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ADS8422: Polarity of AD conversion data

MESH
Intellectual 830 points
Part Number: ADS8422
Other Parts Discussed in Thread: THS4131,

Hello,

I considering AD conversion circuit that circuit is used ADS8422 and THS4131.
I considering this circuit based on figure 29 of ADS8422 data sheet.

THS4131 negative output is connected +IN of ADS8422 on figure 29.
And THS4131 positive output is connected -IN of ADS8422 on figure 29.

I would like to confirm polarity of AD conversion data at below condition.

Conditions:

- Positive input (390ohm side) of THS4131 is positive voltage.
- Negative input (390ohm side) of THS4131 is 0V.

Regards,

MESH

  • +1
    TI__Mastermind 36280 points

    Hello MESH,

    The VIN input polarity for the circuit in Figure 29 is labeled incorrectly; the THS4131 will invert the differential input voltage (gain=-1). 

    Assuming Vref=4.096V.

    +VIN (390ohm side) = 4.096V

    -VIN (390ohm side) = 0V

    +IN (ADC positive input) = 0V

    -IN (ADC negative input) = 4.096V

    Vadc_diff = [(+IN) - (-IN)] = -4.096V : Vadc_code=0x8000h

    If we reverse the input polarity:

    +VIN (390ohm side) = 0V

    -VIN (390ohm side) = 4.096V

    +IN (ADC positive input) = 4.096V

    -IN (ADC negative input) = 0V

    Vadc_diff = [(+IN) - (-IN)] = +4.096V : Vadc_code=0x7FFFh

    Regards,
    Keith Nicholas
    Precision ADC Applications

  • 0 in reply to Keith Nicholas
    Intellectual 830 points

    Hello, Keith

    Thank you for your reply.

    I will design the AD conversion circuit based on your reply.

    Regards, MESH

  • 0 in reply to Keith Nicholas
    Intellectual 830 points

    Hello, Keith
    I have additional question relatded connection of THS4131 to ADS8422 just to make sure.

    <Question>
    I would like to confirm following items in the connection shown below.
    - No problem with each circuit operation with the next connection.
    - If 4.096V is input to + VIN (390ohm side) of THS4131 and 0V is input to -VIN (390ohm side) of THS4131 in the next connection, the AD conversion result will be 0x7fff.

    <Connection of THS4131 to ADS8422>
    - Connect THS4131 Vout+ to ADS8422 +IN
    - Connect THS4131 Vout- to ADS8422 -IN

    Regards,
    MESH

  • +1 in reply to MESH
    TI__Mastermind 36280 points

    Hello MESH,

    Yes, if you change the connections as you described:

    - Connect THS4131 Vout+ to ADS8422 +IN
    - Connect THS4131 Vout- to ADS8422 -IN

    Then you will get 0x7FFFh conversion code with +VIN=4.096V and -VIN=0V.

    Regards,
    Keith

  • 0 in reply to Keith Nicholas
    Intellectual 830 points

    Hello Keith,

    Thank you for your reply.

    I will change the schematic.

    Regards,

    MESH

  • 0 in reply to MESH
    TI__Mastermind 36280 points

    You are welcome!

    Regards,
    Keith