Machine Design

Coming Soon To Circuit Boards: The electronic potentiometer

A new breed of electronically adjustable resistors promises to ease fine-tuning in sensors and other electronic devices.

Nick Tasker
Microbridge Technologies
Montreal, Quebec

The Rejustor electrically adjustable resistor uses low currents to heat resistive elements and change resistance. The resistor and micro heater are suspended over the cavity, which provides low thermal inertia and thermal isolation, and leaves adjacent circuits unaffected.

A schematic shows the interlaced heating and resistance elements. Resistance values range from 200 to several million with trimming times from 1 to 4 sec.

A Rejustor fourdevice die measures 1.6 x 1.8 mm and can withstand shock to 10,000 g's and vibration to 30 gs.

Adjustable resistors have come a long way from mechanical potentiometers set with a screwdriver. The latest technique is an improvement on electrically trimmable potentiometersand promises speedier programming-and in-circuit adjustment without the use of memory chips.

The electronically readjustable resistor, or Rejustor, is a passive, VLSI and MEMS-compatible adjustable resistor. It uses electric current to heat an element and change the resistance value. It doesn't need power to hold resistance value and can be readjusted thousands of times, bidirectionally, to a precision of 0.01%. Adjustments are made using 2-to-6-V signals with power consumption from 5 to 15 mW.

The adjusting of resistor values is the most common method to optimize sensor circuit performance in discrete and integrated circuits. Typically, circuit setup and calibration takes place by adjusting a resistance element using a variety of trimming techniques. These include manual mechanical potentiometers, laser trimming, fuseable passive-resistor arrays, and electrically configurable resistor arrays.

The Rejustor is based on standard CMOS and BiCMOS processes. Postprocessing via a cavity etch creates suspended resistive microstructures. These structures are thermally isolated from the substrate and have low thermal mass, which makes possible the localized, controllable and rapid thermal cycling of electrical resistance elements embedded in the microstructures.

A trimmable functional resistor with an associated interdigitated secondary or auxiliary resistor is built onto the microstructure. The auxiliary is used as a heating element, but is only powered during the actual adjustment step. It can be used once for single adjustment or hundreds or thousands of times for repeated bidirectional trimming.

Locally heating the auxiliary resistor changes the physical properties of the resistor with each heat cycle. The algorithms used to apply the heat cycles are adaptive. They involve a repeating sequence of measuring or inferring the functional resistance at room temperature, computing pulse parameters for the next temperature cycle, and applying the high-temperature sequence.

This adaptive approach would be impractical without the capability to rapidly heat and cool the microstructure so many cycles can be applied in a short time. This eliminates high-temperature furnace operations and prevents damage to other circuits near the chip.

Being electrically trimmed, the parts can replace thick-film resistors. Plus, semiconductor devices can be trimmed after packaging processes. It can also get rid of EEPROMs, currently used in some applications to store resistor value information. The parts avoid some of the problems associated with digital trimming such as limited resolution and limited resistor values.

ASIC designers, for example, no longer need to use digital-trimming methods requiring foundry processes that include EEPROMs. Board designers can replace mechanical trim pots with discrete components, saving labor costs. And MEMS-based sensors can include calibration and thermal compensation on the device.

The Rejustor is not without some limitations. For instance, though it's fabricated using standard CMOS and BiCMOS processes, additional steps of bulk silicon etch and wafer-level capping may be an obstacle to a CMOS-only foundry. However, these two processes are becoming increasingly available as many of these foundries move into manufacturing MEMS devices.

Also, extra pins are needed to apply the trimming signals. With multiplexing circuitry, a number of Rejustor heaters can be multiplexed through a single pin. Without multiplexing, one additional pin is needed per Rejustor. Another issue is that the ability to trim the Rejustor with low power levels rests upon its thermal isolation from the bulk silicon substrate. This means the power dissipation of the Rejustor is tied directly with the trimming power level, although larger Rejustors mean higher power ratings.

Last is trim time. This can vary from fractions of a second to several seconds depending on range and accuracy. Being an electrical trim, it is possible to trim any number of devices in parallel, limited only by the number of pin driver channels on the test system.

Products will be first available as discrete chip resistors and application-specific networks and later designed into analog and mixed-signal integrated circuits.

trim potentiometers

Easy to implement,
Wide adjustment range

Precision devices are costly.
Large form factors.
Prone to drift under vibration and temperature swings.
Manual adjustment with screwdriver.
Limited high-frequency operation.
Laser trimming

Established process,
Thick-film benefits
from thermal dissipation
of ceramic

Trimmed in only one direction.
One-time adjustment limited to wafer-probe level before final packaging for semiconductors.
Packaging stresses cause drift after laser trimming.
Complex process., Expensive capital investment.
Large barrier to entry for new or smaller players.
resistor arrays
Simple to use
High power needed to blow fuse.
One-time adjustment.
Trimmable in only one direction.
Higher precision requires more fuses, increasing cost.
resistor arrays
(Digital trimpots)
Low unit cost
Nonpassive technique needs applied voltage to maintain resistance value.
Needs additional circuitry for charge pumps to generate programming voltages.
EEPROM available in limited foundries and is relatively expensive.
Limited resistance values (to 10 K or higher) due to wiper resistance
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