@miro123:
1. like Kleinstein said, cascaded OPs which drive the resistor networks are used here to avoid thermal feedback in the AZ-OP due to changing load current, which would decrease their excellent specs
2. correct, directly connected to an ADC this part needs to be changed a bit anyway for CM/DM-filtering depending on ADC-type
3. this frontend is intended to be used later with AD7177-2 and AD4030-24 ADCs, all sitting on the same pcb and thermally stabilized like in the HPM7177
4. yes, in this case the ADA4523 are used due to their excellent CMRR and Avol-specs, to minimize their error-contribution. The OPA2625 can drive the ADC directly, but its not suited for this task without working in a cascaded OP-stage due to its worse specs.
5. TDP1603 are used since:
-DIP-package which reduces mechanical strain from pcb-TC/humidity swelling and therefore changing resistor parameters
-8 available resistors allowing many dividers combinations
-proven in stuff like Wavetek/Fluke 7000 reference, CERN PBC 10V/10mA source (assumed due to paper)
-noise-spec was tested by Nikolai Beev
If further thermal coupling seems necessary that is no problem by usage of thermal jumpers. The TDP1603 dont need to be thermally connected to each other, the design was done so that every divider is consisting of only one TDP each time.
6. The autozero/gain-switching stuff isnt yet included in this design, but will sit in front of the jfet-AFE like in the 3458A. I still need to design it.
The JFET-AFE could also be done as a differential input, but i dont know what parameters will be affected, since im not capable of fully designing a suitable differential cascoded AFE myself. Since the 3458A seems to do well with its AFE, i just copied it hoping for the best.
LEMOs + maybe thermal jumpers for even better thermal coupling are indeed a good choice and ill also go that way.
@ Kleinstein:
Indeed RN6 is probably the most problematic network with its several 10mW dissipation swings. I hope the thermal stabilization (like HPM7177) will help with it, but im unsure if were still talking about TC-effects or already power coefficients, which are of course not as easy to deal with.
Increasing the value as suggested should help, but i have to do calculations again how that influences parameters.
For people that dont use TEC based thermal stabilization of their AFE the trick from CERNs PBC might help: keep the network-temperature constant regardless of dissipation changes with a controlled heater thats thermally connected to it
2x VHD200 might also be an option as a last resort.
Need to read the thread again about the switch-discussions between Julian and you, to produce a nice AZ/Gain-switching in front of the JFET-AFE.
Attached is a prototype-picture of my first Rev. 0 HPM7177-variant. Yet without aluminium blocks and not completely working (hence the logic analyzer...).
http://cds.cern.ch/record/643294/files/cer-002399331.pdf CERN PBC-paper