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Data Out from ADS1231 is always zero

Matthew Sipp
Prodigy 10 points
Other Parts Discussed in Thread: ADS1231, ADS1220, ADS1247

I am trying to use the ADS1231 to read a half-bridge strain gauge. My original design used 3.3v for excitation of the strain gauge which was also connected to AVDD, DVDD, and VREFP. The ADS1231 was able to read the differential voltage across the strain gauge fine (received good data on DOUT). I then increased the excitation across the bridge to 8v. Because AVDD and VREFP cannot handle 8v, I left them connected to +3.3v supply. The differential voltage across AINP and AINN is around -1.33mV with no load. (Voltage from AINP to GND is around 4v) The digital output of the ADS1231 is now always zero. If I vary the load on the strain gauge I do not see any change in the data out. It now only seems to output zeros. I do know that the ADS1231 is not completely dead because I do see the DOUT line transition from high to low when the SCLK starts to pulse.

  • 0
    TI__Guru** 113765 points

    Hi Matthew,

    Welcome to the forum!  Basically you are violating two specifications when connecting the 8V excitation.  The first violation is the absolute maximum specification of the analog inputs exceeding AVDD+0.3V.  The second violation is the common-mode specification of having the analog inputs that must be between AVDD-1.5V and AVSS+1.5V.  The common mode as you have it configured is about 4V and with 3.3V AVDD should be between 1.5V and 1.8V.  Even if AVDD was 5V, the 8V excitation is creating a common-mode voltage that is too high for the ADS1231.

    It is difficult to say why you are seeing 0V output.  I would have assumed that you might see something that would be incorrect, but it is difficult to say for certain what the output will be when exceeding the absolute maximum at the inputs.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 10 points
    Bob, thanks for your help! Does TI have any ADCs for bridge sensors that can accept +8v excitation?
  • 0 in reply to Matthew Sipp25
    TI__Guru** 113765 points

    Hi Matthew,

    The answer is yes and no.  It is not so much the 8V of excitation, but rather the input voltage relative to the AGND and input common-mode.  As long as those two requirements are met you are good to go.  This can be achieved in a couple of different ways.  One is to use a supply that is referenced to some value below analog ground.  Another method would be to use an INA (or some other op amp configuration) where you can shift the common-mode to the correct input range.

    From a design perspective you really need to determine if and why 8V excitation is required.  I will make the assumption that you wish to increase the dynamic range of the measurement.  Consider this, if you make a ratiometric measurement where the excitation is common with the reference, the noise and drift of the source will cancel out of the measurement as they are common (as it relates to the ADC transfer function).  If you use different voltage sources for the excitation and reference, then the noise of each source will be reflected in the measurement.  This is a consideration that is often overlooked.  As I don't know anything about the type of measurement or sensor being used, it is difficult to answer what makes more sense.  For example, it may make more sense to use a 5V excitation with 5V AVDD and reference which are all in common as opposed to the 8V excitation and the 3.3V reference.  The dynamic range is less, but what really matters is the measurement resolution.  The measurement resolution is determined largely on noise.  In the end, you need to determine your design requirements and decide which methods may work best.

    As to your specific issue, I believe you are looking for a simple solution where you can connect an 8V excited bridge directly to the ADC.  I am not aware of any of our low noise bridge ADCs that will do this without some external circuitry.  If you use the ADS1247 or ADS1220 with bipolar supplies for the analog supply (AVDD and AVSS at +/-2.5V) and internal reference, you may be able to make a direct connection if the bridge common-mode is at 0V.  This would require that the 8V excitation source be 4V above AGND and -4V below AGND (or an additional supply of +/4V as the excitation) so that the common-mode is at or near AGND.

    You could use the split supply approach for the ADS1231 as well, which would require some odd voltages like +5.5V and -2.5V.  Again, the thought here is to make sure the common-mode is in the correct operating range for the device being used.

    Best regards,

    Bob B