• State TI Thinks Resolved
  • Locked Locked
  • Replies 1 reply
  • Answers 1 answer
  • Subscribers 46 subscribers
  • Views 605 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

ADS125H02EVM: Communication problem

Adrian Ymeraj
Expert 3750 points
Part Number: ADS125H02EVM
Other Parts Discussed in Thread: ADS125H02,

Hello team,

my customer is using the EVM as the following setup:

whereas the jumpers are as in the following picture:

Finally this is the status of the ADC registers that I changed from the default values:

MODE4 (0x10) = 0x05      ----->    Input Multiplexer = AN1-AN0  ;   PGA Gain =0,5V/V

REF (0x06) = 0x03     ----->     Internal Referance DISABLE  ;   Reference Positive Input =AVDD =+5V   ;     Reference Negative Input =AGND

The ADC does not seem to be so accurate: by measuring the same voltage with an external 4 1/2 digit multi-meter I noticed about 3 / 4mV of deviation), is it normal ?

I then tried to perform a calibration with the OFSCAL and GANCAL commands. I would like to understand if the procedure I do is correct:
- short circuit AN1 with AN0 leaving AN0 grounded (as shown in the figure below) and send the OFCAL command from the console

- then I connect AN1 to the + 5V of the card (which based on the setting of the registers represents the VREF and therefore the full scale of the ADC) and send the command GANCAL from the console

1) Is the procedure correct?
2) At the end of the OFCAL command actually the ADC adjusts the reading correcting the zero while at the end of the GANCAL command the ADC adjusts the reading correcting the full scale, but if at this point I go back to reading the zero I find it again moved away as if the GANCAL command had also influenced the offset. How is this possible ??
3) Is there an additional command that I have to send after the OFSCAL and GANCAL commands to confirm the calibration ??
Please let me know.
Thank you in advance!
Best regards,
Adrian

  • 0
    TI__Mastermind 44550 points
    Hi Adrian,

    The ADC and DMM may each have their own offsets and gain errors which combined add up to 0.75 mV difference. I don't see any issues there.


    1) The offset calibration measurement is good. However, I think the gain calibration may be causing some issues...

    With an approximate 5V supply acting as the reference source and a gain of 0.5V, the ADS125H02 can tolerate a maximum +/- 8V input, due to the output swing limitations on the Stage-two PGA. If it weren't for this output swing limitation, you could apply a 10V full-scale signal and perform gain calibration, but because of this limitation you'll may have to calibrate with lower voltage and then compensate for the fact that the voltage is not at 100% full-scale...

    NOTE: Gain calibration is only beneficial if you can supply a precise full-scale signal. The signal must be more precise than the ADC's calibrated gain error, otherwise calibration will make the gain error worse! The ADS125H02 has a typical uncalibrated gain error of +/- 0.1% (and max +/- 0.7%). NOTE: The 5V LDO on the ADS125H02EVM has an output accuracy in the range of +/-2.5%; therefore, it should not be used as a calibration source.

    If you do decide to calibrate the 10V ADC range (8V PGA range), then you'll have to find a precise voltage source somewhere in the range of 5-8V. You can use the GANCAL command; however, note that the ADC will calculate the gain scaling factor under the assumption that you've applied a 100% full-scale signal, and will result in a scaling factor larger than "1". Since the input signal is less than full-scale (for a 5V input this is only 50% full-scale and the computed scaling factor will be approximately "2"), you will need to multiple the FSCAL[2:0] scaling factor by 50% (for a 5V input, or 80% for an 8V signal) to correct for this.


    2) What values were programmed into the OFCAL[2:0] and FSCAL[2:0] registers after you performed calibration, and how much offset did you measure with short-ed input after calibration?


    3) I would recommend reading back the OFCAL[2:0] and FSCAL[2:0] registers to ensure that reasonable values were set. OFCAL should be a small number (note: this is a signed number, so small negative numbers may appear to have a large unsigned decimal value), and FSCAL should not be too far off (i.e. more than +/- 1%) from 400000h (1 V/V).