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ADS8900B: Reference voltage vs input signal range

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Part Number: ADS8900B

Questions for above circuitry.

1. If the input signal range to AINP is from 0V to 5V, and AINM is grounded; what should the reference voltage be applied on REFIN?

2. If the input signal range to AINP is from 0V to 5V, and to AINM is in 0 to -5V; what should the reference voltage be applied on REFIN?

  • Hello Wen,

    The inputs to the ADS8900 are unipolar, fully differential. This means that the allowable input voltage range for each of the inputs, AINP and AINM, is limited to positive voltages with respect to ground (0V to Vref).

    The full scale range is -Vref to +Vref differential. With Vref=5V, the input range will then be -5V to +5V differential.

    AINP=0V, AINN=5V, (AINP-AINN)=-5V differential, (AINP+AINN)/2=2.5V common mode

    AINP=5V, AINN=0V, (AINP-AINN)=+5V differential, (AINP+AINN)/2=2.5V common mode

    Also, since the inputs are fully differential, the input common mode voltage (average of the positive and negative inputs) must be very close to Vref/2, or 2.5V in the case of a 5V reference. (The allowable range is 2.4V to 2.6V.) With this limitation, you cannot connect the negative input to ground and get good performance out of the ADC.

    Please take a look at TI Precision Labs for a better description on these limitations.

    2.1 TI Precision Labs – ADCs: SAR ADC Input Types

    https://training.ti.com/ti-precision-labs-adcs-sar-adc-input-types

    Regards,
    Keith Nicholas
    Precision ADC Applications

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