In another thread a really cool resource was mentioned, the manual to the NG 304 power supply:
https://www.mikrocontroller.net/attachment/412953/DBL_BEHA_NG304_MANUAL_DEUTSCH.PDFSo I looked at the schematic (which I have included a copy of), and there is an interesting feature which I have drawn red boxes around. Normally the current sense resistor is on the low side of the power supply, and so as the current increases the negative output of the supply is lifted off of ground. However, in the NG 304, the sense resistor is on the high side.
Now I was thinking of using this approach, but there are two potential problems I thought I identified:
1. The common-mode rejection of the op-amp must be really good to ensure that the current sensing threshold doesn't depend on the output voltage level.
2. The circuit is not really a difference amplifier, only a comparator. But if used to actually measure the quantitative current in the circuit (for example putting in a meter), the ability of the circuit to reject the common mode would likely be limited by how closely the divider resistor networks to the + and - terminals are matched to each other. This problem occurs in something like an instrumentation amplifier I suppose.
So why is high-side sensing like is done in the NG 304 not done more often? Is it because of the difficultly of the common-mode rejection when the output voltage changes?
I'm just curious but it's not something I've seen in many power supply designs.