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FDC1004 is it possible to use n:1 input

Shinichi Inoue
Guru 21791 points
Other Parts Discussed in Thread: FDC1004, TS3A5017, TS5A9411, TS3A5018

Dear Specialists,

My customer is considering FDC1004.

He is thinking about 8sensors detected by one FDC1004?

Each signal is changed to another channel by the switch. 

Is it possible?

What is tips, tricks and traps.

Could you please see attached file.

Best regards,

Shinichicustomer use case of FDC1004.pptx

  • 0
    TI__Expert 7400 points
    It is possible to multiplex the input of the FDC1004. The issue with multiplexing though is that there will be some parasitic capacitance due to the switch itself, that is usually on the order of 10-20pF. This violates the input capacitance range of the device. You can use the CAPDAC to help offset it within the input capacitive range spec though.

    How exactly are you going to use be using liquid sensing with that configuration. Are you just trying to determine whether there is liquid in the tube or not? Or are you trying to resolve what the actual level is?

    What is the fluid type?

    Thanks
    -David Wang
    Capacitive Sensing Applications
  • 0 in reply to David Wang11
    Guru 21791 points

    Dear David

    Thank you for your reply.

    I understand the idea is possible to achieve.

    I'll send your answer to the customer and confirm actual case.

    I appreciate your great help.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    Guru 21791 points
    Dear David

    I got a feedback from the customer.

    Could you please confirm?

    He is thinking that a water is detected by a sensor pad area(20mm×40mm square).

    The threshould has not been determined yet.

    The liquid type is water, sometimes may be cleaning liquid.

    Also could you recommend which types of switch he should use.

    Are there any limitation Rdson, parasitic capacitance, voltage range and speed.

    I choose two types of combination as a trial as 8:1.

    (1) TS3A5017(SP4T2ch) x1pc and TS5A9411(SPDT1ch) x1pc.

    (2)TS3A5018(SPDT4ch) x2pcs

    I appreciate your great help.

    Best regards,
    Shinichi
  • 0 in reply to Shinichi Inoue
    TI__Expert 7400 points

    Hi Shinichi:

    Typically, the Rdson is very low (a few ohms) so it should not impact the signal waveform. As far as parasitic capacitance, the concern is saturation of the input capacitance of the device. Luckily, there is the CAPDAC that can offset the input capacitance to be within the +/-15pF. Lower parasitic capacitance is better since it will act as a gain error.

    Thanks

    -David Wang

    Capacitive Sensing Applications

  • 0 in reply to David Wang11
    Guru 21791 points

    Dear David

    Thank you for your reply.

    I understand that It should be careful for the input capacitance.

    I'll feedback to the customer.

    I appreciate your great help.

    Best regards,

    Shinichi