Page 41 - Troubleshooting Analog Circuits

28 3. Getting Down to the Component Level

RESISTIVE
MATERIAL
METAL
ALLOY
WIRE
CERAMIC
CORE

Figure 3. I. Film resistors (a) are made by cutting a spiral into a layer of metal or carbon deposited on a
nonconductive core. Carbon composition types (b) have a solid core of resistive material;
wirewound resistors are formed by winding resistive wire on a nonconductive core (c).

100 or 50 ppm/"C, these resistors have TCs as good as 20,10,5 or 2 ppm/OC and
accuracies as good as 0.01%. These resistors are comparable to small precision wire-
wound resistors but are generally smaller and (slightly) less expensive. They also
have much less inductance than the wirewound types and, thus, are suitable for
higher-speed operation. A few spirals on a film substrate add negligible inductance
compared to the hundreds or thousands of turns on a wirewound resistor's bobbin.
Recision-film resistors are also available in matched sets of discrete resistors whose
relative accuracy and TC tracking are better than those of individual resistors.
You can also buy custom thin-film resistor networks on a single substrate if your
requirements are critical and your bankroll large. A more economical route is to use
four, seven, or eight matched precision thin-film resistors in a DIP. I have found the
TC tracking of these devices from several manufacturers to be better than 1 ppm/"C.'
These sets are ideal for precision amplifier stages and D/A converters. (For a good
example of where they are useful, refer to the matched resistors in the thermocouple
amplifier in Figure 2.10.) Also, when you buy film resistors, leave them on the tape.
When you need matched sets, you can pull off adjacent resistors and be reasonably
confident that they will match and track well.
Thick-film resistors are usually found in hybrid circuits, but are also available as
small networks. They are made of Cermet or other proprietary materials, and are
baked and fired after being screened onto a ceramic substrate. Their TC is not quite
as good as thin-film resistors, but they are popular because they have good TC
tracking, and they are inexpensive and easy to trim to 1 or 1/2%.
Traditionally, the best, most stable resistors have been wirewound resistors
(Figure 3.1~). These days, precision film resistors can match wirewound resistors
quite well for almost any set of specifications. However, for a resistor whose value is
between 200 kR and 1 MR, wirewound resistors are more expensive and come only
in larger packages. Wirewound resistors also have one major disadvantage: the in-
ductance of an ordinary wirewound resistor makes achieving fast (sub-microsecond)
settling almost impossible. However, you can specify a special winding pattern that
can greatly cut down the inductance of the windings. This type is listed in several
manufacturers' catalogs as "Type HS." But I've found that there are two different

1. For example: Allen Bradley FOSB 103A; Beckman 694-3-R- IOK-A, and Caddock T-914- 10k-100-
05, all consist of four 10 kfl resistors, matched to 0.05% with 50 ppWC and 5 ppmPC tracking, in an 8-
pin mini-dip package.