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ADS1262 Missing Parameters in Datasheet Rev. B

Oliver Wiecha
Prodigy 20 points
Other Parts Discussed in Thread: ADS1262

What's the noise performance of the internal 2.5 V reference? This parameter is not stated, but it is critical when using the internal reference for other precision circuits on the same board.

What's the tolerance of the 280 ohm resistors located after the PGA (also connected to CAPP and CAPN)? The manufacturing tolerance of those resistors together with the tolerance of the external capacitor determine how short the settling time can be, which is crucial in applications with tight timing.

  • 0
    TI__Mastermind 44680 points

    Hi Oliver,

    Welcome to the TI E2E forums - Those are good questions!

     

    I've looked at the ADS1262's internal reference's noise performance and found it to perform as following:

    • 0.1 to 10 Hz noise: 13.8 uVpp
    • Broadband noise density @ 1kHz: 290 nVrms/sqrt(Hz)

    ...Keep in mind that only a fraction of the reference noise may actually appear in the ADC's output result:

    • Reference noise scales with the input signal. For a 0V input there is no reference noise seen in the ADC output result, while an input signal equal to VREF will have reference noise. Therefore,
    • The ADC's digital filter will filter out much of the reference noise.
    • The PGA gain will only gain up the input signal (not the reference noise), so using a larger PGA gain will improve overall resolution.
    • If the circuit is configured ratiometrically (i.e. the reference voltage is used to excite the input sensor), the reference noise will be common to both the input and reference signals; therefore, the noise will tend to cancel in the ratio (Vin / Vref).

    I recommend checking out the following TI Design: "High-Resolution, Low-Drift, Precision Weigh-Scale Reference Design with AC Bridge Excitation", to see an example of the measured noise performance of the ADS1262 with an applied input signal.

     
     
    Regarding the tolerance of the 280 Ohm resistors, I think 10-15% would be a safe estimate.
    In general, I don't think this should create an issue (even with the resistor tolerance error) because the cutoff frequency of this RC circuit is well outside of the digital filter's -3dB bandwidth (even at the 38.4 kSPS data rate), and also because the the ADS1262 has an inherent 50us conversion time delay (when DELAY[3:0] = 0000b) to allow for the PGA to settle.
     
    Best Regards,
    Chris