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How does the CMRR of difference amplifier ICs change over time?

This is a question I have recently asked at (here is a link), but without a good answer so far. I am reposting it here because I settled on using the AD8276 now, and it has been suggested that I try asking the manufacturer directly.

When designing precision analog circuits, I often come across parts which seem to be more than accurate enough for my purposes, but where the datasheet does not specify how key parameters will change over time.

Right now, I am looking at datasheets for difference amplifiers, and the CMRR looks better than what I would achieve by using affordable matched resistor dividers (e.g. the MAX5490). However, the resistor ratios will drift over time, which will reduce the CMRR.

Resistor dividers often give a typical value for this drift in ratio, so I can estimate how long my circuit can go without recalibration. However, while some of the difference amplifiers I saw specify input offset drift over time, I didn't see one yet which specifies the change in CMRR or the resistor ratio matching over time.

I'd assume that the parameters won't drift much beyond the initial limits over time, and this seems to be true e.g. for the offset voltage of many op amps, but on the other hand, I remember seeing 0.1% resitors which were only specified to drift less than 2% (or something along that magnitude) within a few thousand hours.

Now I'm wondering: Is there some rule of thumb for estimating how the CMRR (or similar parameters without aging specification) will develop? Can I assume that it will remain above the "minimum" specification even after some years of use? If not, for how many hours of use does the datasheet specification actually remain valid?

If there is no good general answer, how long can I expect the AD8276 to stay above its minimum CMRR of 80 dB / 86 dB (depending on grade)?

  • Hi Simeon,

    The summary answer, is that unless we have specifically measured a part for this data, we don't have absolute certainty. We do, however, have a number of features which give us a significant amount of confidence in the product: unlike discretes, the thin-film resistors are effectively identical due to the processing on the wafer, we also have relative-trimming to get the parts to spec, which means the actual deviations from resistor-to-resistor is small. In this case, the parts track each one another very well, which is how we're able to obtain performance that is otherwise very difficult to get with discrete solutions (even with very expensive precision parts).

    The specific measurements which embodies these sorts of measurements, are called "Long-term Drift." At ADI, we do actually perform Long-term drift on a few select parts, and the number of parts which are being measured this way is growing. Typically, however, these are the higher precision parts (references, ultra-low VOS amplifiers, etc) so the lower cost parts like the AD8276 would not normally receive this treatment (due to test cost). All is not lost however, the Industry standard test for observation is 1000-hours of use. From many precision references (not just from ADI), to amplifiers and even actual bench-top equipment, much of the real error accumulation happens over the first 1000 hours of life, after which the majority of errors settle out and the drift takes on a small, but non-zero error as a function of time.

    The positive take-away here, is that the worst of the errors will settle themselves out in roughly the first month of service life (if always powered). The device should still be within specifications at that time, and from there on the total error over the lifetime should be negligible.

    Please let me know if I can clarify anything further!



  • Hello David.

    Thank you for the insightful answer, this is pretty much what I was hoping to hear. Just one clarification, what exactly do you mean by "the Industry standard test for observation is 1000-hours of use"? Are the min/max specifications derived from measurements taken after that time?

    Thanks for your time,