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TMUX1133: Using analog switch in front of transimpedance amplifier for small current - high impendence source

dragisa jankovic
Prodigy 10 points
Part Number: TMUX1133


Hello,

I'm trying to use this switch in front of transimpedance amplifier to select between sensor (~1nA high impendence current source) and test circuit which is a single resistor. Basically I'm using 

two channels where D1 is connected to fixed voltage and D2 goes to transimpedance amplifier and on S1A/S2A I have sensor and S1B and S2B 1meg resistor. I thought this should work

because of ~5pA leakage current and low resistance but my problem is that when I do select test channel and I touch sensor I see that sensor noise is transferred to transimpedance amplifier even though channel is not selected. Do you have and advice or recommendation?:

Thank you

  • 0
    TI__Mastermind 24430 points

    Hello,

    I think your device is experiencing crosstalk- A measurement of unwanted signal coupling from an ON channel to an OFF channel. This is the ratio of VS2 to VS1 measured in dB at a specific frequency. You can mitigate this in your system( a lot of the time this is caused by poor layout, high frequencies, or large loading):

    1. What are the frequency  and voltages of the signals you are sending into the multiplexer?

    Regards

  • 0 in reply to Kameron Hill
    Prodigy 10 points

    In my test setup, I have 1meg resistor connected to S1A and S2A and 10meg resistor connected to S1B and S2B. D1 is connected to 1.6V and D2 goes to negative terminal of transimpedance amplifier with 10meg resistor in its feedback loop. Positive terminal of transimpedance amplifier is connected to 1.5V which is virtual ground. Entire system is running from 3V power supply.

    I'm injecting 60Hz  1V amplitude signal in series with 1meg resistor that is touching S2B (channel A is selected) and I can see 60Hz signal on transimpedance amp. Same happens if I touch S1A and select channel B.  

    I makes sense that I have strong cross talk. My RL is kind of zero at 60Hz because it's the input of transimpedance amplifier but if parasitic capacitance is like 1pF that could transfer about 340pA current to the amplifier from channel that is not selected. I probably have more parasitic capacitance on pcb which could move parasitic current into nA range.

    Thank you very much for helping me on this !!!!!