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MUX506: Pulldown Resistors on the Signal Inputs to Reduce Crosstalk

Shane Jiang
Prodigy 50 points
Part Number: MUX506
Other Parts Discussed in Thread: FDC2214

Hi team,

We would like to use pulldown resistors on the Sx pins to reduce crosstalk between signal traces. As only one input will be active at a time, we would like to ground all non-active inputs to reduce signal interference. We are trying to interface several multiplexers with the FDC2x1x for a larger scanning area for a capacitive sensing application. There is a large array of sensors we want to read one at a time. In our application, how should we determine the value of the pull-down resistors for each signal input? We would require 80 pulldown resistors for 80 mux inputs.

Any advice is appreciated. Thanks!

Shane

  • 0
    TI__Intellectual 1156 points

    Hi Shane,

    What is your operating frequency? The pulldown is mainly used for 2 things: to make sure your switch input is a determined voltage when the switch is off, and reduce crosstalk when the switch is off from external noise. The value of the pull down will depend on your system impedance, typically 50 Ohms. 

    Thanks,

    Stephen 

  • 0 in reply to Stephen Scheramic
    Prodigy 50 points

    Hi Stephen,

    Thank you for your response. We are trying to reduce crosstalk when the switch is off from external noise. We are deciding to use a 40MHz external clock for (fref) and close to 6MHz for the LC Tank sensor frequency (fin). We are aiming to maximize the CDC sample rate, which is a maximum of 13.3 kilosamples per second according to the FDC2x1x datasheet. May I ask what is a good method to follow in order to determine our system impedance and correlate it to the value for the pulldown resistor?

    Best,

    Shane

  • 0 in reply to Shane Jiang
    TI__Intellectual 1156 points

    Hi Shane,

    Looks like you are driving a high impedance, capacitive load. So here any value of pulldown would probably be fine. But you should make sure that it doesn't have any issues with your LC filter. Unless I am understanding your application wrong, the resistor will be in parallel with the LC filter. There are many calculators online that can help you see the impact of different RLC values. here is a commonly used one. 

    https://www.electronics-tutorials.ws/accircuits/parallel-resonance.html

    Thanks,

    Stephen 

  • 0 in reply to Stephen Scheramic
    Prodigy 50 points

    Hi Stephen,

    We are trying to drive/measure a high impedance (3.2 to 4 kOhms) and very low capacitive (~2pF) load. I have found from another TI forum post on the FDC2214 that 

    • Rp = L / (C * Rs)​
    • where​
    • Rp is the equivalent parallel resistance at the sensor resonant frequency​
    • L is the sensor coil inductance​
    • C is the sensor capacitor (including parasitic capacitance)​
    • Rs is the series resistance of the inductor at the sensor resonant frequency

    Another post also claims that the FDC2214 requires a sensor Rp of at least 1kohm to keep the LC tank oscillating. So, since the pulldown resistor is essentially treated as a parallel impedance to the LC tank, my hypothesis is that this won't be a concern for the oscillation of the LC tank. To my understanding, we want to make the pulldown resistor large in order to keep Rp large since they are in parallel. Please correct me if I'm wrong.

  • 0 in reply to Shane Jiang
    TI__Intellectual 1156 points

    Hi Shane,

    yes that is correct a larger pulldown resistor will keep Rp large as well. 

    Thanks,

    Stephen

  • 0 in reply to Stephen Scheramic
    Prodigy 50 points

    Thanks Stephen, I have an understanding of what the lower limit of the pulldown resistor should be. Ideally, the pulldown resistor is large to keep Rp large, but what would be a good estimate for the upper limit value? Perhaps approaching the MOhms range, the pulldown resistor will effectively create an open circuit between the mux input and ground due to its large impedance, preventing the filtering of noise and/or crosstalk. My current guess is that a value in the 100s of kOhms would suffice but does this seem reasonable to you?

    Thank you for your feedback!

    Shane

  • +1 in reply to Shane Jiang
    TI__Intellectual 1156 points

    Hi Shane,

    100s of kohm should be fine, even 10kOhm may be fine. It may require some experimentation to ensure it doesn't interfere with your LC filter, but that range is a good starting point. 

    Thanks,

    Stephen 

  • 0 in reply to Stephen Scheramic
    Prodigy 50 points

    Hi Stephen,

    This resolves my issue. Thank you for your patience and help. I wanted to be thorough with the circuit design and you've answered a lot of my concerns.

    I can now make an informed decision about this part of the circuit which is very crucial to design cautiously due to its potential interference with the sensor performance.

    It is much appreciated,

    Shane