Page 134 - Troubleshooting Analog Circuits

Time for Timers 121

100 MR or more. Just be careful about board leakage and socket leakage-as you
would with a high-impedance op-amp circuit. Then you can use a smaller. higher-
quality capacitor.
Furthermore, it is a nontrivial statement that not all 555s work similarly; some
manufacturers’ 555s have different internal circuits and different logic flow charts.
So be careful to check things out-555s from different manufacturers can act quite
differently.
At high speeds, the timers don’t just respond in a time 0.693 R X C; the response
time is more like 0.693 R X (C + CSTRAY) + T,,,,,. Most books never mention this
fact-most data sheets don’t, either. So, although you can usually get a fast timer
circuit tofunction, to get it to work the way you want it to, you still have to be
careful. These designs are not always trivial, and Ref. 1 may help you avoid some
pitfalls. A timer is, after all, just an aggregation of parts that includes a comparator.
so many of the techniques you use with comparators work with timers, and vice
versa.

Digital ICs: Not Purely Digital

Although timers are partly digital, the more classic digital ICs perform purely logical
functions. Nevertheless, in the hands of a clever “linear” designer, some digital ICs
can be very useful for performing analog functions. For example, CD4066 quad
analog switches make excellent low-leakage switches and a 74C74 makes an excel-
lent phase detector for a Phase-Locked Loop (Ref. 2). And not only is the price
right-so is the power drain. Even when ordinary CMOS ICs aren’t fast enough, you
can often substitute a high-speed CMOS or 74ALS or 74AS counterpart to get more
speed. I won’t belabor the point; instead, I’ll go straight to the litany of Troubles and
Problems that you-whether an analog or a digital designer-an encounter with
digital ICs.
First, unless proven otherwise, you should have one ceramic power-supply bypass
capacitor in the range 0.02 to 0.2 pF-or even 1 pF, if the IC manufacturer requires
it-for each digital IC, plus a tantalum capacitor in the range 2 to 10 pF for every
two, three, or four ICs. The ceramic capacitors provide good local high-frequency
bypassing; the tantalum parts damp out the ringing on the power-supply bus. If you
can’t use a tantalum capacitor, you can use 10 or 20 pF of aluminum electrolytic, or
if you are desperate you can try a 1 or 2 pF extended-foil Mylar unit in series with a
I carbon resistor, to provide the needed lossiness. If your linear circuit really de-
pends on clean, crisp digital outputs (CMOS outputs make dandy square-wave gener-
ators, as long as the power supply isn’t ringing and bouncing) you may even want
more bypassing-possibly hundreds of microfarads.

Floating Inputs Can Leave You at Sea

On lTL parts, you can leave an unused input floating and it will normally go HIGH:
on CMOS, you must tie unused inputs (such as the preset and clear inputs of a flip-
flop) to the positive supply or ground, as appropriate. Otherwise, these inputs will
float around and give you the screwiest intermittent problems. Also, when these
inputs float, for example, on unused gates, they can cause considerable unwanted
power drain and self-heating.
With CMOS, people used to tell you that you can use an inverter as an amplifier by
tying a few megohms from the input to the output. At low voltages, you can make a