Page 136 - Troubleshooting Analog Circuits

Perfect Waveforms Don’t Exist I23
n

PULSE OUTPUT

COMPARATOR

Figure IO. I. Runt pulses cause problems in this simple ADC (a). The comparator state changes at random
times. Occasionally, the state will change at precisely the wrong tim-n the clock‘s rising
edge-making the output pulse narrower or wider than normal. You can solve the problem
by using two flip-flops with the clocks separated by a delay.

counter makes a carry from 01 11 to 1O00, for a few nanoseconds the output code will
be oo00, and the decoder can spit out a narrow pulse of perhaps 6-8 ns in duration
corresponding to oo00. Even if you are observing with a good scope, such a pulse can
be just narrow enough to escape detection. If the decoder were merely feeding an
LED display, you would never see the sub-microsecond light pulse, but if the de-
coded output goes to a digital counter, a false count can occur. In digital systems,
engineers often use logic analyzers, storage scopes, and scopes with very broad band-
widths to look for glitches or runt pulses and the conditions that cause them. In
analog systems, you may not have a logic analyzer, but these nasty narrow pulses
often do exist, and you have to think about them and be prepared to look for them.
Another thing to know about digital ICs is that many CMOS ICs have the same
pinouts as ‘ITL parts. For example. the 74193,74LS193, and 74C193 have the same
pinouts. On the other hand, some of the older CMOS parts have pinouts that differ
from those of similarly numbered TTL devices. The 74C86’s pinout is the same as
the 74L86’s but differs from the 7486’s. Beware!