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Problem with ADS1281 input channel connection

Yu-Hung
Expert 1130 points
Other Parts Discussed in Thread: ADS1281, OPA1632

Hi,

ADS1281 has one differential analog input channel.

But my sensor is 3 pins. (Vin,Vout,GND)

So, the ground of sensor and ADC are the same.

Is the connection in the following figure right? 

(I feel strange because it likes single-end input, not differential input)

  • 0
    TI__Mastermind 44680 points
    Hi Yu,

    That is correct. The connection you've shown is a single-ended input signal (NOTE: A single-ended signal technically is still differential - it is just measured with respect to ground). Unless you use another sensor with a differential output (Vout+ and Vout-), you'll have to measure the single-ended signal, as you've shown.

    Note: Your sensor needs to provide a buffered output since the ADS1281 does not have a internal PGA. However, if there was a PGA, you would not be able to connect VIN- to GND.

    Best Regards,
    Chris
  • 0 in reply to Christopher Hall
    Expert 1130 points
    Hi Chris,

    The TI ADS1281 EVM has a buffer which is composed of OPA1632.
    Maybe I can reference the circuit of EVM board.

    As you say, my understanding is
    1. If the ADC has internal PGA, then VIN- can "not" be able to connect to GND. (Do you know why?)
    2. If the ADC does not have internal PGA, then VIN- can be able to connect to GND.
    Is it right?

    Regards,
    YHH
  • 0 in reply to Yu-Hung
    TI__Mastermind 44680 points

    Hi Yu,

    You can certainly reference the EVM circuit!

    Regarding your questions...

    1. Let me clarify this point...The circuit you showed was powered with a unipolar supply (i.e. AVSS = GND). If the negative input is connected to AVSS (GND), than the PGA's negative output would head towards the negative supply voltage; however, a PGA would saturate and not be able to output 0V.

      If instead you had a bipolar supply (i.e. AVDD = +2.5V, AVSS = -2.5V), then connecting the negative input to ground (Vin- = GND) would be okay, but Vin- still cannot be connected to AVSS for the same reason as above. We also refer to this limitation as the common-mode input voltage range.

    2. Correct, the Delta-Sigma's modulator is capable of measuring signals near AVSS, however, a PGA is not capable of outputting voltages near AVSS. (An integrated PGA is usually powered by AVDD and AVSS; the same power rails as the ADC).

    Note: The OPA1632 on the ADS1281EVM is externally powered by +/-10V supplies; therefore, it has can achieve a wider output voltage range!

    Best Regards,
    Chris

  • 0 in reply to Christopher Hall
    Expert 1130 points
    Hi Chris,

    It's really helpful for me.

    Thanks for your reply. :)

    YHH