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Original question:

OPA189: input current noise density spectrum of OPAx189

OPA189: Offset current common mode stability and thermal drift.

Ralph Loader
Prodigy 10 points
Part Number: OPA189

Hi,

The question of input current noise density was usefully answered here: https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1269686/opa189-input-current-noise-density-spectrum-of-opax189?tisearch=e2e-sitesearch&keymatch=input%252520current%252520noise#

Two related questions that are relevant especially at low frequency:

  • What is the linearity, or just stability, of the input offset current w.r.t., to common mode input voltage.
  • What is the temperature coefficient of input offset current.

I suspect the answer may well be that they are low, but given the current noise is orders of magnitude lower than the total current, it can't be taken for granted - even modest values might have more practical effect than noise.

(In both cases, the offset current is much more relevant than the bias current if it makes any difference - with proper impedance matching the bias cancels and only offset contributes to the noise.)

  • 0
    TI__Mastermind 21247 points

    Ralph,

    • The offset versus common mode is specified by CMRR (CMRR = 20log(ΔVcm/ΔVos)).  Also, a typical graph is given in figure 7-6.  The change in offset versus common mode is linear.
    • The input bias current and bias offse will double every 10ºC.  The input current offset can be much lower than this as shown in figure 7-11.  The maximum specified input current is 3nA at 125ºC.  You can estimate the current at any temperature using the relationship given in Optimizing Chopper Amplifier Accuracy  (section 6).  The article also recommends maximum source and feedback network impedances for most TI chopper amplifiers.

    I hope this helps!  Best regards, Art