Creating my own high voltage divided diff amp. what resistance should I put in?

[kevin.cheung]

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.

[KevinIV]

Diff amps are still more like opamps. The instrumentation amplifier your designing is going to be inherently poor because the voltage is too high. The problem is that the ouput will be noise. Only a fragment of the signal will remain, but it may be useful.

[kevin.cheung]

I solved the problem. I guess I'm just very rusted on my op-amp math.

@Kevin, what's your recommendation?

[KevinIV]

I dont' recommend any circuits I know of. If your design is functional, it may be worth developing. There are many electronics products I have no experience with.