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FDC1004EVM: Application Inquiry

Renan Santos Adriano
Mastermind 22380 points
Part Number: FDC1004EVM
Other Parts Discussed in Thread: FDC1004

Hello Team,

I'm posting on behalf of my customer.

I saw a manual named Capacitive Sensing: Out-of-Phase Liquid Level Technique. I met a question on Page 6. why should we calculates Meas1 & 2 through CIN4? The CIN 4 has no electrode attached.

Link: www.ti.com/.../snoa925.pdf

If I use two parallel copper plates, one is positive, another is ground. I wanna measure the capacitance between the parallel capacitor. For my understanding, I connect positive copper to CIN1 (CHA), for the ground plate, I plan to connect it to CIN2(CHB). Am I right about the connection? And I found that even with no connection with wire, there is still some capacitance like 0.4pF. May I know how to calibrate that value into 0 pF?

Regards,

Renan

  • 0
    TI__Mastermind 40295 points

    Renan,

    Your connections are correct.
    You can account for offset using the offset calibration registers as described in section 8.6.4 of the  FDC1004 data sheet .
    If your customer has one of our EVMs, they can experiment with the offset calibration using the Sensing Solutions EVM GUI.
    It has a field to adjust the offset calibration.

    Regards,
    John

  • 0
    Prodigy 31 points

    Hi John,

    Thanks so much for your kindly help and detailed explanations.

    I am clear about the connection method and also the offset calibration. I have a FDC1004 EVM from your company. And I already found the 'offset capacitance' in GUI. Your suggestions are really helpful.

    May I ask one more thing which is 'I saw a manual named Capacitive Sensing: Out-of-Phase Liquid Level Technique. I met a question on Page 6. why should we calculate Meas1 & 2 through CIN4? The CIN 4 has no electrode attached' mentioned above? Thank you sooo much for your help.

    Best regards,

    Pei

  • 0 in reply to Pei Zhang
    TI__Mastermind 40295 points

    Pei,

    It is configured that way because of the active shields are to be configured for the OOP technique.
    The measurement is to be configured as described in section 3.2 of the app note:
    "Meas1 = CIN1 (CHA) – CIN4 (CHB). CIN1 is set as the positive input channel, and CIN4 is set as the negative input channel."
    This will cause SHLD1 to be driven with the same waveform as the positive channel (CIN1), and SHILD2 will be driven with the same negative signal as CIN4 (which is not connected). 
    The positive/negative signals on the shields will keep the fluid being measured at constant potential, thereby improving the accuracy of the measurement.

    Regards,
    John

  • 0
    Prodigy 31 points

    Dear John,

    Thank you so much for your kindly reply.

    I think I understand your explanations. I understand we keep the CIN4 without any connection. In Figure 8 in section 4.1, I also notice no connection related to CIN4. So, in such a way, I also could use CIN3(CHB) to replay the CIN4. Am I correct about this point? I will use the active shield method for my experiment, so I have highly interested in this part. Thanks so much again.

    Best regards,

    Pei

  • 0 in reply to Pei Zhang
    TI__Mastermind 40295 points

    Pei,

    You could use CIN3 as a substitute to CIN4 and make the appropriate SHLD assignments.

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 31 points

    Dear John,

    I am clear about this point.

    Thank you so much! Have a good day!

    Best regards,

    Pei