), along with EUSART, I2C and SPI interface peripherals. They also feature XLP technology for typical active and sleep currents of just 35 µA/MHz and 30 nA, respectively, helping to extend battery life and reduce standby current consumption.

The PIC16F170X/171X family is supported by Microchip’s standard suite of world-class development tools, including the PICkit 3 (part # PG164130, $44.95), MPLAB ICD 3 (part # DV164035, $189.99), PICkit 3 Low Pin Count Demo Board (part # DM164130-9, $25.99), PICDEM Lab Development Kit (part # DM163045, $134.99) and PICDEM 2 Plus (part # DM163022-1, $99.99). The MPLAB Code Configurator is a free tool that generates seamless, easy-to-understand C code that is inserted into your project. It currently supports the PIC16F1704/08, and is expected to support the PIC16F1713/16 in April, along with all remaining microcontrollers in this family soon thereafter.

The PIC16(L)F1703/1704/1705 microcontrollers are available now for sampling and production in 14-pin PDIP, TSSOP, SOIC and QFN (4 x 4 x 0.9 mm) packages. The PIC16F1707/1708/1709 microcontrollers are available now for sampling and production in 20-pin PDIP, SSOP, SOIC and QFN (4 x 4 x 0.9 mm) packages. The PIC16F1713/16 MCUs are available now for sampling and production in 28-pin PDIP, SSOP, SOIC, QFN (6 x 6 x 0.9 mm) and UQFN (4 x 4 x 0.5 mm) packages. The PIC16F1718 microcontrollers are expected to be available for sampling and production in May 2014, in 28-pin PDIP, SSOP, SOIC, QFN (6 x 6 x 0.9 mm) and UQFN (4 x 4 x 0.5 mm) packages. The PIC16F1717/19 microcontrollers are expected to be available for sampling and production in May 2014, in 40/44-pin PDIP, TQFP and UQFN (5 x 5 x 0.5 mm). Pricing starts at $0.59 each, in 10,000-unit quantities.

Source: Microchip Technology, Inc.

A Look at Low-Noise Amplifiers

Maurizio Di Paolo Emilio, who has a PhD in Physics, is an Italian telecommunications engineer who works mainly as a software developer with a focus on data acquisition systems. Emilio has authored articles about electronic designs, data acquisition systems, power supplies, and photovoltaic systems. In this article, he provides an overview of what is generally available in low-noise amplifiers (LNAs) and some of the applications.

By Maurizio Di Paolo Emilio
An LNA, or preamplifier, is an electronic amplifier used to amplify sometimes very weak signals. To minimize signal power loss, it is usually located close to the signal source (antenna or sensor). An LNA is ideal for many applications including low-temperature measurements, optical detection, and audio engineering. This article presents LNA systems and ICs.

Signal amplifiers are electronic devices that can amplify a relatively small signal from a sensor (e.g., temperature sensors and magnetic-field sensors). The parameters that describe an amplifier’s quality are:

  • Gain: The ratio between output and input power or amplitude, usually measured in decibels
  • Bandwidth: The range of frequencies in which the amplifier works correctly
  • Noise: The noise level introduced in the amplification process
  • Slew rate: The maximum rate of voltage change per unit of time
  • Overshoot: The tendency of the output to swing beyond its final value before settling down

Feedback amplifiers combine the output and input so a negative feedback opposes the original signal (see Figure 1). Feedback in amplifiers provides better performance. In particular, it increases amplification stability, reduces distortion, and increases the amplifier’s bandwidth.

 Figure 1: A feedback amplifier model is shown here.


Figure 1: A feedback amplifier model is shown.

A preamplifier amplifies an analog signal, generally in the stage that precedes a higher-power amplifier.

IC LOW-NOISE PREAMPLIFIERS
Op-amps are widely used as AC amplifiers. Linear Technology’s LT1028 or LT1128 and Analog Devices’s ADA4898 or AD8597 are especially suitable ultra-low-noise amplifiers. The LT1128 is an ultra-low-noise, high-speed op-amp. Its main characteristics are:

  • Noise voltage: 0.85 nV/√Hz at 1 kHz
  • Bandwidth: 13 MHz
  • Slew rate: 5 V/µs
  • Offset voltage: 40 µV

Both the Linear Technology and Analog Devices amplifiers have voltage noise density at 1 kHz at around 1 nV/√Hz  and also offer excellent DC precision. Texas Instruments (TI)  offers some very low-noise amplifiers. They include the OPA211, which has 1.1 nV/√Hz  noise density at a  3.6 mA from 5 V supply current and the LME49990, which has very low distortion. Maxim Integrated offers the MAX9632 with noise below 1nV/√Hz.

