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FDC2214EVM: Please tell me about ENOB

user6183735
Prodigy 180 points
Part Number: FDC2214EVM 
We are currently using FDC2214 as a touch sensor.

Our application uses all 4 channels and needs 1000sample / s (Tsample = 1ms) in total, so we set the register values ​​as shown below. (Values ​​such as Idrive have also been adjusted)
I have a question here.
In this state, I performed "DATA Streaming", touched it several times, and saved it in a csv file.

Stop once,
When I check the csv file, the value (CH0 MAX) when not touching is
29758020, the value when touched (CH0 MIN) was 29581431.
* Both are raw data
 
Since the ENOB is 14.2 bits, I expected that the value would be 18821 (2 ^ 14.2) or less, but it seems that 28 bits of data are taken, so I do not know what this ENOB represents It was.

If there is a register setting that can increase the sensitivity of data, please let me know.

There may be places where I lack knowledge, but I would appreciate it if you could respond.

Regards,

MASATO

  • 0
    Prodigy 180 points 
    I will also paste the CSV file
    See column H
    20200508_EVM.csv
  • 0 in reply to user6183735
    TI__Expert 3435 points

    Hi,

    Thanks for providing detailed information and screenshots. 

    Are you using the EVM to test this setup or your own application board?

    The default value for the Parallel inductance should be 18uH. Is there a reason why it was changed to 22uH. The reference count and settle count has been changed as well. Is there any reason for it? When you use low ref count very low, the resolution of the measurement decreases, which shows up as low ENOB. It is a calculated value based on the ref count used. The device always outputs 28 bits of code, but the noise is very high so that the ENOB is 14.2 bits. Please use the default values on the GUI to get the maximum sensitivity.

    Regards

    Sharath.

  • 0 in reply to Sharath Vallamkonda
    Prodigy 180 points

    Hello Sharath

    Thank you for your reply.

    Are you using EVM to test this setup or your own application board?
    → Right.

    The default value of parallel inductance is 18uH. Is there any reason why you changed to 22uH?
    →Since the size of the 18μH coil mounted on the evaluation board was large and our board did not meet the specifications, I searched for the same 18μH small size, but could not find it.
    Therefore, I put the 22uH one instead.

    The reference count and resolution count have also changed. Is there any reason?
    → The sampling time must be set to 1ms for the total of 4 channels, and the reference count was calculated based on this, and this value was obtained.

    The device always outputs a 28-bit code, but the noise is so high that ENOB is 14.2 bits. Use the GUI defaults for maximum sensitivity.
    → Even with 28-bit display, the 14.2 bit part is effective.
    Is it possible to suppress noise or increase ENOB without using the default value?

    Regards,

    Masato.

  • 0 in reply to user6183735
    TI__Expert 3435 points

    Hi

    The referance count code is 65535 is the default value. With this, the ENOB is 21 bits. 

    Let me know if this solves your ENOB issue.

    Regards

    Sharath.