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ADS8482: Implications of falling outside of the common-mode input range on ADS8482 pseudo-bipolar A/D

Part Number: ADS8482
Other Parts Discussed in Thread: ADS8881

This part is useful for my particular application due to the differential input, 18-bit resolution, throughput and parallel data output.  However, I also have a strange use case where I plan to mux two measurements into this A/D:

  1. The first is a fully-differential signal centered around VREF/2 that may span the full input range of the A/D.  This signal meets all of the guidelines in the ADS8482 datasheet.
  2. The second is a single-ended signal that can range between 0.5V and VREF-0.5V.  I want to supply this signal to the IN+ pin and VREF/2 to the IN- pin of the A/D.   In this case my input signals fall outside of the recommended common-mode input range for the part.  

The common-mode input range for the part is specified as 200mV above or below VREF/2.  In the "Analog Input" section of the datasheet there is a paragraph that states "Care must be taken regarding the absolute analog input voltage. To maintain the linearity of the converter, the +IN and –IN inputs and the span [+IN – (–IN)] must be within the limits specified. Outside of these ranges, the converter's linearity may not meet specifications."

Can you provide any more clarification on what might happen if I try to measure single-ended signals that fall outside of the recommended common-mode input range?  Is it a just a hit to linearity or will the part function incorrectly?  Are there any changes in the small signal measurement bandwidth if my common mode input is moving in and out of the specified range?  Thanks for your help! 

  • HI Eric,

    As the differential signal starts moving outside of the common-mode voltage range specification, the linearity (and THD performance) of the device will start to degrade gradually. If the signal is grossly outside of the common-mode specification, the device will be outside its linear range, the analog input stage may run out of headroom, and gross distortion/ erroneous conversion results will occur.

    You may also want to consider the ADS8881, this is a 18-bit, 1-MSPS , True-Differential Input, SAR ADC (although this device has a serial interface). The device provides a very flexible common-mode specification. Provided that the absolute voltage of both inputs (AINM, AINP) remain between -0.1V and VREF+0.1V, the device will convert the differential voltage signal and meet performance.

    Please let me know if you have additional questions.


    Thank you and Best Regards,
    Luis