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ADS1281: Configuration of the ADC for Unipolar Operation?

Matthew Bloom
Prodigy 10 points
Other Parts Discussed in Thread: ADS1281

Dear TI Support,

I am currently using the ADS1281 in a unipolar mode of operation. 

The ADS1281 is set up as follows:

*AVDD = +5V, AVSS = 0V

*VREFP = +5V, VREFN = 0V

*ADC driven by an AD8138 differential op-amp with VCOMMON = +2.5V. VIN- of the op-amp is set to GND and VIN+ can be anywhere between 0V to +5V (the input into the system is single-ended, but is converted to differential signals for the ADC). At VIN+ of 0V, VOUT+ = VOUT- = +2.5V. As a side-note, the ADC is currently configured in FIR-mode only.

When analyzing the output of the ADC in continuous conversion mode, I am only analyzing 24-bits rather than 32-bits of data. Analyzing analog input values between 0V to +2.5V correlates to hex values between 000000h to 7FFFFEh. As soon as I try to inject values of +2.5V to +5V, the ADC output clips at 7FFFFFh, which actually correlates to the characteristics listed in Table 14 of the datasheet for FIR filtering mode. Therefore, my ADC looks like it is currently setup for a bipolar mode of operation where the FSR is +/-2.5V.

I tried searching the datasheet to see if there was a way to configure the ADC to work in unipolar mode, where values greater than VREF = +5V result in the expected clipped hex values. Is there a way to set the ADC into this type of unipolar mode, or do the ADC outputs only correspond to a bipolar mode of operation? Will the ADC output values always clip for values > VREF/2?

Regards,

Matthew Bloom

  • 0
    TI__Mastermind 44680 points

    Hi Matthew,

    Welcome to the TI E2E Forums!

    The ADS1281 (analog) supplies can be configured as unipolar (AVDD = +5V, AVSS = 0V), or bipolar (AVDD = +2.5V, AVSS = -2.5V).

    The ADS1281 (analog) inputs are fully-differential and the input range is referenced to the analog supplies (AINP & AINN are "rail-to-rail" inputs).

    No matter what supply mode you use, the analog inputs are always fully differential. So to answer your question, there is no other input mode.

    What that means is your positive full-scale input is limited to +Vref/2 (+2.5V in your application), and your negative full-scale input is limited to -Vref/2 (-2.5V in your application).

    Right now, you are only making use of 1/2 of the ADC's input range (from 0 to +2.5V). You are limited to the positive output codes.

     

    To make full use of the ADC's dynamic range, you need the AD8138 to:

    • Output -2.5V differentially (+OUT=1.25V, -OUT=3.75V), for a 0V single-ended input.
    • Output +2.5V differentially (+OUT=3.75V, -OUT=1.25V), for a 5V single-ended input.

    This will make use of all the 2's compliment output codes.

    Best Regards,
    Chris