THE SENSIBLE USE OF
PRECISION RESISTORS
TYCO ELECTRONICS offers SMD and
wire-leaded precision resistors that serve a multitude of applications, as well as
assistance for their proper use!
In analog circuit design, many situations call for resistors that have been specified very
tightly with respect to tolerance and Temperature Coefficient of Resistance (TCR). These
"high-precision" (or simply "precision") resistors are commonly used
to control the gain in op amps or to provide a highly accurate reference voltages. They
are made using thin-film manufacturing processes and provide stability over a longer term
than standard thick-film resistors.
The table below lists the key differences between precision and standard thick-film
resistors. Even as a low-cost commodity, thick-film resistors can carry tolerance ratings
as low as ±1% or ±5%. However, thin-film versions are typically available with tolerance
levels down to ±0.05%. Precision resistors furthermore have a far lower temperature
coefficient, which means they are suitable for environments that experience extreme
temperature fluctuations - a typical example is in automobiles. What's more, thin-film
resistors provide more stable and reliable operation in moist environments. The advantage
of thick-film resistors, on the other hand, is their lower cost and their ability to
handle higher power levels.
Parametric Differences between
Thick-Film and Thin-Film Resistors
Thick-film and thin-film resistors are made using vastly dissimilar manufacturing
processes, with the primary difference being the manner in which the resistive layer is
applied. With thick-film technology, specially formulated pastes are printed onto a
substrate then fired in a high-temperature furnace. The pastes are usually applied onto a
96% alumina ceramic substrate using a silk-screen process.
In thin-film manufacturing, the substrate is etched with an appropriate pattern and a
layer of metallization is sputtered onto it. This metallization is normally made of nickel
chromium, while the substrates can be 99.5% alumina ceramic, silicon or glass.
The use of photolithographic processes to form thin-film patterns produces much finer
lines than do thick-film processes, thus permitting the resistance and TCR of the
resistive elements to be controlled more accurately. Over the years, these manufacturing
processes and the accuracy of the thin-film deposits have been continually improved,
resulting in ever more precise thin-film products. In the past, an off-the-shelf precision
resistor with a tolerance of 0.5% and a TCR of 25ppm would have been considered a high-end
product. Today, however, resistors in both wire-leaded and surface mount designs are
available with tolerances of 0.05% and TCRs of 5ppm.
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When two or more thin-film resistors are needed in a circuit, they can be closely matched
in terms of resistance value and TCR. As a result, the circuit can provide improved signal
processing - a major advantage for instrumentation and industrial control applications,
among others.
One common application for precision thin-film resistors is in circuits that use
differential op amps. For example, when such an op amp must function in an electrically
noisy environment, precision resistors can be used to tightly symmetrize the inputs and
thus suppress unwanted interference without influencing the useful signal. Precision
resistors are also advantageous in voltage divider circuitry, where they are used to step
down voltages within exacting limits while ensuring that no errors occur. Another typical
area of application for precision resistors is A/D and D/A converters. For example, in D/A
converters, the precision of the most significant bit has a highly weighted influence on
the overall accuracy of the conversion process. Thus, thin-film resistors are the
preferred choice for circuitry used in processing higher-value bits, while standard
thick-film resistors are often used for lower-value bits.
TYCO ELECTRONICS, through its Holsworthy brand, offers one of the most extensive precision
resistor ranges on the market. Devices are available in both surface-mount and wire-leaded
designs. The SMD versions are packaged in tape and reel, with 250-piece strips and 1000-
and 5000-piece reels offered. Standard resistance tolerance is 0.1%; the standard TCR
value is 10ppm. Tolerance levels of 0.01%, 0.05%, 0.25% and 0.5% are also supported, as
are TCR values of 5ppm, 25ppm, 50ppm and 100ppm.
Wire-leaded precision resistors from TYCO come in ammo packs of 250, with a rated power of
0.125W, 0.25W, 0.5W and 1W. Standard versions are available with a resistance tolerance of
0.1% and TCR of 15ppm, or 0.1% tolerance and 25ppm TCR. Tolerance levels of 0.05%, 0.25%,
0.5% and 1% are also available, in addition to TCR values of 5ppm, 50ppm and 100ppm.
Not only are precision resistors getting better, they are also increasingly cost-effective
to use. As a result, an ever greater number of circuit designs can benefit from the
stability and tight tolerance offered by these devices.
Don't hesitate to order the comprehensive overview catalog and/or detailed data sheets for
these precision products from CODICO. Or, send us your application details and we would be
pleased to assist you further!
Arnold Geitzenauer, EXT 24
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