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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 

  • Hi,

    If you look at Figure 28 in the datasheet of the AD623, you’ll see the peaking as you’ve observed. But look at Figure 29 too: there is no peaking. The only difference is the package. Therefore I think the solution is the stray capacitance at the inverting inputs of the internal opamps (where you connect the gain setting resistor). This can depend on the package, but also on the external wiring. If you put the jumpers on, the stray capacitance is higher. This results in slightly higher gain at higher frequencies, since the feedback factor is lower, the noise gain is higher.

    I hope it can help to understand the reason.

  • Hi, 

    Thanks to your replay. 

    I think I understand a bit more.

    Do you have a internal schematic more detailed to see about the internal of the In Amp?


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