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FDC2214EVM: Sensor not detected

Jonathan Navor
Genius 9475 points
Part Number: FDC2214EVM
Other Parts Discussed in Thread: FDC2214

Hi Team,

Good day! I'm posting this in behalf of the customer. 

As per customer below:

I'm using the FDC2214 module on the ch2 and i want to detect the change of capacitance from a sensor for liquid sensing. I've already measued the capacitance of same capacitors and, with the correct idrive, the measurement are good, comaped to a multimeter so the FD2214 works fine.
My sensor is created from my university and it's fabricated with the FFF tecnology (Fused Filament Fabrication) with 3D print, ad it consist in a series of electrode are fabricated with conductive polylactic acid (PLA) doped with carbon black and carbon nanotubes (CNTs) (an immage is attacched in the article of the caracterization of the sensor). When i attached the sensor to the FDC2214 the change is not visible, 3/4 pF only for the connection, with the multimeter the change in capacitance is 100pF. even when i insert the liquid nothing change, whit the multimeter..

on the other hand, the change is relevant. So, with other experiment, like RC series and RC parallel, the sensor capacitance was never detected with the FDC2214, only with the multimeter. Do you have and ideas why the sensor was not detected?

Here is my email: r.vocale@studenti.poliba.it

2021 Stano (under review) - Fused Filament Fabrication, a New Fabrication Paradigm for 3D Printi (1).pdf

Thank you in advance for the support. 
Best regards,
Jonathan
  • 0
    TI__Mastermind 36615 points

    Jonathan,

    Thank you for your post and the interesting content.

    The FDC devices do not work like a traditional multimeter or capacitance meter. In addition, they are really intended to estimate a change in capacitance, rather than a single capacitance value. For more information, please see the  FDC2x1x Capacitive Sensing FAQs.

    Having said all that, the devices do not measure capacitance directly.
    The FDC2214 is based on a resonant LC circuit that is formed between the sensor's capacitance, an inductor on the PCB, and any on-board and/or stray capacitance.
    This is implemented on our EVM for channel 2 by L2(18uH), C10(33pF), and the combinations of C19/C21/C35/C36(18pF).
    You can see this on on page 45 of the EVM User's Guide.

    The FDC2214 estimates the change in the sensor's capacitance by measuring the resonant frequency shift (of the waveform) as the liquid level changes.
    The supported amplitudes and frequencies of the sensor waveform are given in the data sheet.

    As a troubleshooting step, please try measuring the amplitude and frequency of the sensor waveform with a high impedance probe and an oscilloscope, and compare what you see to the values given in the data sheet. This might offer a clue as to why it isn't working with your sensor.

    As a baseline test case, it might be helpful to measure the sensor waveform using the EVM's built-in sensor pads, and noting the sensor waveform amplitude & frequency as you place conductive targets at different distances from the sensor.

    Please try those steps and update this thread with any questions you have.

    regards,
    John

  • 0 in reply to John Miller - Sensing
    TI__Genius 9475 points

    Hi John,

    Here is the cusotmer's resonpose:

    First of all thanks for the reply.
    Regarding the first test you mean to generate a signal with the oscilloscope and the the amplitude and the frequency with the sensor attacched? (i was planning to do this in these upcoming days)

    Regarding the second test you have suggested I've already measured the amplitude in the ch2 with some capacitors and i see the change in frequency and amplitude, if is that what you mean.
    In these day i discovered something on the sensor that can clarify why the FDC2214 wasn't able to read it. With the multimeter i was able to measure the the resistance of the sensor wich i suppose is in series with the C that i want to measure. ( the test measurement is the second picuture on the word attacched, sorry, i wasn't able to take a photo because i am alone in the lab now). As i move the second probe further down the sensor the R increase. Now, even with only one couple of electrode, the R is 1KΩ.

    So I tried to replicate this, supposed, RC series circuit with a test capacitance and various value of R (you can see the test circuit in the first picture of the word attached). With the multimeter I was able to measure C correctly with an R up to 10KΩ. On the other and, with the FDC2214, I was able to measure C only with an R up to 100Ω, already with 350Ω the C measured was off. With 1KΩ the C wasn’t measured at all, so I think this is the problem with my sensor, it’s like the current is too low to overcome the R series so the FDC2214 is not able to see the capacitance. Is it possible that this is the problem? And if so how can I overcome this problem? Thanks again for the support"

    Parallel.docx


    Thanks, 

    Best regards,

    Jonathan

  • 0 in reply to Jonathan Navor
    TI__Mastermind 36615 points

    Jonathan,
    What happens to the sensor waveform when you change the resistance?
    Also, what sensor drive current settings are you using (e.g. register DRIVE_CURRENT_CHx)?

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    TI__Genius 9475 points

    Hi John,

    Here is the customer's response below:

    The idrive i'm currently using is 27, because i did some testing to find the perfect value of idrive for a range of capacitance with the oscilloscope ( a table with the result on the range of capacitance is in the word attacched).
    I've done the testing with the RC series (as you can see in the word) and you can see how the aplitude decreases with much higher R. With R = 1kohm the fluctuation of frequency are too big to measure a value (the same happens when i tried to use a value of idrive too low when i tested the measurament in the table). So it's like the FDC2214 doen't see the capacitance, i think. I saw also in some other thread that it's not optimal to increase the current flow from the FDC2214. What do you think? Thanks for the support.

    Idrive.docx

    Best regards,

    Jonathan

  • 0 in reply to John Miller - Sensing
    TI__Genius 9475 points

    Hi John,

    Here is the customer's response below:

    The idrive i'm currently using is 27, because i did some testing to find the perfect value of idrive for a range of capacitance with the oscilloscope ( a table with the result on the range of capacitance is in the word attacched).
    I've done the testing with the RC series (as you can see in the word) and you can see how the aplitude decreases with much higher R. With R = 1kohm the fluctuation of frequency are too big to measure a value (the same happens when i tried to use a value of idrive too low when i tested the measurament in the table). So it's like the FDC2214 doen't see the capacitance, i think. I saw also in some other thread that it's not optimal to increase the current flow from the FDC2214. What do you think? Thanks for the support.

    0714.Idrive.docx

    Best regards,

    Jonathan

  • 0 in reply to Jonathan Navor
    TI__Mastermind 36615 points

    Thanks Jonathan.
    Something isn't completely clear to me.
    Are the measurements you described in your .doc done with an actual sensor, or with an physical capacitor and resistor?
    Regards,
    John

  • 0 in reply to John Miller - Sensing
    TI__Genius 9475 points

    Hi John, 

    Here is the response from the customer. 

    the measurement in the doc are with the capacitor of 100 pF, not with the sensor. With the sensor, with any idrive i don't see anything

    Best regards,

    Jonathan

  • 0 in reply to Jonathan Navor
    TI__Mastermind 36615 points

    Jonathan,

    If the waveform looks okay with a physical capacitor, but not with the actual sensor, then I'd say there needs to be some troubleshooting with the sensor.
    The explanation from 4 days ago sounds like the (capacitive) sensor has a measurable amount of resistance. If the equivalent series resistance in the sensor is too high, then then the LC circuit formed by the sensor and the PCB inductor won't be able to oscillate. This may be why no waveform is observed when the sensor is attached: the sensor may have too much series resistance to support an LC oscillation.

    We have no experience with the kind of sensor described in this thread.

    Please take a look at our FDC2x1x Capacitive Sensing FAQ page. There cold be some information there that will help.

    Regards,
    John