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Op amp drift over time?

Other Parts Discussed in Thread: OPA333, OPA2188

I have an opamp setup in a voltage follower (buffer) configuration with the input 
voltage tied to the '+' channel well between the supply rails of the opamp.

This is for an essentially DC application-- a buffer for an A/D vref line.

My application has to have extremely good long term stability, and in the 
context of this buffer, I want to make sure that the input voltage always follows 
the output voltage as precisely as possible.   Will this still be the case after 
temperature cycling from -40 to +85 deg C, vibration, and aging in general?

The circuit is set up ratiometrically, but at this point I'm afraid this vref buffer 
could introduce long-term offet/ratiometric error.
(I'm potentially looking for a single supply opamp recommendation as well)

Any thoughts would be appreciated!

  • Andy,
    The long-term stability of TI op amps is determined during the product life-test where the parametric shift of large sample size of parts is measured using accelerated aging process in high temperature ovens.  Based on this data, the op amps input offset voltage (Vos) and Vos drift may shift after ten years by the full maximum initial spec guaranteed in the PDS.  Thus, for example, an op amp with initial Vos of 70uV and the max PDS Vos spec of +/-120uV may at most shift after ten years to either 190uV (70uV+120uV) or -50uV (70uV-120uV); similarly, an op amp with initial Vos drift of .5uV/C and the max PDS Vos drift of +/-1.0uV/C may at most shift after ten years to either 1.5uV/C (0.5uV/C+1.0uV/C) or -0.5uV/C (0.5uV/C-1.0uV/C) - see examples below.
    Therefore, in general, you may estimate the maximum expected shift of any parameter over any given period of time by using:
    * 100% of the max (min) PDS guaranteed value in the case of specs centered around a mean value (Vos, Vref, Vos Drift, etc).
    * 10% of the max (min) guaranteed value for parameters specified as a fixed positive value (IQ, AOL, PSRR, CMRR, etc).
    and pro-rate them based on the expected ten-year life of the product.
    You need to keep in mind that the long-term shift is not exactly a linear function of time - it is steeper (shifts faster) in the first year and slows down in the later years. It also usually excludes the first 30 days due to continuing self-curing of the molding compound used for packaging of IC. 
  • Hello Andy,

    Marek already gave a very detailed explanation on what you can expect for the long term drift of the amplifier.  I am very interested in learning more about the level of precision you are trying to achieve with the voltage reference + buffer amplifier over time and temperature.

    What level of accuracy are you looking for? Around 1%, 0.1%, 100ppm or 10ppm?

    Thank you!


  • Hi Thomas (and thanks to Marek for the clear answer)-

    I'd ideally like to be in the neighborhood of 10-30 ppm.   Stability over temperature is good, but repeatability over temperature and time is really what I am after since I explicitly calibrate the units at given temperatures.   Once the system is calibrated I hope to estimate how well the calibration will hold up over time.

    All of the sensors wired to the A/D are ratiometric to the supply voltage, however the supply voltage (low noise supply) has capacitance, and when this same supply is connected to the VREF line of the A/D it has the potential to incur A/D gain errors due to the source impedance.  So my options are either (a) deal with the gain errors (which may vary over time due to caps changing over time), or (b) add a buffer to isolate the VREF line from the circuit, which is what I was getting at in this post.


  • You've been given the technical answers but no recommendation. ?????????

    Try a Zero-Drift OPA333.


  • Thanks Neil,

    the OPA333 would work well, and after looking at that part I noticed the newly released OPA2188 which looks great too-