I'm currently using a differential op amp to get a high common mode voltage (200Volts+) down to a reasonable range. I've been using the AD629 http://www.analog.com/static/imported-files/data_sheets/AD629.pdf i want to modify this and get a even higher voltage range. What resistance do I need to add in order to get this to at least 400V? If the output is just a simple scaling off, I can compensate it in software. The circuit is also a bit lacking, so in the future I"m looking to upgrade it. The input impedance is too low for my purpose, and I couldn't find another one that fits my accuracy and voltage requirement, so I'm looking to create my own. Below is my plan: i'm going to create a single stage diff amp with voltage divided input and precision op amp. But I need to find the resistance needed to get the voltage divided diff amp circuit working. By that I mean I couldn't find the transfer function. I was using the node equation for calculation, and I couldn't even verify the AD629's circuit of why the one reference is 21.1K, while the other is 20K, while all others are 380K. I know it has to do with balancing the ratio of (380+20)/380, with 21.1K/20K, can someone help me make sense of this? I really really appreciate this. http://cds.linear.com/docs/Datasheet/1052fa.pdf Besides getting some high precision low temp co-efficient resistance, and a good precision op amp.