Page 109 - Troubleshooting Analog Circuits

96 8. Operational Amplifiers-The Supreme Activators

in the air (which is an excellent insulator) you may be able to get decent bandwidth,
but you should be aware that you are probably measuring the AC CMRR of your set-
up, not of the op amp.
I was discussing this circuit with a colleague, and I realized the right way to make
this RlOO is to solder, for example, 100 R to the + input and 100 R to the -input, and
then just clip their tips together to make 200 R-balanced strays, and all that.
If you have an op amp with low gain or low g,,,, you may want to add in a buffer
follower at a-b, so the amplifier does not generate a big error due to its low gain. The
National LM6361 would need a buffer as it only has a gain of 3000 with a load of
10 kR, and its CMRR is a lot higher than 3000.
Altogether, I find this circuit has better resolution and gives less trouble than any
other circuit for measuring CM error. And the price is right: a few resistors and a
minigator clip.

Single-Supply Operation??
One of the biggest applications problems we have is that of customer confusion about
single-supply operation. Every week or so, we get a call from a customer: “Can I run
your LM108 (or LF356 or LM4250) on a single power supply? Your data sheet
doesn’t say whether I can. . . .”
Sigh. We cheerfully and dutifully explain that you can run any operational ampli-
fier on a single supply. An LM108 does not have any “ground” pin. It can’t tell if
your power supplies are labelled “+15 V and -15V” or “+30 V and ground,” or
“ground and -30 V.” That is merely a matter of nomenclature-a matter of your
standpoint-a matter of which bus you might choose to call ground.
Now it is true that you have to bias your signals and amplifier inputs in a reason-
able way. The LM108LM308 can amplify signals that are not too close to either
supply-the inputs must be at least a couple volts from either supply rail. So if you
need a circuit that can handle inputs near the minus rail, the LM108 is not suitable,
but the LM324, LM358, LMC660, LMC662, and LMlO are suitable. If you need an
amplifier that works near the + rail, the LMlOlA and LM107 are guaranteed to work
there. (The LF355 and LF356 typically work well up there but are not guaranteed.)
But if you keep the inputs biassed about halfway between the rails, just about any
op amp will work “on a single supply.” It’s just a matter of labels! Someday we plan
to write an applications note to get these silly questions off our back, and to answer
the customers’ reasonable questions, but right now everybody’s a little too busy to
write it.

Measure Bias Current Rather Than Impedance
Another op amp spec you don’t need to worry about is the differential input impe-
dance. Every year I still get asked, “How do we measure the input impedance of an
op amp?” And every year I trot out the same answer: “We don’t.’’ Instead, we mea-
sure the bias current. There’s a close correlation between the bias current and the input
impedance of most op amps, so if the bias current is low enough, the input impedance
(differential and common mode) must be high enough. So, let’s not even think about
how to measure the low-frequency differential input impedance (or, input resistance)
because I haven’t measured it in the last seven years.
Generally, an ordinary differential bipolar stage has a differential input impedance
of 1420 X I,), where 1, is the bias current. But this number varies if the op amp in-
cludes emitter-degeneration resistors or internal bias-compensation circuitry. You can
easily test the common-mode input resistance by measuring 1, as a function of Vcw