The op-amp can be realized with a bipolar junction transistor (BJT), as in the case of the LT1128, or a MOSFET, which works at higher frequencies and with a higher input impedance and a lower energy consumption. The differential structure is used in applications where it is necessary to eliminate the undesired common components to the two inputs. Because of this, low-frequency and DC common-mode signals (e.g., thermal drift) are eliminated at the output. A differential gain can be defined as (Ad = A2 – A1) and a common-mode gain can be defined as (Ac = A1 + A2 = 2).

An important parameter is the common-mode rejection ratio (CMRR), which is the ratio of common-mode gain to the differential-mode gain. This parameter is used to measure the  differential amplifier’s performance.

Figure 2: The design of a simple preamplifier is shown. Its main components are the Linear Technology LT112 and the Interfet IF3602 junction field-effect transistor (JFET).

Figure 2: The design of a simple preamplifier is shown. Its main components are the Linear Technology LT1128 and the Interfet IF3602 junction field-effect transistor (JFET).

Figure 2 shows a simple preamplifier’s design with 0.8 nV/√Hz at 1 kHz background noise. Its main components are the LT1128 and the Interfet IF3602 junction field-effect transistor (JFET).  The IF3602 is a dual N-channel JFET used as stage for the op-amp’s input. Figure 3 shows the gain and Figure 4 shows the noise response.

Figure 3: The gain of a low-noise preamplifier.

Figure 3: The is a low-noise preamplifier’s gain.

 

Figure 4: The noise response of a low-noise preamplifier

Figure 4: A low-noise preamplifier’s noise response is shown.

LOW NOISE PREAMPLIFIER SYSTEMS
The Stanford Research Systems SR560 low-noise voltage preamplifier has a differential front end with 4nV/√Hz input noise and a 100-MΩ input impedance (see Photo 1a). Input offset nulling is accomplished by a front-panel potentiometer, which is accessible with a small screwdriver. In addition to the signal inputs, a rear-panel TTL blanking input enables you to quickly turn the instrument’s gain on and off (see Photo 1b).

Photo 1a:The Stanford Research Systems SR560 low-noise voltage preamplifier

Photo 1a: The Stanford Research Systems SR560 low-noise voltage preamplifier. (Photo courtesy of Stanford Research Systems)

Photo 1 b: A rear-panel TTL blanking input enables you to quickly turn the Stanford Research Systems SR560 gain on and off.

Photo 1b: A rear-panel TTL blanking input enables you to quickly turn the Stanford Research Systems SR560 gain on and off. (Photo courtesy of Stanford Research Systems)

The Picotest J2180A low-noise preamplifier provides a fixed 20-dB gain while converting a 1-MΩ input impedance to a 50-Ω output impedance and 0.1-Hz to 100-MHz bandwidth (see Photo 2). The preamplifier is used to improve the sensitivity of oscilloscopes, network analyzers, and spectrum analyzers while reducing the effective noise floor and spurious response.

Photo 2: The Picotest J2180A low-noise preamplifier is shown.

Photo 2: The Picotest J2180A low-noise preamplifier is shown. (Photo courtesy of picotest.com)

Signal Recovery’s Model 5113 is among the best low-noise preamplifier systems. Its principal characteristics are:

  • Single-ended or differential input modes
  • DC to 1-MHz frequency response
  • Optional low-pass, band-pass, or high-pass signal channel filtering
  • Sleep mode to eliminate digital noise
  • Optically isolated RS-232 control interface
  • Battery or line power

The 5113 (see Photo 3 and Figure 5) is used in applications as diverse as radio astronomy, audiometry, test and measurement, process control, and general-purpose signal amplification. It’s also ideally suited to work with a range of lock-in amplifiers.

Photo 3: This is the Signal Recovery Model 5113 low-noise pre-amplifier.

Photo 3: This is the Signal Recovery Model 5113 low-noise preamplifier. (Photo courtesy of Signal Recovery)

Figure 5: Noise contour figures are shown for the Signal Recovery Model 5113.

Figure 5: Noise contour figures are shown for the Signal Recovery Model 5113.

WRAPPING UP
This article briefly introduced low-noise amplifiers, in particular IC system designs utilized in simple or more complex systems such as the Signal Recovery Model 5113, which is a classic amplifier able to obtain different frequency bands with relative gain. A similar device is the SR560, which is a high-performance, low-noise preamplifier that is ideal for a wide variety of applications including low-temperature measurements, optical detection, and audio engineering.

