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ADG794 Synchronously Rectified Charge Injection

Hello.

I'm trying to synchronously rectify and integrate a high-impedance AC signal using an ADG794 switch in the circuit shown below.

It works for lower values of R1, but once the current drops below about 100nA RMS, the integrator goes the wrong way (i.e., the rectified current is in the opposite direction of what it should be).

The signal frequency (V1) is much higher than the R1 C1 time constant, so the phase shift should be negligible. I've tried offsetting the switch and signal frequency clocks in time. The system is effectively dual supply and nothing exceeds the ADG794 supply rails. The issue does not seem to be DC leakage, because the integrator holds a constant charge for a long time if the switch remains off. Using a low input-offset op amp does not seem to improve anything. The only thing I've been able to come up with is that the switch's charge injection is also being synchronously rectified.

Does this make sense? Is there a better explanation I could test? Is there a model for this switch or a similar one that would show the problem? Is there a solution besides adding a current amplifier before the switch?

Thanks,

Dan

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  • That is correct. However, the problem is still there even without any excitation signal. The switch output with the integrator unconnected shows the expected charge injection square wave.

    However, I think I figured it out. The circuit below uses 3.3V rails, and the switch switches at 100KHz. Cpar is the parasitic trace on my board.

    Any difference in charge injection between the two switches will be dumped into the integrator each cycle. Additionally, any op amp input offset will have the same effect, but with the op amp I'm using that effect is much smaller.

    If this makes sense to you, are you aware of a solution other than amplifying my signal to overwhelm the problem?

    Thanks for all your time.

    --Dan

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  • That is correct. However, the problem is still there even without any excitation signal. The switch output with the integrator unconnected shows the expected charge injection square wave.

    However, I think I figured it out. The circuit below uses 3.3V rails, and the switch switches at 100KHz. Cpar is the parasitic trace on my board.

    Any difference in charge injection between the two switches will be dumped into the integrator each cycle. Additionally, any op amp input offset will have the same effect, but with the op amp I'm using that effect is much smaller.

    If this makes sense to you, are you aware of a solution other than amplifying my signal to overwhelm the problem?

    Thanks for all your time.

    --Dan

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