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I'm building a device to, among other things, measure a few voltages from different power supplies that are expected to be floating. Because the user might do silly things like connecting them in series to get a negative voltage, the measurement circuit should not connect any of them to its own supply ground.

The measurement logic itself uses a voltage-to-PWM converter, which measures the voltage above its own supply ground, so I need a low-current supply for this IC and an optocoupler that can float together with the measured value.

Do I have to build a chopper+transformer+rectifier+comparator+optocoupler circuit for this, or is there a simpler/cheaper topology that allows me to supply 5V/20mA (my pessimistic estimate) without connecting the ground reference?

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There are several options for obtaining isolated low power supplies.

The simplest, though not necessarily the cheapest, is to buy an 'isolated DC-DC converter' from your favourite online parts supplier.

A partial isolation solution would be a pair of small capacitors, driven differentially with a high frequency, going to a full bridge rectifier. This is not a high voltage or safety isolation, but does allow the grounds to be at different DC potentials, up to the DC rating of the capacitors. The higher the drive frequency, the smaller the capacitors can be to transmit the required current. The differential drive minimises the residual drive signal that appears across the grounds.

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