Decouple Rprog setting from internal GND of DCP405

[fietser28]

In the current implementation of the DCP405 Rprog setting is done relative to the internal GND of the DCP405. This makes it impossible to connect a AWG with GND to protective earth (PE) AND also connect a scope (with PE to GND) to the - terminal fo the DCP405 due to the voltage difference between the GND and - output of the DCP405.

Attached schematic solves this use case: Both Rprog and the negative potential of the Load can be connected to PE.

• I did not test or design this for cases where PE is connected to the + side of the load (e.q. use the DCP405 to provide negative voltage).
• It looks like there is an 50mV offset in Rprog programming. This probably can be solved with the remark in the schematic. I did not test this.

This issue can be combined with #87?