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It has nothing to do with the feedback loop, as it's neither inside, nor parallel to the feedback loop.
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Oh, it most definitely is across the feedback loop. Read the circuit diagram very carefully, paying attention to all the common terminal symbols marked 'B'. You'll find that 'B' is connected to the output of the class B output stage (at the same place that all the other feedback connections start from at a star point), one end of that 250G resistor (via an insignificant 2.2k, where it also connects to that trim circuit) and the centre rail of the 5V bootstrap supply. The other end of the 250G resistor is attached to the amp's summing point.
Nope, in Ampere mode, Ground "B" is 'bootstrapped ground', not the signal output!
Signal output is created as explained on page 6-6.
It's really a bit tricky, to understand, how this is accomplished, as the whole driver circuit is intended also for volt and ohm circuit which may float +/- 210V. Carefully check the relays settings for the different modes.
In U, R modes, this amplifier output works differently, than in current and charge mode.
But we're discussing Ampere mode, anyhow.
The feedback loop, or in other words the shunt resistors, are R312, R322, R331, or R330, i.e. 100 Ohm, 100k, 100M or 100G. These create a voltage, proportional to the input current.
This amplifier output is at last created over RL, see figure 6-7 on page 6-6!
The Ground B you're referring to, is left to RL, and is signal output for Volt and Ohm mode only, like in figure 6-6. This bootstrap circuit accounts for 200TOhm input resistance in Volt mode, for all ranges up to 200V, which is very special.
R332, the 250G resistor, has absolutely nothing to do with with this current measurement / feedback loop.
You can check this, if you carefully read the bias calibration paragraph 7.4.9 on page 7-5, and following the schematic in parallel.
I have to admit, that this part of the circuit is not well explained, like the whole "Theory of Operation" chapter is very bad. No fun to read, in contrast to other (HP) manuals.
Frank