Page 107 - Troubleshooting Analog Circuits

94 8. Operational Amplifiers-The Supreme Activators

100,OOO at 1 kHz or any lower frequency. Note also that, on this unit, the CM error is
not really linear-as you get near -9 V, the error gets more nonlinear. (This is a
-9-V/+12-V CM range on a 12-V supply; I chose a f12-V supply so my function
generator could over-drive the inputs.) So, the business of CMRR is not trivial- at
least, not to do it right.

How to Do It Right.. .

As we discussed in the previous section, there are circuits that people use to try to test
for CMRR, that do not give valid results. Just how, then, can we test for CMRR and
get the right results??
Figure 8.7 is a darned fine circuit, even if I did invent it myself about 22 years ago.
It has limitations, but it’s the best circuit I’ve seen. Let’s choose Rl = Rl = 1 k, R2 =
R12 = 10 k, and R3 = 200 k and R4 = a 500 R pot, single-turn carbon or similar. In this
case, the noise gain is defined as 1 + [R$ Ri,], or about 11. See pages 100-101 for
discussion of noise gain. Let’s put a fl l-V sine wave into the signal input so the
CM voltage is about f10 V. The output error signal will be about 11 times the error
voltage plus some function of the mismatch of all those resistors. Okay, first connect
the output to a scope in cross-plot (X-Y) mode and trim that pot until the output error
is very small-until the slope is nominally flat. We don’t know if the CMRR error is
balanced out by the resistor error, or what; but, as it turns out, we don’t care. Just
observe that the output error, as viewed on a cross-plot scope, is quite small. Now
connect in R100a, a nice low value such as 200 Q. If you sit down and compute it,
the noise gain rises from 1 1 to 1 1 1. Namely, the noise gain was (1 + R2/Rl), and it
then increases to (1 + R2/R1) plus (R2 + R12)/R1~. In this example, that is an increase
of 100. So, you are now looking at a change of V,,, equal to 100 times the input error
voltage, (and that is VC., divided by CMRR).
Of course, it is unlikely for this error voltage to be a linear function of VcM, and
that is why I recommend that you look at it with a scope in cross-plot (X-Y) mode.
Too many people make a pretend game, that CMRR is constant at all levels, that CM
error is a linear function of V,,, so they just look at two points and assume every
other voltage has a linear error; and that’s just too silly. Even if you want to use some
ATE (Automatic Test Equipment) you will want to look at this error at least three
places-maybe at four or five voltages. Another good reason to use a scope in the X-
Y mode is so you can use your eyeball to subtract out the noise. You certainly can’t
use an AC voltmeter to detect the CMRR error. For example, in Figure 8.6, the CM
error is fairly stated as 0.2 mV p-p, not 0.3 mV p-p (as it might be if you used a meter
that counted the noise).
Anyhow, if you have a good amplifier with a CMRR of about 100 dB, the CM
error will be about 200 pV p-p, and as this is magnified by 100, you can easily see an
output error of 20 mV p-p. If you have a really good unit with CMRR of 120 or 140
dB, you’ll want to clip in the R100b, such as 20 R, and then the A (noise gain) will be
1OOO. The noise will be magnified by lOOO, but so will the error and you can see what
you need to see. Now, I shall not get embroiled in the question, are you trying to see
exactly how good the CMRR really is, or just if the CMRR is better than the data-
sheet value; in either case, this is the best way I have seen.
For use with ATE, you do not have to look with a scope; you can use a step or
trapezoidal wave and look just at the DC levels at the ends or the middle or wherever
you need. Note that you do not have to trim that resistor network all the time, nor do
you have to mm it perfectly. All you have to know is that when the noise gain
changes from a low value to a high value, and the output error changes, it is the