Getting signals across isolation barriers is a common problem in electronics. In the case of digital signals this is relatively easy and cheap. Optocouplers, transformers or dedicated digital isolator chips are readily available, easy to use and generally low in cost. Unfortunately, things quickly get more complex and expensive if you need to isolate analog signals.
You can in theory use run-of-the-mill optocouplers to transfer analog signals since the LEDs and the phototransistors or photodiodes they are comprised of are essentially analog components. The problem is that their light-current relationship is far from linear and varies widely from device to device.
This can be overcome with specialist analog optocouplers such as the LOC11x from IXYS or the HCNR20x from Broadcom. These include an LED and two matched photodiodes as shown in Figure 1. The matched photodiodes allow us to use an op amp to linearise the input-output current relationship and to virtually eliminate the effect of variations in characteristics between devices.
Analog optocouplers such as the LOC11x from IXYS or the HCNR20x from Broadcom have one LED and two matched photodiodes as shown here. One of the photodiodes is used to linearise the transfer function which would otherwise be extremely non-linear.
Figure 2 shows how this can be done. The op amp U1 drives the optocoupler LED, and photodiode D1 is used to provide negative feedback that ensures the photodiode current ID1 is equal to Vin / R1. Since the diodes are very well matched, the current in the other diode (ID2) will be identical. The op amp U2 amplifies this current to reconstruct the output voltage Vout such that it is equal to Vin * R2/R1. Resistor R limits the LED current and capacitor C may be needed for stability.
The LED is driven by U1 such that negative feedback ensures a current of Vin/R1 passes through photodiode D1. A similar current therefore passes through the matched photodiode D2. Op amp U2 amplifies this current to produce a voltage at the output equal to Vin (R2/R1).
The upside to this approach is its low cost – probably less than $5 in low quantities – and its low noise. The bandwidth is good to a few hundred kilohertz for the circuit shown, but this could be extended to around 1 MHz with very careful design. On the downside this circuit requires a power supply on both sides of the isolation barrier and the absolute gain accuracy is typically around 5%. The circuit shown is only suitable for unipolar input, however it can be adapted to support bipolar input with the addition of a second optocoupler and a few other components.
Another approach is to digitize the analog input and transfer it across the isolation barrier using capacitive or inductive coupling. Devices such as the AMC1350 from TI take the capacitive coupling approach. Figure 3 is a diagram from the data sheet that shows how this device works. The differential input signals are serialized via a delta-sigma modulator, transferred across the isolation barrier, and reconstituted via an analog filter.
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The AMC1350 is a typical example of modern capacitively-couples signal isolators. A differential input signal is amplified and modulated at high frequency so it can be capacitively coupled across the high-voltage isolation barrier. A demodulator/filter reconstructs the original signal on the output side.
This isolator can accept a ±5V input signal and produces a ±2V output with a maximum bandwidth of around 300kHz. The cost of the device is hefty at about $10 apiece in low quantities. Like the first solution we do need a power supply on the input side. Output noise is relatively high as you might expect given the way the device works.
To complete the survey, we should look at an inductively coupled solution. The Analog Devices AD215 shown in Figure 4 is a typical example. This works similarly to the AMC1350 in that the input signal is modulated, but in this case, it is transferred across the isolation barrier via a transformer before being demodulated, filtered, and buffered. Similar to the AMC1350, the AD215 has some impressive specifications making it suitable for precision applications but is limited to a bandwidth of 120kHz.
The AD215 is a typical example of a magnetically coupled isolation amplifier. It does have the advantage of including an isolated power supply to drive the front end, but the cost of around $100 makes it an expensive solution when compared with its capacitively-couples counterparts.
This device is also quite expensive at around $100 each, but unlike the other solutions, it includes an isolated power supply. The high cost of these devices is related to their hybrid nature. Combining magnetic components with silicon requires several manufacturing processes and an assembly step that the all-silicon capacitive coupling approached do not.
I have not looked at some important characteristics such as isolation voltage, input/output capacitance and many others that you will almost certainly have to consider in your application. That said, it is my personal view that magnetically coupled isolators are slowly becoming obsolete and I would avoid them for most new designs, especially given their cost disadvantage.
References
“High Linearity Analog Optocouplers.” Accessed August 12, 2022. https://www.broadcom.com/products/optocouplers/industrial-plastic/specific-function/high-linearity-analog.
“AMC1350-Q1 Data Sheet, Product Information and Support | TI.Com.” Accessed August 12, 2022. https://www.ti.com/product/AMC1350-Q1.
“AD215 Datasheet and Product Info | Analog Devices.” Accessed August 12, 2022. https://www.analog.com/en/products/ad215.html#product-overview.
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Andrew Levido (andrew.levido@gmail.com) earned a bachelor’s degree in Electrical Engineering in Sydney, Australia, in 1986. He worked for several years in R&D for power electronics and telecommunication companies before moving into management roles. Andrew has maintained a hands-on interest in electronics, particularly embedded systems, power electronics, and control theory in his free time. Over the years he has written a number of articles for various electronics publications and occasionally provides consulting services as time allows.
