Page 43 - Troubleshooting Analog Circuits

30 3. Getting Down to the Component Level

“voltage coefficient.” This last term refers to the nonlinearities in Ohm’s Law that
occur when there is a large voltage drop across a resistor; the effect is most common
in resistors with large values and small values-nes built with high densities.
Therefore, when you drive the reference input to a D/A converter, you should be
aware that the Rin will only shift 1-3% over the entire temperature range. However,
there may still be a broad tolerance, as it is not easy to keep tight tolerances on the
“sheet rho,” or resistivity, during the IC’s production. For example, a typical D/A
converter’s Rin specification is 15 kll+33%. These film resistors have even better
tracking TC than diffused resistors, often better than 1 ppm/”C.
In addition to the TC, you might also be concerned with the shunt capacitance of a
resistor. Recently (back in Chapter 2), I was trying to build a high-impedance probe
with low shunt capacitance. I wanted to put a number of 2.5 MQ resistors in series to
make 10 Ma. I measured the shunt capacitance of several resistors with our lab’s
impedance bridge. A single Allen-Bradley carbon-composition resistor had a 0.3 pF
capacitance, so the effective capacitance of four in series would be down near 0.08
pF-not bad (Figure 3.3). Then I measured a Beyschlag carbon-film resistor. Its
capacitance was slightly lower, 0.26 pF. The capacitance of a Dale RN60D was 0.08
pF; the capacitance of four in series would be almost unmeasurable.
It would be an improper generalization to state that certain resistor types (films)
always have less shunt capacitance than others. However, the main point is that if
you need a resistance with low shunt capacitance, you can connect lower-value resis-
tors in series; and if you evaluate several different manufacturers’ resistors, you may
find a pleasant surprise.

Variable Resistors and Pots

As with the fixed resistors discussed so far, there are many kinds and types of
variable resistors, such as trimming potentiometers, potentiometers, and rheostats.
These resistors are made with many different resistive elements, such as carbon,
cermet, conductive plastic, and wire. As with fixed resistors, be careful of inexpen-
sive carbon resistors, which may have such poor TC that the manufacturer avoids any
mention of it on the data sheet. These carbon resistors would have a poor TC when
used as a rheostat but might have a good TC when used as a variable voltage divider
or a potentiometer. Recently I ran an old operational amplifier where the offset trim
pot had a range of 100 mV. Yet for 4 hours in a row, the amplifier’s offset held better
than f10 pV. That’s an amazing M.O1% stability for a carbon composition pot! On
the other hand, some of the cermet resistors have many excellent characteristics but
are not recommended for applications that involve many hundreds of wiper cycles.
For example, a cermet resistor would be inappropriate for a volume control on a radio.
The major problem area for variable resistors is their resolution, or “settability.”
Some variable resistors claim to have “infinite” resolution; but, if you apply 2 V
across a variable resistor’s ends and try to trim the wiper voltage to any or every
millivolt in between, you may find that there are some voltage levels you can’t
achieve. So much for “infinite resolution.” As a rule of thumb, a good pot can usually
be set to a resolution of 0.1 %, or every 2 mV in the previous case. Thus, counting on
a settability of 0.2% is conservative.
Good settability includes not only being able to set the wiper to any desired posi-
tion but also having it stay there. But, I still see people advertising multi-turn pots
with the claimed advantage of superior settability. The next time you need a pot with
superior settability, evaluate a multi-turn pot and a single-turn pot. Set each one to
the desired value, tap the pots with a pencil, and tell me which one stays put. I nor-
mally expect a multi-turn pot, whether it has a linear or circular layout, to be a factor