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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 

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