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ADS1274 Quasi-Differential Input?

Ronald Gatzke
Prodigy 80 points
Other Parts Discussed in Thread: ADS1274, THS4521, TLC2272

Can I drive the ADS1274 in a "quasi-differential" mode, where I drive the + input single ended, and drive the -input with a Reference voltage at mid-voltage of the input range? What down-side is there to this method of input drive?

  • 0
    TI__Guru 57296 points

    Hi Ronald,

    Thank you for your post and welcome to our forum!

    I suppose to begin answering that question, I need to understand what is driving the ADS1274 and what are the common-mode and peak-to-peak characteristics of your input signal? Typically, we want the INP and INN inputs of the ADC to sit at the same common-mode voltage. If the common-mode voltage is equal to mid-supply, then we allow the signals on each input to swing in the positive and negative directions and use as much of the full-scale input range as possible.

    If you are using an FDA like the THS4521, you can take a single-ended input and drive the ADS1274 inputs differentially with a common-mode voltage set by the Vocm pin, which is mid-supply if not driven to another voltage. Because the ADS1274 is an un-buffered ADC, this would be the preferred way to go. Is that something you could use?

    Best Regards,

  • 0 in reply to Ryan Andrews
    Prodigy 80 points
    Hi Ryan,
    Thanks for this suggestion. I'm using the AD1274 for recording Very low frequency signals (ECG - 1 mVp-p, down to 0.05 Hz. Low frequency noise is a primary concern. The Fully Differential amplifiers are typically not spec'd for frequencies below 10Hz, which is the high end of my range. (The THS4521 looks like its 0.1-100 Hz noise will be around 5uVp-p, a little high.) I must also simultaneously record accompanying signals of high amplitude (4Vp-p) at frequencies up to 30 KHz. Hence the need for a fast, wide dynamic range ADC.

    I was thinking of driving the ADS1274 from a low noise OpAmp (TLC2272) which is spec'd for low noise 0.1 - 10 Hz, ~~ 1 uVp-p. I was thinking of driving one input to the ADC from a fixed Reference voltage of +2.5VDC by a TLC2272. The active input would be driven by a TLC2272 at +2.5VDC nominal, with a +- 2.5VAC signal swing (total absolute swing from 0V to +5V). This should meet the operating specs. of the AD1274, and be accomplished at low noise levels.

    The down side to this is the lack of symmetrical voltage swings around a fixed common mode voltage level. (The common mode voltage would effectively have a large signal swing.

    Would my quasi-Differential drive signal be acceptable without compromising the performance of the ADC?
    Thanks for your help! Ron
  • 0 in reply to Ronald Gatzke
    TI__Guru 57296 points

    Hi Ron,

    Thanks for the insight into your application!

    Essentially, what I believe you are describing is this picture below. The ADS1274 has a minimum of 90dB CMR (typical 108dB). I think that the swinging common-mode input should be ok.

    Does that help answer your question?