Page 103 - Troubleshooting Analog Circuits

90 8. Operational Amplifiers-The Supreme Activators

whereas more modem amplifiers like the NSC OP-07 and the LM607 (gain =
6,000,000 min) have much less nonlinearity than older amplifiers.
Similarly, an op amp may have an offset-voltage temperature-coefficient specifi-
cation of 1 pVPC, but the op amp’s drift may actually be 0.33 pV/OC at some tem-
peratures and 1.2 pVPC at others. Twenty or thirty years ago, battles and wars were
fought over this kind of specsmanship, but these days, most engineers agree that you
don’t need to sweat the small stuff. Most applications don’t require an offset drift less
than 0.98 pV for each and every degree-most cases are quite happy when a 1 pV/T
op amp drifts less than 49 pV over 50 degrees.
Also, you don’t often need to worry about bias current and its temperature coeffi-
cient, or the gain error’s TC. If the errors are well behaved and fit inside a small box,
well, that’s a pretty good part.
There is one classical caveat, and I’ll include it here because it never got men-
tioned in the EDN series. I showed my typed draft to 45 people, and then thousands
of people looked at my articles, and nobody told me that I had forgotten this. Namely:
If you run an op amp in a high-impedance circuit, so that the bias current causes
significant errors when it flows through the input and feedback resistors, do not use
the Vos pot to get the circuit’s output to zero. Example, if you have an LM741 as a
unity-gain follower, with a source impedance of 500 kQ and a feedback resistor of
470 kR, the 741’s offset current of 200 nA (worst-case) could cause an output offset
of 100 mV. If you try to use the VOS mm pot to mm out that error, it won’t be able to
do it. If you have only 20 or 40 mV of this I X R error, you may be able to trim it out,
but the TC and stability will be lousy. So, you should be aware that in any case where
the Ios X R is more than a few millivolts, you have a potential for bad DC error, and
there’s hardly any way to trim out those errors without causing other errors. When
you get a case like this, unless you are willing to accept a crude error, then these
errors are trying to tell you that you ought to be using a better op amp with lower bias
currents.
And where did I find this reminder, not to trim out an I X R with a Vos pot?
There’s a chapter in Analog Devices’ Data Converter Handbook-it’s in there
(Ref. 1). Now, I’ve known about this trim problem for 25 years, but this isn’t a
problem that a customer asks us about even every year, these days, and I guess that’s
true for my colleagues, too. As it was not fresh in anybody’s mind, well-we forgot
to include it-we didn’t notice it when it was missing. It just shows why you have to
write things down!

An Uncommon Mode

A good example of misconstrued specs is the common-mode error. We often speak
of an op amp as having a CMRR (Common Mode Rejection Ratio) of 100 dB. Does
this number mean that the common-mode error is exactly one part in 100,OOO and has
a nice linear error of 10 pV per volt? Well, this performance is possible, but not
likely. It’s more likely that the offset-voltage error as a function of common-mode
voltage is nonlinear (Figure 8.2). In some regions, the slope of Avos will be much
better than 1 part in 100,000. In other regions, it may be worse.
It really bugs me when people say, “The op amp has a common-mode gain, Avc,
and a differential gain, Am, and the CMRR is the ratio of the two.” This statement is
silly business: It’s not reasonable to say that the op amp has a differential gain or
common-mode gain that can be represented by a single number. Neither of these gain
numbers could ever be observed or measured with any precision or repeatability on
any modem op amp. Avoid the absurdity of trying to measure a “common-mode gain
of zero” to compute that your CMRR slope is infinite. You’ll get more meaningful