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SAR ADC Input Types Pseudo Differential ADC why

HARI T O1
Expert 1265 points
Other Parts Discussed in Thread: ADS8370, THS4031

Dear Team,

I was going through this document, which describes different SAR ADC input types.

Below are the images for some pseudo differential configuration

I have couple of questions regarding this configuration.

  1. Can I ground IN+ and connect the input signal to IN-.Any special care is required for this
  2. Why this configuration is required, it is not giving the any benefit of differential signaling
  3. Can you please tell some real life examples where this type of configuration is used

Regards

HARI

  • 0
    TI__Guru**** 171726 points

    Hi Hari,

    1. It depends on the device.  Some pseudo differential devices have a MAX input voltage of +/-100mV on the (-) input (see example above).  Also, the conversion results would be inverted in the case where you could apply a fixed DC (like Vref/2 for example).
    2. Are you referring specifically to the ADS8370?  That device has a +/-200mV limit for the (-) input (see datasheet page 3 Absolute input voltage).  The image above shows a 0-4V input versus +/-2V with a DC bias applied to the THS4031.
    3. One example might be an application where you have a signal from an off board sensor via twisted pair.  Common mode noise that is picked up could be attenuated using a pseudo differential input device.
  • 0 in reply to Tom Hendrick
    Expert 1265 points

    Hi Tom,

    Tom Hendrick said:
    Are you referring specifically to the ADS8370

    No that is a general question.When we ground one input we won't get the benefits of differential signalling.

    Then why companies are making pseudo diff input ADC's.Single ended or fully differential will be enough.

    This was my 2nd question.

    Regards

    HARI

  • 0 in reply to HARI T O1
    TI__Guru**** 171726 points

    It's just another tool to help with signal integrity.  If you ground the (-) input, you essentially have a single ended ADC.

  • 0 in reply to Tom Hendrick
    Expert 1265 points

    Hi Tom,

    Thank you very much.

    May I know how it is helping with SI.It is not differential since one input is grounded.

    Tom Hendrick said:
    If you ground the (-) input, you essentially have a single ended ADC

    Then why companies are making this ADC's with pseudo diff input .They can only make single ended.

    Regards

    HARI

  • +1 in reply to HARI T O1
    TI__Guru**** 171726 points

    Because some applications require the ability to remove unwanted low level common mode voltage.