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linear gain instrument amplifier

xyz xyz
Intellectual 1000 points

Other Parts Discussed in Thread: OPA657, OPA656, VCA821, TINA-TI

Hello

I want to measure a signal within 50mVp-p and with the frequency range between 10KHz to 10MHz.

I decided to measure the signal with two unity gain OPA657 (I mean the 1st stage of instrument Amp). then I connected the signals to a Difference Amplifier (AD8129)

what should I do to have very linear gain in the desired band? I have a medical application and it's very important. Please help me.

it's my email: golhaaa@gmail.com

Regards

  • 0
    TI__Genius 15975 points

    Hi,

    I think the main limitation of linear gain in the desired band is coming from the AD8129 which is operating at G=20. To achieve less than 0.1dB gain flatness in the 10kHz to 10MHz range, I think you would need the difference amplifier to have higher bandwidth. If you look at Figure 33 in the datasheet (attached below), the gain is starting to roll-off beyond 3MHz for G=+20. Also, you have used the OPA657 in unity-gain configuration, which is actually a decompensated amplifier (or stable for gains greater than 10V/V only). I would recommend using the OPA656 instead of that. 

    I have attached a TINA-TI schematic of how you could achieve <0.1dB gain flatness using OPA656 and VCA821. I have adjusted the first stage OPA656 gain to 2V/V and difference amplifier VCA821 gain to 10V/V for this.  OPA656_VCA821_DiffAmp.TSC

    Please let us know if there are questions.

    Best Regards,

    Rohit