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ADS1298 Single-Ended Input Range

Nate
Genius 11680 points
Other Parts Discussed in Thread: ADS1298

Team,

Can you confirm that with a 3.3V supply and 2.4V Vref that the INP range for a single ended channel would be .45V to 2.85V w.r.t. GND?

Thanks!

  • 0
    TI__Guru 57326 points

    Hi Nate,

    Thanks for your question. The short answer is that 0.45 V to 2.85 V is a perfectly acceptable range for a 3.3 V supply and VREF = 2.4 V (I'm assuming single-supply? AVDD = 3.3 V, AVSS = GND).

    However, I think that the INP range values you mention are coming from a datasheet example that is incorrect. Just to confirm, are you using the following from Figures 26 and 27:

    • INN = Common-Mode = 1.65 V (mid-supply)
    • INP = CM +/- 1/2 VREF = 0.45 V - 2.85 V

    The input range to the ADC is dictated by two restrictions:

    1. Each analog input pin is limited to AVSS - 0.3 V to AVDD + 0.3 V
    2. The full-scale differential input range for the ADS1298 is actually +/- VREF / Gain

    Figures 26 and 27 in the ADS1298 datasheet are being updated to the following:

    Therefore, if your VREF is 2.4 V, your input of INP = 0.45 V to 2.85 V is only using half of the available output codes of the ADS1298 (assuming Gain = 1). In theory, with INN fixed at CM = 1.65 V, INP could range from -0.75 V - 4.05 V, but the voltage limits on each analog input pin prevent you from doing this. The best option, if possible, would be to decrease the reference voltage such that the input range uses more of the available output codes.

    To maximize resolution with a 3.3 V supply, I suggest you decrease VREF to 1.2 V, keeping INN = CM = 1.65 V. Then, all of your available 224 output codes would be spread across a differential input range of +/- 1.2 V and INP could range from 0.45 V to 2.85 V.

    Best Regards,

  • 0 in reply to Ryan Andrews
    TI__Genius 11680 points

    Thanks, Ryan!

  • 0 in reply to Nate
    TI__Genius 11680 points

    Hi Ryan,

    If they instead wanted to maximize the range over the full range of the amplifier (.3V - 3V with 3.3V supply), they should use a common mode voltage of 1.65V and a Vref of 1.35V, correct?

    Thanks.

  • 0 in reply to Nate
    TI__Guru 57326 points

    Hi Nate, 

    Are you referring to the internal PGA of the ADS1298, or some external amplifier that the customer is using?

    Keep in mind that what's really important here is that your input signal to the ADC (post-gain) should use as much of the CM +/-Vref range as possible in order to maximize resolution. 

    Regards,