Electronic – Measuring voltage while maintaining isolation

isolationpower supply

I'm building a variable power supply, and want to provide the ability to digitally measure the outputted voltage and current. The digital power supply used to drive the ADC, microcontroller and friends is separate from the unregulated power supply used to derive output voltages.

The power supply for the microcontroller is galvanically isolated from the one for the output voltages — the first is an unregulated transformer, while the second one is an SMPS supply.

Now, measuring the current while maintaining galvanic isolation is easy, since I'm using a Hall Effect sensor for that, but I don't see a similar method for doing that for voltage measurements. My plan was to use a resistor divider and shoving that voltage into the ADC. However, that completely ruins all the effort I put into galvanic isolation of the two rails, since the downscaled voltage output would get into the ADC, causing all kinds of problems.

Is there any kind of solution out there that will let me measure that voltage, while still maintaining galvanic isolation?

I don't think optocouplers will work, seeing as they're either on or off, and I'm not familiar with any other kind of isolation, and I'm not really familiar with any other kind of isolation techniques, but I'm thinking that perhaps transformers could be useful in this situation?

Best Answer

You're looking for an isolation amplifier. These are great if you're looking for a monolithic solution that is easy to integrate, but might not be cost-optimized. Some even provide a little isolated (HV-side) power via an internal DC/DC converter to run scaling op-amps, etc.

I've had good luck with the Burr Brown ISO series of isolation amplifiers (acquired by TI a few years back). The ISO122 is a general use product, and the ISO124 has higher accuracy.

Analog Devices also makes a few general purpose isolation amps.

Just be aware that some sort of modulation is required to move the signal across the isolation barrier within an isolation amplifier. Some do it inductively, some optically. Either way, plan on a little ripple on the output signal. The specs usually do a good job of outlining the limitations.