Page 146 - Troubleshooting Analog Circuits

Not Much Agreement on Acquisition-Time Definition I33

Vi,, in the SAMPLE mode. However, the outputs of many S/H circuits can settle to a
new DC value faster than the hold capacitor charges to the correct value. If the S/H
doesn’t have to go back to HOLD, the data may give false results, even if the output
seemed to give the correct answer when it was still in SAMPLE mode. To avoid con-
fusion, we define acquisition time as the pulse width required for precise sample-and-
hold action. If the circuit SAMPLES and settles and then goes into HOLD and gives
you the wrong answer, the SAMPLE pulse should have been wider-right? Right.
There may be some S/H circuits whose output voltage won’t change if you switch
them to the HOLD mode as soon as their output reaches a value that corresponds to a
new Vi”. But if I had an analog switch that couldn’t hold at all, I could still get it to
“acquire” a signal according to the data-sheet definition just cited. I consider the test
implied by that definition to be too easy. I believe some users and manufacturers in
this field agree with my definition, but the situation isn’t really clear. (I would appre-
ciate reader comments. You folks are getting all sorts of good ideas from me, and if
you have some good comments, it’s only fair that you bounce them off me.)

E Pluribus Unum: The Multiplexer

Another type of circuit that depends on analog switches is the analog multiplexer.
As mentioned already, a multiplexer can draw big transients if you suddenly con-
nect it across big signals at low impedances. So be careful not to overdo operating
a multiplexer in this manner, as excessive current could flow and cause damage or
confusion. It’s well known that multiplexers, like most other forms of analog
switches, are imperfect due to leakages, on-resistance, and response time. But they
are popular and won’t give you much trouble until you turn the power supplies
OFF and keep the signals going. I recall that in the past few years, at least one or
two manufacturers have brought out new designs that could survive some fairly
tough over-voltages with the power removed. I’m not sure what the designs in-
volved other than adding thin-film resistors and diode clamps on the inputs-
ahead of the FET switches. But if you add discrete resistors ahead of any mono-
lithic multiplexer’s inputs, the resistors can help the multiplexer survive the loss
of power.
One other problem with multiplexers is that you don’t have a whole lot of con-
trol over the break-before-make margin. And if you should want make-before-
break action, I don’t think it’s an available option. So, sometimes you may have to
“roll your own” multiplexer.
If your signal levels are less than 15 V p-p, you may be able to use the popular
CD405 1 and CD4053 multiplexers and the CD4066 CMOS analog switches.
which are inexpensive and quick and usually exhibit low leakage. However, if you
need a guarantee of very low leakage, you will have to test and select the devices
yourself, as many people do.

Digital Computers
To avoid making unpleasant comments, I will simply say that I hope somebody else
writes a good book on troubleshooting these.

Software

NO comment.