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ADS131E06: why does input referred noise with PGA gain

Dave Knight
Intellectual 590 points
Part Number: ADS131E06


Page 16 of the ADS131E06 datasheet states that increasing PGA reduces input-referred noise. Why is this? Is there a formula to determine how much the input-referred noise is reduced?

I'd guess the input referred noise is reduced by the gain.

Regards,

Dave

  • 0
    TI__Guru 66706 points

    Hi Dave,

    The input-referred noise of the ADS131E06 is simply the noise observed at the input pins of the device. This precedes the PGA gain stage. Since the ADC thermal noise is assumed to be constant, the noise as seen at the device inputs is effectively the thermal noise divided by the PGA gain.

    Some of the device thermal noise also comes from the PGA itself, so as the gain doubles, the input-referred noise does not reduce by exactly one-half, but it's usually a close approximation.

    Lower input-referred noise allows you to measure even smaller signals, thus increasing your system resolution. However, it should be noted that it also reduces your dynamic range (i.e. the difference between smallest and largest measurable signals). This is why you see ENOB decrease as gain increases.

    Best Regards,

  • 0 in reply to Ryan Andrews
    Intellectual 590 points

    Great answer.

    How does offset error drift of 0.65uV/C change with gain?

    Regards,

    Dave

  • 0 in reply to Dave Knight
    TI__Guru 66706 points

    Hi Dave,

    Offset error drift also reduces with gain. See Figure 13 below:

    Best Regards,

  • 0 in reply to Ryan Andrews
    Intellectual 590 points
    analog inputs have a DC input impedance of 200Mohm. What's the temperature coefficient of this impedance? Does the input impedance change with gain? Does the temperature coefficient of the input impedance change with gain?
  • 0 in reply to Dave Knight
    TI__Guru 66706 points
    Hi Dave,

    The input impedance of the PGA does not change with temperature or gain, so the typical 200M impedance will still hold true.

    Best Regards,