Page 42 - Troubleshooting Analog Circuits

Match the TC to the Application 29

Figure 3.2. Bob uses carefully selected precision resistors to model the resistance of the metal runs of a
FET op amp. (Photo by Steve Allen.)

types of HS: One type has almost zero inductance and greatly increased inter-
winding capacitance; the other type has low inductance and low capacitance and is
well suited for fast-settling amplifiers. Be cautious of manufacturers’ oversimplified
statements.
A tricky problem popped up a couple of years ago when we assembled a precision
amplifier with wirewound resistors. The output was drifting all over the place, but the
amplifier, zeners, and transistors were stable. What was drifting? It turned out that a
wirewound resistor was “drifting” because we had mistakenly used a special temper-
ature-compensating resistor with a TC of +3300 ppm/”C. This type of temperature-
compensating resistor is often used for correcting the TC of transistor-logging cir-
cuits, but it wasn’t labelled in an obvious way. When we put this resistor in a circuit
where a low-TC resistor was required, it took us a couple of hours of troubleshooting
to pinpoint the problem.

Match the TC to the Application
Diffused resistors, commonly used in ICs, have some strange characteristics. Their
TC is high-around +1600 ppm/“C-and includes a nonlinear, or quadratic, term.
Thus, the resistance goes up faster at high temperatures than it falls at cold tempera-
tures. These resistors would be useless except for one minor detail: They track at the
rate of approximately +1 ppm/”C. Since it is very inexpensive to make matched pairs
or sets of these resistors in a monolithic integrated circuit, their use is popular among
IC designers. If you’re not designing an IC, though, you probably won’t meet up with
diffused resistors very often.
Many ICs, such as D/A converters and voltage references, are made with thin-film
(sichrome or nichrome) resistors on the chip. Compared with most other resistor
types, these resistors have the somewhat lower TCs of 50 to 350 ppmPC, closer
ratios, better long-term stability, better TC tracking, and less nonlinearity of the