Controlling an electronic load across two ground domains

[gnurf]

Note slew rate now referenced to Vcom [...]

I'm trying to understand why you made this subtle change. In practice I'd think a simple RC filter referenced to ground would do the job quite nicely, but by referencing it to Vcom I think you've now provided a more direct way to couple a fast rate of change in the DAC output voltage to the negative input. The end result is that the unwanted fast rate of voltage change now appears as common mode on both inputs of the opamp and is rejected. Is that correct or thereabouts?

 

[Baluncore]

Yes. I think I was trying to prevent power supply noise from showing up on the mosfet gate.

I did a quick redesign using an isolated DC to ±12 VDC converter. That eliminates the level shifting resistor ratio errors, and all load-supply quiescent current.

See attached.

 

[Baluncore]

Another minimisation here. If instead of amplifying the sense voltage the output from the isolation amplifier is attenuated, the error amplifier inputs are smaller. That requires a precision op-amp be used as the error amplifier, which was the case. There are two associated benefits. Firstly the AD629 offset voltage is attenuated by 40, from 1mV to 25uV. Secondly it eliminates the offset voltage and CMRR of the sense amplifier being multiplied by 40. So two errors that might have summed have been eliminated.
Attached is the new circuit.