• State Resolved
  • Locked Locked
  • Replies 6 replies
  • Subscribers 46 subscribers
  • Views 1768 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

DDC1128EVM: DDC1128EVM raw output code to current

Sjors Tiemissen
Prodigy 185 points
Part Number: DDC1128EVM
Other Parts Discussed in Thread: DDC1128

Dear Sir/Madam,

Currently we are evaluating a DDC1128 Current-Input Analog-to-Digital Converter using the DDC1128EVM evaluation board. I would like to convert the raw output codes to a current in A (nA, uA, etc.). I can’t figure out how to calculate this.

This is how I tried to calculate it:

  1. Full-scale 100% is equal to 150pC according table 7 in the datasheet.
  2. 100% is equal to 0xFFFFF (decimal 1048575)
  3. 150pC / 1048575 = 0,000143051284 pC/step
  4. Capture data using the PC software, this gives me a raw value of e.g. decimal 4011
  5. 011 x 0,000143051284 = 0,57 pC while the PC software says -0,01220 pC. What am I doing wrong?

Does the Vref influence the value in Coulombs? I am using the onboard reference (REF3040, 4.096V).

Using the value in Coulombs and Tint (integration time), the current in Amps can be calculated.

So in short: What’s the equation for converting the raw values to amps?

Thank you very much in advance!

  • 0
    TI__Guru 50036 points
    Hi Sjors,

    The serial output data are provided in an offset binary code. An offset is included in the output to allow slightly negative inputs (for example,
    from board leakages) from clipping the reading. This offset is approximately 0.4% of the positive full-scale. The offset value in codes corresponds to 4096 (decimal).

    The charge would be (ADC output code - offset) * 0.000096% * FS charge
    The input current would be the charge over the total integration time.

    For example: let us say FS charge = 150pC, total integration time = 500 us and ADC output code = 75000.
    Integrated charge = (75000 - 4096) * 0.000096% * 150pC = 10.2 pC
    Input current = 10.2pC / 500 us = 20.42 nA
  • 0 in reply to Praveen Aroul
    Prodigy 185 points

    Hi Praveen,

    Thanks for your quick reply and clear explanation. Now I am able to convert my raw values from the exported csv file to a usable format.

    Thanks!

  • 0 in reply to Sjors Tiemissen
    Prodigy 185 points

    Hi Pavreen,

    Another question. We are developing a sort of interface board which applies currents to the EVM board. We are using just a few inputs. What should we do with the unused inputs? Are we allowed to let it float or do we have to short it to GND? If shorting to ground is needed, is a resistor neccesary?

    Thanks!

  • 0 in reply to Sjors Tiemissen
    TI__Guru 50036 points

    Hi Sjors,

    With the unused inputs, you could leave them open (floating).

  • 0 in reply to Praveen Aroul
    Prodigy 185 points

    Then we will leave them floating, thanks!

  • 0 in reply to Sjors Tiemissen
    Prodigy 185 points

    Hi Praveen,

    I can't get the EVM board to work at the highest sample rate. When analyzing the data, the actual sample frequency seems the set frequency/2. Could you please tell me how to configure the EVM software to use a sample rate of 6.25kHz. Please see the file attached for the settings I'm currently using.

    I'm looking forward to a fast response.

    Thanks!