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ADS1282EVM-PDK: ADS1282EVM-PDK

Dean Gacita
Intellectual 300 points
Part Number: ADS1282EVM-PDK
Other Parts Discussed in Thread: ADS1282

Dear Forum;

I am developing a sensor that produces a DC output range of +20mVDC to +80mVDC. The plan is to interface this sensor to the ADS1282EVM to evaluate the ADS1282 as a solution for data conversion. My question centers on the J6 pinout. Since the signal will be unipolar A1N, A2N, REFN and VCOM are grounded. The question I have is centered around REFP. I would like to set it to 1VDC to capture the highest resolution between +20mVDC - +80mVDC as possible. Reviewing the datasheet it appears the min reference voltage is .5vdc.

Are there any obvious issues that anyone sees?

Thanks

Dean Gacita

  • 0
    TI__Guru* 92715 points

    Dean,


    The problem that I see with your application is the input range of the ADC. Connecting A1N (or A2N) to ground is already outside the range of the PGA. The absolute input range of the ADC is AVSS+0.7V to AVDD-1.25V. To be in the range of the PGA, the negative input must be at least at 0.7V.

    If the PGA is in gain (particularly high gain), both positive and negative inputs should be near the midpoint of the supplies. Look at Figure 29 of the ADS1282 datasheet. The output of the PGA must be within AVSS+0.4V to AVDD-0.4V. The input and output common mode must match.

    Most ADS1282 applications are for seismic sensors. Do you really need this combination of resolution, and data rate?


    Joseph Wu

  • 0 in reply to Joseph Wu
    Intellectual 300 points
    Joseph;

    Our application measures the gap between two metallic surfaces in both static and dynamic up to 5KHz. The ADS1282 was chosen for its capability to measure very small delta voltages.

    Thanks for the heads up on A1N and A2N. I will drive both A1N and A2N with a .7V min reference. It appears that I will need to amplify my signal to avoid the output appearing negative.

    At this point I would like to discuss Vref. What would you recommend I drive Vref on the evaluation board? I stated initially that I was going to ground Vrefn and set Vrefp to 1VDC. Do you think this is the way to go? You did not mention it as a problem just want to get your thoughts on it.

    Thanks
    Dean Gacita
  • 0 in reply to Dean Gacita
    TI__Guru* 92715 points
    Dean,


    Based on what you've said so far, and assuming a 5V supply, I would probably use a 2.5V reference. Note that the reference should be a low noise precision reference. Any error in the reference becomes a gain error in the measurement.

    The A1N and A2N inputs could also be tied to the 2.5V reference, which would set the input voltages near mid-supply, allowing for the largest possible gain for the PGA. With a 2.5V reference, and PGA set to 16. The largest measurable input would be VREF/(2x16) = 78.1mV. It's close, but this is a bit short of the 80mV that you said you need. If you really need to get to 80mV you would need to go to the next lowest gain or use a 3.0V reference with a gain 16 (that would make the positive input range about 94mV).

    Note that using a 0.7V setting for A1N and A2N would be ok for PGA=1, but not be high enough for other gains because the output may be limited by the PGA output swing. With an input of 80mV, the common-mode input voltage would be 0.74V. If the PGA were in gain of 16 then the output of the PGA would be 80mV x 16 = 1.28V. However, with the symmetrical nature of the PGA, the common-mode output voltage can't match the common-mode input voltage without running into the negative rail.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Intellectual 300 points
    Joseph - This is very helpful. I think I can move forward with this info. However, as time progresses I believe I will have additional questions.

    Thanks
    Dean Gacita