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

Category: Hardware

hello, 

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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  • 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?

    BR

  • Hi seth1er,

    I would agree with ZoltanDTI's answer. Just a few notes, could you verify that your operation of frequency is at 500kHz? For AD623, operating at unity gain 500kHz is not recommended as seen in figure 28 that there is some peeking at around that area.

    Even in figure 29 that there is no seen peaking, the flat response is only at around 100kHz.

    Cutoff frequency is usually set as 1decade roll-off to assure stability of the operation. I'm not quite sure that your operation is at 500kHz, if it is, I'll be suggesting that we look for another pin-compatible/footprint compatible InAmp.

    Best regards,

    Kristine

  • hi, 

    Thanks for your answer.

    my operating frequency is really at 500KHz ( and I already test until 1MHz) I noticed that the cut off is around 700KHz (-3dB). I don't understand why working at 500KHz is not recommanded for AD623.

    if you have another inAmp better I'm interested. 

    It's for a low power application (ad623 is under 1mA)  with human skin(noise environnement). 

    Thanks for your help

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  • hi, 

    Thanks for your answer.

    my operating frequency is really at 500KHz ( and I already test until 1MHz) I noticed that the cut off is around 700KHz (-3dB). I don't understand why working at 500KHz is not recommanded for AD623.

    if you have another inAmp better I'm interested. 

    It's for a low power application (ad623 is under 1mA)  with human skin(noise environnement). 

    Thanks for your help

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