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FDC2214EVM: Please tell me about the value when using Data Streaming of GUI.

user6183735
Prodigy 180 points
Part Number: FDC2214EVM

I use FDC2214EVM for a touch sensor of a robot.

From the terminal of the evaluation board, 
I extend the sensor electrode pad with a cable and paste it inside the robot.
I confirm changes in capacitance when the electrode pad is touched by hand from outside 

When measuring using [Data Streaming] of GUI 
It shows 53pF when not touching the sensor, and 0.1 ~ 0.5pF capacitance increases when touching from outside.

My concern is that the sensor is reacting, but this 0.1 ~ 0.5pF is probably too small.

Can you tell me because I don't know if it's weak?

Regards

＜Conditions＞

Electrode size: 5mm x 10mm
Electrode thickness:0.2~0.3mm
Robot material: Plastic
Robot thickness: 0.2 ~ 0.3mm
Cable length: 10cm

<Configuration,DataStreaming>
  • 0
    TI__Genius 17440 points

    Hello,

    Whether or not your target response is too small depends on your noise floor. You can measure your noise floor using the following steps:

    1. To measure the noise floor using an evaluation module, first download and open the Sensing Solutions GUI. For more detailed installation instructions, see the User’s Guide associated with the EVM.
    2. Once the GUI is open, navigate to the Configuration tab. Configure the RCOUNT setting so that the LDC or FDC device will sample the sensor frequency at the desired rate. Note: for non-default sensors, at this point the user will need to use an oscilloscope to verify that the sensor oscillation amplitude and the sensor frequency are within the acceptable range for the specific device, as specified in the datasheet. If the sensor oscillation amplitude is outside of the recommended range, the sensor drive current will need to be modified. Please see the Register Map in the datasheet for the registers that control the drive current settings. If the sensor oscillation frequency is outside of the frequency range supported by the device, either the inductor or the capacitor in the LC tank will need to be modified.
    3. Next, navigate to the Data Streaming tab.  Click the “Start Streaming” button to begin the noise floor measurement. Ensure that the graph displays “Detected Sensor Frequency (MHz),” which is controlled by the drop down menu in the upper left corner. Click on the “Show Statistics” button to display the average and standard deviation of the data. Increase the number of decimals of the standard deviation until the last digit is rapidly changing. Use three times the standard deviation as the noise floor.
    We recommend that your SNR is at least 10 for robust sensing.
    Best Regards,
  • 0 in reply to Kristin Jones93
    Prodigy 180 points

    Hi! Kristin

    Thank you for answering kindly.

    I checked step 1,2,or 3 you instructed.

    Regarding Step 3 [Detected Sensor Frequency], the value is 5.332MHz when not touched, and the graph below shows when touched.

    There are three questions.

    ① What does the noise floor mean?
    Is this Detected Sensor Frequency related?

    ② If the standard deviation is 3 times, will the noise floor value be 1 based on this graph?

    ③ Where should I check if the SNR is 10?

    I would appreciate your reply.

    < 1:touch 2:no touch>

  • 0 in reply to user6183735
    TI__Genius 17440 points

    Hello,

    1. The noise floor is the measured changes in the sensor frequency without the target present. The noise can come from a variety of sources, including aggressor signals within the sensor's passband, changes in temperature that cause changes in sensor inductance or capacitance, and quantization noise.

    2. You should use the second graph, where there are no touches on the sensor. You also need to increase the number of decimal places of your Std Deviation measurement. Once you have a non-zero number in the Std Deviation column, use 3 times this number.

    3. In graph 1, your signal magnitude is the difference between the default sensor frequency and the sensor frequency when you touch the sensor. Use this to calculate the SNR.

    Best Regards,

  • 0 in reply to Kristin Jones93
    Prodigy 180 points

    Hi!!! Kristin

    Thank you for answering kindly.

    1. I understood the noise floor to some extent. I will investigate the details in my own way. Thank you very much.

    2. I checked you instructed again (Please see the chart below). I can't understand yet what you said "use 3 times this number".

       What use number??

    3.Is the default value of the sensor correct with the value set in “Configuration” of the GUI?

      Could you tell me the specific SNR calculation method?

      It may be common sense, but I am happy if you can tell me because I don't have enough knowledge.

     Regards,

    <touch /no touch sensor>

      

      

  • 0 in reply to user6183735
    TI__Genius 17440 points

    Hello,

    Thanks for your patience while I was out for the Thanksgiving holiday.

    2. Use the number right under "Std. Deviation." In your no touch image, this number is ~0.000126MHz, or ~126Hz. Use 3*126Hz = 378Hz as your noise floor.

    3. SNR = (signal magnitude) / (noise floor) = ( (sensor frequency before touch) - (sensor frequency after touch) ) / 378Hz

    I hope this helps.

    Best Regards,

        

  • 0 in reply to Kristin Jones93
    Prodigy 180 points

    Hello Kristin,

    Thank you for replying.

    3. SNR = (signal magnitude) / (noise floor) = ( (sensor frequency before touch) - (sensor frequency after touch) ) / 378Hz

    →I understand very well.

       Actually,I calculated .

    (sensor frequency before touch)　=　5.122MHz

    (sensor frequency after touch)       =   5.152MHz

    SNR = (5.122MHz - 5.152MHz ) / 378Hz = 79.365...

    Since the SNR is at least 10, is there any problem with the sensor response?

    Best Regards,

    Masato

  • 0 in reply to user6183735
    TI__Genius 17440 points

    Hello Masato,

    No, there is no problem with your sensor response. You have a very good SNR. 

    Best Regards,

  • 0 in reply to Kristin Jones93
    Prodigy 180 points

    Hello Kristin,

    OK. I resolved!

    Thank you for teaching me a lot!

    If there is anything else, I would appreciate it if you could respond.

    Best Regards,

    Masato