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ADS1293EVM testing

Bill Tyler
Intellectual 290 points

Other Parts Discussed in Thread: ADS1293EVM, ADS1293

Hi, I am trying to understand the ads1293evm test mode.  when I set a channel to test is the output supposed to be a flat line at the indicated voltage?

What I am trying is:

- Click test ref block in app - set TST1 to zero test

- Write registers

With nothing hooked up to the input, I get a very noisey line which is indeed centered on zero.

The board is hooked up via USB only, no other attachments.

Thanks

  • 0
    Intellectual 290 points

    hey guys, I really need a hand here?

  • 0 in reply to Bill Tyler
    TI__Expert 3350 points
    Please see page 17 of the ADS1293, section 8.3.3. In this section the test mode and the operation of the device in this mode is provided.
  • 0 in reply to Bahram Mirshab
    Intellectual 290 points
    Ok, I figured out that in the default Auto Y Axis Scale setting the signal seems to be zoomed in on quite a bit and looks like a very noisy signal rather than a flat line. By setting the Fixed Y Axis Scale to .3 to -.3 it gives a nice clean signal and I can have all three channels displayed in the different test modes.

    When I set the Output format to display in Output Code I get a range of 7865450 to 4283900. I don't see how to convert the output code to Input Voltage. Can you suggest where I can look?
  • 0 in reply to Bill Tyler
    TI__Expert 3350 points

    Please see section 8.4.3 on page 36 of the ADS1293 datasheet.

    To convert from ADC code to the analog quantity at the input, use the following relation:

    V=2 * VREF(ADC code/ADC_Max - 0.5)

    where

    VREF = +2.4V

    ADC_MAX  is derived from the selection of SDM (sigma-delta-modulator) frequency and decimation factor R1, R2 and R3 from Table 8,Table9, Table 10, and Table 11 (page 33 to  36) in the ADS1293 datasheet.

    B. Mirshab

  • 0 in reply to Bahram Mirshab
    Intellectual 290 points
    This doesn't seem to work. In working with the test modes I observed these output codes:
    +0.2v - 7865350
    0.0v - 6075100
    -0.2v - 4283975

    I determined ADC_Max to be 12150000 from the table you mentioned.

    When I run these output codes through the formula you mention I get the following results:
    +0.2v - 7865350 - 0.7072987654321
    0.0v - 6075100 - 0.00003950617282
    -0.2v - 4283975 - -0.70756543209878

    So it seems to work at 0v but its off by .5v at the .2v level. Does this make sense?
  • 0 in reply to Bill Tyler
    Intellectual 290 points
    ok, I got it worked out. Your equation did not account for the 3.5 multiplier so it should be:
    V=(2 * VREF(ADC code/ADC_Max - 0.5))/3.5
    Let me know if this is not correct, but it works for the +/-.2 v as well.
  • 0 in reply to Bill Tyler
    TI__Expert 3350 points

    Correct, you need to scale down the final value by the 3.5X gain of the INA's in the signal chain.