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AD623 unity gain, different results depending the length of Rg path?

Category: Hardware


I made a pcb to test different in Amp, for different gain and I have a incomprehensible result.

below you can see the scematic and how it is on the PCB.

I test with different resistor and it's ok. then, I remove the resitor to have a unity gain, but keep the 2 jumper connected, and test my circuit at different frequency (from 1K to 1MHz)

I have a bode plot, with a amplification around 500KHz of +3dB.

later, I make the same test but without the 2 jumper and the result is different. the amplifiacation around 500KHz is now +0.5dB

I try diffferent thing, cut all wire to reduce at maximum the patch but still have a difference with or without the jumper connection without any resistor. 

I don't understand why, because RG1 and RG2 are not connected. the resistance must be infiny. why the AD623 out change with a longer patch. 

If someone has an explaination please, let me know. 

thank you 

Parents Reply
  • If you need gain of 1 only, you don't have to use an inamp. If higher gains are also required, then you may not find an IC inamp that have sufficient bandwidth. So you may consider to build your own inamp (or, if gain of 1 is enough, a difference amplifier with input followers) using opamps. You can easily find many opamps that can meet all of the requirements. 

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