Page 133 - Troubleshooting Analog Circuits

IO. The Analog/Digital Boundary

A Never-Never Land?

Previous chapters have dealt with circuit elements and circuits usually thought of as
purely analog. Now we turn to an area that confounds and frightens all too many
engineers-the boundary between the analog and digital worlds. Armed with a solid
theoretical foundation and the insights presented here, you can keep your journey
into the analog/digital interface from seeming like a visit to an unreal world.
Many classes of circuits are neither entirely analog nor entirely digital. Of course,
as an analog engineer, I don’t have a lot of trouble thinking of all circuits as analog.
Indeed, when problems develop in circuits containing both analog and digital ele-
ments, finding a solution is more likely to require that you summon your analog
expertise than your digital knowledge. Timers, D/A and A/D converters, V/F and F/V
converters, and SP circuits all fall right on the boundary line between the analog and
digital worlds. Digital ICs have more than a few analog subtleties. And even multi-
plexers, which you may have thought of as purely analog, have some quirks that
result from their close association with the digital world.

Time for Timers

A timer is basically a special connection of a comparator and some logic, which is
usually built with analog circuit techniques. The familiar 555 timer can do a lot of
useful things, but it sure does get involved in a great deal of trouble. I’ll treat the most
classical fiascoes.
For one thing, people try to make timers with the crummiest, leakiest-usually
electrolytic4apacitors. Then they complain because the timers are not accurate or
their timing isn’t repeatable. Some people insist on building timers to run for many
seconds and then have trouble tweaking the time to be “exactly right.” Sigh. These
days I tell people, “Yes, you could make a 2-minute timer with an LM555 or a 10-
minute timer with an LM322, but that would be wrong.” Instead, you could make a
simple 4-Hz oscillator using one-quarter of an LM324 or LM339 and cheap, small
components. This oscillator can drive a CD4020 or CD4040; the last output of that
counter, Q12 or Q 14, can time very accurately and conveniently.
Such an arrangement is cheaper and much more accurate and compact than what
you get if you blow a lot of money on a 47 pF polyester capacitor for a long-interval
timer, or put up with the leakages of a tantalum capacitor, which no manufacturer
wants to guarantee. In addition, in just a few seconds, you can trim the moderate-
frequency oscillator by looking at an early stage of the divider; trimming a long-
interval timer can take hours. The CMOS counters are inexpensive enough, and these
days for 2- to 20-minute timer applications, I can usually convince customers not to
buy the linear part. The LM555 data sheets tell you to avoid timing resistors with
values higher than 20 MR. Nowadays, though, you can get a CMOS version
(LMC555 or equivalent) or use a CMOS comparator or a CMOS op amp to work at

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