Electronic Design

Resistor Trimming

What kinds of applications utilize precision resistor trimming?

Obvious applications include setting operational-amplifier gain and offset and voltage regulator output (Fig. 1a and 1b). But there are many others, such as sensor bridges and various specialty IC applications (Fig. 1c).

What are the options for resistor trimming?

Designers can use precision discretes, though practically speaking, this approach is limited to prototyping. For production trimming, the classic choices have been laser-trimming, using either resistors fabricated alongside active devices on an integrated-circuit die or trimmable discrete devices, and digital potentiometers. Recently, a third alternative known as the rejustor has become available.

How does laser-trimming work, and what are its pluses and minuses?

As part of the back end of IC manufacturing, trimming is accomplished by burning away part of the resistor structure using a laser beam. As the resistor's effective cross-section is reduced, resistance increases. The trimming is frequently done in conjunction with wafer probing, so it isn't so much the resistance that is being monitored to control how much material is being removed as it is some characteristic (e.g., amplifier gain) of the circuit being probed that is adjusted. Most of the advantages of laser trimming lie in the decades of experience the technology has accumulated. On the downside, laser equipment is fairly capital-intensive.

How about digital pots?

A digital potentiometer is essentially a ladder network whose switches are controlled by a digital input. Some digital pots incorporate nonvolatile memory to maintain their setting when power is removed, and some do not. Digital pots are very handy when a voltage divider is called for.

But since they're digital, their controllability is more granular than what would be possible with the highestprecision laser trimming. Also, designers must deal with a fixed amount of "wiper resistance." Their maximum allowable current varies with wiper position. And, digital pots often are frequency-limited by their packaging and generate the most thermal and 1/f noise of any adjustable resistor technology.

What about rejustors?

As a new technology, rejustors take a little more explanation. Although they can be processed at the IC fab level, the only products that have been announced to date are discretes, packaged in pairs to form voltage dividers.

Think of these products as something like digital potentiometers, but with lots of bandwidth (up to 2 GHz) and low noise-roughly equivalent to a metal-film resistor. They're continuously adjustable, like a physical pot, but without any wiper resistance. As with lasers, the object of trimming is usually to adjust some performance characteristic.

Each rejustor has two components: a thermally isolated poly film resistor and an adjacent power resistor (Fig. 2). For trimming, the power resistor is pulsed in a controlled fashion, briefly raising the temperature of the rejustor resistor. The result is an annealing that changes the rejustor's resistance in a controlled and predictable manner.

Why do it that way?

Rejustors make it possible to adjust resistance and temperature coefficient (TC) to independent targets. The adjustment software makes it possible to recursively adjust both those characteristics incircuit. In practice, designers pick a nominal value for the rejustor. During production, each circuit is fine tuned not just for trim point, but for TC as well. Also, rejustors have lower capital-equipment costs than trimming lasers.

Doesn't a resistor that gets set by thermal means imply drift in operation?

No. The annealing takes place at temperatures far above equipment operating (or soldering) temperatures.

When rejustors are eventually integrated inside monolithic devices, won't the annealing process damage adjacent semiconductor structures?

Again, no. The rejustors are thermally isolated. During fabrication, one or more dopant implant masks are used to tailor the rejustor resistor-poly film. (This is in addition to the other masks used to fab the IC.) At the end of fabrication, the resistive microstructures are released by a bulk-silicon etch process, leaving them suspended over a cavity, providing thermal isolation and low thermal mass.

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