Other Parts Discussed in Thread: TMUX1134
Hello, I have taken over an amplifier design, on which questions have already been asked regarding stability (see THP210: Stabilisation of a current-to-voltage differential amplifier (with a 20KHz bandwidth and a pass-band gain of +25dB) - Amplifiers forum - Amplifiers - TI E2E support forums).
On a THP210 development board, the circuit did not initially perform well - we had a large DC offset on the output and both input and outputs were slowly moving around by a volt or so. We stabilised this by adding 1Meg resistors between Vocm and the +ve and -ve inputs to the device.
This now performs with excellent symmetry on the output signals and, when floating, the inputs before our input series resistors, are at Vocm (2.5V) as expected. THD is good - typically better than -105dB
However, in our design, we have 24-channels of this circuit on a small 32mm x 150mm PCB, and for test purposes we have the input signals going through analogue switches. We cannot get a channel on our PCB to have a zero DC offset. The inputs sit at around 4.4V, and there is often a difference of a few 100mV in the mean output signal levels.
If I take the analogue switches out of the circuit and inject the signal directly onto the 500R series resistors, then the circuit behaves as expected.
If I simulate 10nA of leakage (e.g. a possible leakage path from an analogue switch) into the circuit (see attached) then a transient simulation is very close to what I see with a large DC offset at the output
I appreciate that this then isn't really a TI problem. but I wonder if you may be able to offer some advice regarding the use of analogue switches in this kind of circuit / how it might be desensitized to such unwanted currents. Clearly adding a 2nd current (flowing in the opposite direction) to the other input cancels the effect, but I can't see a practical way to achieve that.
I am in contact with Vishay to get a better understanding of the leakage paths involved (DGQ2788AEN-T1-GE4).
Many thanks in advance,
Nick
