Page 117 - Troubleshooting Analog Circuits

104 8. Operational Amplifiers-The Supreme Activators

the broadband noise. A sample time of 0.1 second works pretty well; the ones that
pass a 0.1 -second test but fail a 0.5-second test are uncommon.

Popcorn Noise Can Rattle Sensitive Circuits

Flicker noise, also known as l/f noise, is AC noise that exists at low frequencies. And
even more insidious than l/f noise is popcorn noise-a type of electrical noise in
which bursts of square steps are added to the normal thermal noise at random times.
Popcorn noise occurs rarely these days, but, unfortunately, it’s not at 0%, not even
with the cleanest processing and the best manufacturers. I’ve been chastised and told
that some of my amplifiers are noisy compared with those of certain competitors. But
when I look at the competitor’s data and plots, I see l/f and popcorn noise lurking
unnoticed in a comer. On high-performance parts, we try to screen out the noisy
ones. But when a few parts have a spacing of 2 to 10 seconds between bursts of pop-
corn, it’s not cost-effective to look for those parts. Only a small percentage of our
customers would want to reject that one noisy part and pay for the testing of all the
good parts, too. Remember, 10 seconds of testing time equals 30 cents; time equals
money.
Although oscillation and noise problems may be the most common ones you’ll
encounter when you use op amps, there’s a host of other characteristics that are wise
to look out for. These characteristics include overload or short-circuit recovery, set-
tling time, and thermal response. Many op amps have a fairly prompt recovery from
overdrive when you make the output go into the stops-that is, when you force the
output into the power-supply rails. For most op amps, this recovery characteristic is
not defined or specified. One recently advertised op amp claims to require only 12 ns
to come out of the stops. Just about all other op amps are slower to one degree or
another. The recovery time for chopper-stabilized amplifiers can be seconds.
Even if you have a fast op amp that doesn’t have a delay coming out of limit, there
may be circuits, such as integrators, that take a long time to recover if you overdrive
the output and inputs. To avoid these cases, a feedback bound made of zeners and
other diodes may be helpful (Ref. 7). However, if you have a differential amplifier,
you may not be able to use any zener diode feedback limiters. I recall the time I de-
signed a detector circuit using a fast, dielectrically isolated op amp. When I went to
put it into production, nothing worked right. It turned out that the manufacturer had
just recently redesigned the chip to cut the die size by 50%. The new and improved
layoutjust happened to slow down the op amp’s overdrive-recovery time. I wound
up redesigning the circuit to use an LM709. I saved a lot of pennies in the long run,
but the need to change parts didn’t make me very happy at the time.

Rely Only on Guaranteed Specs

Don’t rely on characteristics that aren’t specified or guaranteed by the manufacturer.
It’s perfectly possible for you to test a set of samples and find that they feature some
desired performance characteristic that is not specified by the manufacturer. But if
the next batch doesn’t fulfill your requirements, whom are you going to get angry at?
Don’t get mad at me, because I’m warning you now. Any unspecified conditions may
cause a test result to vary considerably compared to a guaranteed tested specification.
If you have to work in an unspecified range, you should keep a store of tested good
ICs in a safe as insurance. If a new batch comes in and tests “bad” you’ll have some
backup devices. I recall a complaint from a user of LM3046 transistor arrays: A
fraction of the parts failed to log accurately over a wide range. The “bad” ones turned