Setting the absolute potential of an isolated DC power supply

groundpower supply

I have a circuit that, for reasons out of my control, is powered by an bipolar +/-4V supply. This circuit includes instrumentation amplifiers and so forth that have input headroom requirements of over 1V.

I use an isolated DC/DC converter to produce a floating 5V supply from +4V/0. Now, I'd like to be able to reference the mid-voltage of this floating supply (2.5V) to the circuits ground, so that the 5V output actually looks like a +/-2.5V supply from the circuit's ground. The device running off the 5V supply is only going to send analog signals back into the main circuit, and that into high-impedance inputs.

How can I do this sort of biasing? All the currents involved are low, but precision is important to me. I'd like to avoid ground loops and the like as far as possible.

Best Answer

Conceptually, you could do something like this:


simulate this circuit – Schematic created using CircuitLab

In reality, depending on the op-amp type and the value and positions of bypass capacitors on the supplies, you'd have to take some precautions to keep the op-amp from oscillating, most likely.

Since the output voltage of the op-amp is nominally 1.5V above the negative rail, pick an op-amp that can sink or source all necessary current at that voltage with some margin. If there is significant bypassing on the supplies, either pick an op-amp that has enough phase margin with a heavy capacitive load or decouple the output with a smallish resistor and establish an AC feedback path.

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