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OPA333: OPA333 input-referred voltage noise variance with temperature

Part Number: OPA333
Other Parts Discussed in Thread: OPA388

From one of my customers:

"Is there any information available for how the OPA333 input-referred voltage noise varies with temperature? I am interested mainly in the frequency range of 1 to 100Hz. I’ve seen some indications with the OPA333 that there can be noise maxima at certain temperatures in the 20C to 50C range. The amplitude and frequency of these maxima vary from part to part. I suspect the OPA333 vs other parts of the circuit since I can swap the OP333 between circuits and the noise signature vs temperature follows the OPA333."

An additional question to the customer question: Should we be looking at a different amplifier, and if so, which would you recommend?

Thanks

  • The low frequency, 1/f, input voltage noise in the CMOS transistors is NOT a function of temperature so the only temperature related variation in the noise will come from the variation in the broadband (thermal) noise - see below.  

    Additonally, chopper amplifiers like OPA333 have no 1/f noise component (it's removed by chopping) so the only noise even at the low frequency is the broadband noise.  Thus, the variation in the noise is proporional to the squre-root function of absolute temperature so if you change the temperature from 25C (298K) to 50C (323K), OPA333 typical noise spectral density of 55nV/rt-Hz will increase by factor of sq-rt(323/298) or ~4% to 57.2nV/sq-rt.

    The higher the speed of the amplifier the lower the thermal noise (~sq-rt(1/gm) so if you wish to find the lowest noise among 5V auto-zero type of amplifiers you may want to take a look at OPA388 with 10MHz bandwidth and typical spectral density noise of 7nv/rt-Hz at 25C, which may increse to 7.3nV/sq-rt at 50 deg C.

  • I have emailed you additional information...
  • All the recent parts we have are marked "CHQ" which is the appropriate marking for the OPA333AMDCKREP part that we are using. We don't see any other marking that we could interpret as a lot code.