Moreover, the Krohn-Hite custom Models 7000 and 7008 low-noise differential preamplifiers provide a high gain amplification to 1 MHz with an AC output derived from a very-low-noise FET instrumentation amplifier.

One common LNA amplifier is a satellite communications system. The ground station receiving antenna will connect to an LNA, which is needed because the received signal is weak. The received signal is usually a little above background noise. Satellites have limited power, so they use low-power transmitters.

Telecommunications engineer Maurizio Di Paolo Emilio was born in Pescara, Italy. Working mainly as a software developer with a focus on data acquisition systems, he helped design the thermal compensation system (TCS) for the optical system used in the Virgo Experiment (an experiment for detecting gravitational waves). Maurizio currently collaborates with researchers at the University of L’Aquila on X-ray technology. He also develops data acquisition hardware and software for industrial applications and manages technical training courses. To learn more about Maurizio and his expertise, read his essay on “The Future of Data Acquisition Technology.”

DSP vs. RISC Processors (EE Tip #110)

There are a few fundamental differences between DSP and RISC processors. One difference has to do with arithmetic. In the analog domain, saturation, or clipping, isn’t recommended. But it generally comes with a design when, for example, an op-amp is driven high with an input signal. In the digital domain, saturation should be prevented because it causes distortion of the signal being analyzed. But some saturation is better than overflow or wrap-around. Generally speaking, a RISC processor will not saturate, but a DSP will. This is an important feature if you want to do signal processing.

Let’s take a look at an example. Consider a 16-bit processor working with unsigned numbers. The minimum value that can be represented is 0 (0×0000), and the maximum is 65535 (0xFFFF). Compute:

out = 2 × x

where x is an input value (or an intermediate value in a series of calculations). With a generic processor, you’re in trouble when x is greater than 32767.

If x = 33000 (0x80E8), the result is out = 66000 (0x101D0). Because this value can’t be represented with 16 bits, the out = 2 × x processor will truncate the value:

out = 2 × 333000 = 464(0x01D0)

From that point on, all the calculations will be off. On the other end, a DSP (or an arithmetic unit with saturation) will saturate the value to its maximum (or minimum) capability:

out = 2 × 333000 = 65535(0xFFFF)

In the first case, looking at out, it would be wrong to assume that x is a small value. With saturation, the out is still incorrect, although it accurately shows that the input is a large number. Trends in the signal can be tracked with saturation. If the saturation isn’t severe (affecting only a few samples), the signal might be demodulated correctly.

Generic RISC processors like the NXP (Philips) LPC2138 don’t have a saturation function, so it’s important to ensure that the input values or the size of the variable are scaled correctly to prevent overflow. This problem can be avoided with a thorough simulation process.—Circuit Cellar 190, Bernard Debbasch, “ARM-Based Modern Answering Machine,” 2006.

This piece originally appeared in Circuit Cellar 190, 2006. 

Voltage Regulator Protection (EE Tip #103)

In many cases, the load connected to a voltage regulator is not returned to ground. It goes to an even lower voltage or perhaps even the negative power supply voltage. (Here we make the assumption of using positive voltages, when using voltage regulators with negative output voltages the reverse is true.)

Op-amps and level-shifters come to mind. In such cases, a diode (1N4001 or equivalent) connected across the output of the regulator IC usually provides sufficient protection (see Figure 1).

Source:Ton Giesberts, Elektor, 080943-I, 4/2009

Source:Ton Giesberts, Elektor, 080943-I, 4/2009

Polarity inversions which could occur, for example, during power on or during a short circuit could prove fatal for the regulator IC, but such a diode prevents the output of the IC going lower than ground (well, minus 0.7 V, to be accurate).

A short-circuit proof voltage regulator (such as the 78xx series) will survive such a situation without any problems. It is also possible for the input voltage of a voltage regulator to drop quicker than the output voltage—for example, when there is a protection circuit that shorts the input power supply voltage as a result of an overvoltage at the output.

If the output voltage of the regulator is more than 7 V higher than the input voltage, the emitter-base junction of the internal power transistor can break down and cause the transistor to fail.

You can use a shunt diode to prevent this condition (see Figure 2). This ensures that any higher voltage at the output of the regulator is shorted to the input.

—Ton Giesberts, Elektor, 080943-I, 4/2009