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Original question:

ADS112C04: Trouble reading REFP/REFN (possibly a symptom of other problems)

ADS112C04: Understanding the input voltage limits on ADS112C04

Siddharth Krishnamoorthy
Prodigy 20 points
Part Number: ADS112C04


Hello,

I'm working with the ADS112C04. I have a device that will at most output 80mV. Therefore, I can tune the PGA gain fairly high to improve resolution and reduce noise. The current output as measured by a volt meter is 50 mV. At gains 1, 2, 4 I see 50 mV from the ADC but at gain 8, I start seeing 32 mV. At gain 16, I start seeing 19 mV.

Is there a different formula to calculate the output for gains above 4? I see some suggestion of this in the datasheet on page 24, but I'm not entirely clear on this.

Thanks!

Siddharth

  • +1
    TI__Guru** 113765 points

    Hi Siddharth,

    Each output code relates to formula 8 in section 8.5.2 where the device full-scale range is 2*VREF/GAIN.

    Most likely you are seeing an issue with respect to the PGA.  Note that even with the PGA disabled and bypassed that as soon as you select gains greater than 4 the PGA becomes automatically enabled.  The input range is specified in section 6.3.  

    If the input range is exceeded there is degradation of the measurement due to the non-linear operation of the amplifier.  Measure the AINN input relative to AGND and see what the DC voltage is at that input.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 20 points

    Thank you, Bob. I was able to disable the PGA and get this to work with a gain of 4.