Correct RN500 is for OHM i just double checked it,
I did discover the signal path, from jack to ADC,
I am now mapping temperature dependend parts in the 20V range,
in this quest i find a few things of interest
the Analog muxes are very sensitive to temperature, i did not expect that.
the large resistors R302-R309 are the voltage dividers for 200mV and 2V range.
the Resistor network RN1 is in use 20V-200V-1000V range, and is also very sensitive to temperature,
and it is very sensitive it is to anything comming near it, maybe a good idea to shield it ?
and to temperature isolate it from ambient ?
the zero drift opamp U406 is both the impedance buffer for ADC and LO ref drive.. it is also sensitive to temperature.
Refferance chip and ADC it self are also sensitive, but not as much as i expected,
right next to the ref chip, there is a TL072 running at +/-15V it is running hot, i did not figure out its function yet.
I am preparing to explain my findings in more detail soon.
signals explain
in 20V DC range, 10.0V input
GND of RN1 is+2.5V (buffered refferance)
LO black terminal for voltage input is +2.5V same buffered ref.
divided voltage from RN1 (pin3 is selected via ADG1408 U number unknown
then the voltage is buffered with left side U406 AD8629 zero drift opamp
right side of U406 is also a buffer, for the ref, both outputs go via high value resistor to a cap just over U406
this differential signal is 2.6V and 2.5V (the measured 10V input, is now only 100mV dif signal)
zero input, is 2.5V on both.
on the ADC, pin11 is the 2.6V, voltage measure
pin 12 is the buffered ref 2.5V
pin 13 and 15 is a direct connection to ref ic pin 6 = 2.5V
i find it a little bit odd, 20V range, and 200V range is 100% the same hardware configuration,
the only differance is internal ADC gain,
this means the 20V range, with a 10V signal, will have 10 times as much noise,
compared to the 200V range with a 100V input,
i would have loved it to be the other way :-)