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ADG794 Synchronously Rectified Charge Injection

Question asked by dmoyer on May 7, 2012
Latest reply on May 15, 2012 by seanbrown

Hello.

 

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

ADG794 Synchronous Rectifier.png

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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