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ADS8924B: Data Acquisition Board Integration

Part Number: ADS8924B
Other Parts Discussed in Thread: THS4551, REF5045

e2e,

I am currently working on a proof of concept board for NASA that focuses on low noise (<.1mV) low frequency signals (.01 - 100 Hz). In our current design the preferred signal to digitize and communicate via SPI a +- 5V AC signal operating near DC frequencies. We are currently using the ADS8924B, however, it would require a schematic rework to fit the full signal into the 0 - 5V range since this is an unipolar ADC.

From here the only solution I see outside of a rework (correct me if I am wrong please) is to find another precision ADC that accepts a bipolar differential input range of +- 5V and communicates using SPI. I have combed through the internet trying to find an ADC that fits these signal input specs and doesn't suffer with small signals in our frequency ranges.

Other Information:

  • Reference Voltage: preferably ground, potentially 2.5V.
  • Digital control pins Used: SOMI, MISO, CS, CLK, CONVST, REFBUFOUT.
  • Supply voltages: 5V, 2.5V, -5V.
  • Side Note: as this is a proof of concept the ADC does not need to be military/space grade for this application.

Your help is much appreciated,

-Karl texasinnovation@gmail.com

  • Hi Karl,

    One possible solution is to use a fully differential amplifier (FDA) to scale and shift the signal to the 0-5V unipolar range.  Attached is a TINA simulation file showing the THS4551 (FDA) powered by the +5V supply with the appropriate gain resistors to scale and shift the input signal.  This example scales a bipolar differential input signal (from -5V to +5V bipolar supply range to a 0-5V supply unipolar range. In this example, the ADS8924B uses VREF=4.5V reference (REF5045) powered with the VCC=+5V supply. The THS4551 VOCM pin is biased to VREF/2+100mV or a common mode of +2.35V to maximize output swing. Noise of the THS4551 circuit is ~6.5uVRMS.

    Please review the attached schematic/TINA simulation file.  Please let me know if you have any questions or if the circuit requires modifications.

    Many Thanks,

    Kind Regards,

    Luis Chioye

    SAR ADC Applications

    FORUM_ADS8924_THS4551_9-19-17.TSC