Using ADALM1000 AIN and CHA as kelvin connection

Question

Hi, I m having some issue trying to use ADALM1000 as a kelvin connector. I want to get accurate force voltage at CHA. 
 At board revision F, I found that I can open S4 with: 
 devx.ctrl_transfer(DevID, 0x40, 0x51, 34, 0, 0, 0, 100) 
 
 Then I made a proof test with 2 series resistors like below: 
 
 In this case, If I apply 1V at CHA and measuring half of this voltage at AIN, I was expecting to have something close to 2V at CHA, while having accurate 1V at AIN. But I always get 1V at CHA. What am I missing? 
 Thanks in advance.

dmercer · Accepted Answer

Hello: 
 Sorry but the AIN Split Input/Outpup mode does not work that way. That schematic of the SMU output driver is incomplete. The analog feedback path from the driver output (CHA) is always connected back to the driver inverting input (with the non-inverting input always connected to the set point DAC). So it is not possible to use the AIN as a SENSE input as you are attempting. I would add a link to the full design schematic of the F revision but I can't find it right now. I think it is some where on the ADI Wiki. 
 The purpose of the AIN pin is to open the SMU feature of simultaneous sourcing and measuring on one single pin and allow more conventional measuring and sourcing on separate individual pins. 
 Sorry if this was somehow confusing. 
 Doug)

dmercer · Answer

Not really, the PCB layout is multi-layer and very closely packed with fine pitch lines and IC pins. If you cut feedback connection an bodge in a wire to an external connection you will break the source current output mode and render the factory calibration in valid. 
 The only real way to do this would be to add another SPDT analog switch properly controlled by the micro controller to switch things in the proper order etc. 
 So I wouldn't recommend doing such hacking on a board. Likely to break the board in an un-recoverable way. 
 BTW just for completeness of this discussion in case you want to understand in more detail the Rev F design files can be downloaded using the links at the bottom of this Wiki page: 
 https://wiki.analog.com/university/tools/m1k/hw

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Using ADALM1000 AIN and CHA as kelvin connection