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I'm trying to figure out how to implement an auto range by changing the resistors values for R2 and R3 with a MCU. First I was thinking of using a mux like the ADG5208BCPZ, but I think I might fry the chip depending on the input voltage (like 100v). I am considering on making a mux using relays, but I'd like to try a more compact option. Can someone explain to me how add this functionality to my circuit?

I have designed a board with an AD7794 with inputs to read analog signals, but each differential pair had a set range (+-0-1.17V, 2x +-0-100V, 0-5A, 0-100kOhm (they all have input protection)) and I used the ADC's gain to have a pseudo auto range. But there are some things I want to improve, like adding an instrumentation amp between the signal and the ADC and an actual range select.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Start with a decent specification of what you want. \$\endgroup\$ – Andy aka May 11 at 7:27
  • \$\begingroup\$ I just want 4 inputs to measure 0-500v and 0-5A, and the 5th channel for resistance measurements using the ADC's 10uA, 210uA and 1mA current sources for 4 or 2 point resistance measurement. if possible it would be nice to have the other 4 inputs to be able to do 2 point resistance measurements using an external current source. How to implement these things individually I know I just dont know how get this mux. If i didnt answer your question could you be more specific? I'm a physicist not an engineer so I dont exactly know what to say. \$\endgroup\$ – user169808 May 11 at 13:02
  • \$\begingroup\$ just to add I'm trying to use the service manual for the fluke 27 as a general guide. here is the link if you want testmart.com/webdata/mfr_pdfs/FLU/27______smeng0100.pdf \$\endgroup\$ – user169808 May 11 at 19:01
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following this tutorial form analog devices I came up with the following that seems like it will work. I had to change the op-amp to the AD8226.

schematic

simulate this circuit – Schematic created using CircuitLab

I only need to compare the error between using the internal gain of the ADC's instrumentation amplifiers and using the AD8226. I plan on using a solid state relay like the TLP3409S or TLP175A.

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