FAQ

A Rejustor is a re-adjustable resistor. It is a semiconductor device with the physical properties of a resistor. That is, it impedes current-flow and creates a voltage-drop independent from frequency. It is a passive, bi-directional device that retains its resistance value throughout its operating range or storage life span.

More importantly, the resistance value of the Rejustor is adjustable. Resistance can be reduced from the nominal as-manufactured value by at least 30%. Resistance can be adjusted up or down within the active range. The programmed resistance value is stored in the physical properties of the semiconductor and retained until it is reprogrammed. No power is required to maintain the resistance value – it is strictly a passive circuit element.

Rejustors have two-terminals for the resistive element and two-terminals for the trimming element. Figure 1, below, shows the schematic symbol for a single Rejustor element.

Figure 1 : Rejustor Schematic Symbol
Currently Rejustors are available as standard products with two Rejustors per package available in SOIC-8 or 16-pin QFN packages.

Rejustor technology is available as Intellectual Property for inclusion with other semiconductor functionality. Consult Microbridge Technologies Inc. for more information.

The Rejustor can fulfill any application that requires a precision resistor, or an application that requires selecting a specific value for resistance, where the value is different from unit-to-unit or batch-to-batch.

There are lots of applications for the Rejustor. For example, a power supply vendor may choose to implement Voltage Regulators using Rejustors. A single, adjustable regulator can be used to create fixed regulators at 5V, 3.3V, 2.5V and 1.8V by adjusting the ratio of Rejustors controlling the adjustment pin of the regulator. Extremely accurate output voltages can be provided as required to meet the voltage regulation. A single integrated assembly using a regulator with Rejustors can be used for any voltage, reducing inventory of custom-voltage supplies and improving time to market.

Rejustors are adjusted from the nominal as-manufactured value using hardware and software available from Microbridge Technologies Inc. The Rejust-it software executes a control system that iteratively measures the target resistance (or controlled output) and adjusts the resistance to achieve the desired precision.

The physical resistance of the Rejustor is changed using a stream of pulses generated by the hardware. Patented software from Microbridge makes Rejustor trimming quick and easy.

Rejust-it software can also be adapted to work with commercial off-the-shelf hardware, such as National Instruments test and measurement modules. Microbridge is currently developing a system based on the NI cDAQ 9172 Test System.

Yes. Once trimmed, the Rejustor will retain its resistance precision to within a precision of 0.3%. High temperatures storage tests were performed at 150ºC for 1000 hours duration, as per JESD22-A108, and devices were found stable to within 1% of their trimmed resistance value.

All standard Rejustor products are RoHS compliant, Pb-free and green.

Different types of resistors have different Temperature Coefficient of Resistance (TCR). Resistance increases (or decreases) as the temperature increases (much like a thermistor) for some devices. TCR is measured in ppm/ºC (parts-per-million change in resistance per degree Celsius (or Kelvin) change in temperature). Certain applications require low-TCR for resistance stability across temperature. For example, if a thermistor is measuring a change in temperature relative to a stable resistance, it's important to minimize the temperature related change in the stable resistor (or Rejustor).

Low-TCR Rejustors offer environmentally stable TCR performance with 0ppm/ºC resistance change within ±100 ppm/ºC from the adjusted or unadjusted resistance. Rejustors within the same package provide additional benefit in that they closely track each other. These Rejustors provide TCR= 0±10ppm/ºC. Close tracking is well suited for Rejustor divider configurations where any environmental change in one Rejustor is mirrored in the other Rejustor to maintain a constant ratio between the two.

Low-TCR Rejustors are best for applications where the system being controlled is intended to be immune to temperature induced changes.

Relative Temperature Coefficient of Resistance:  the TCR difference between two or more devices. When two or more Rejustors are packaged together they have an RTCR of ±10ppm/ºC. Individual Rejustors have ±100ppm/ºC TCR, allowing a maximum worst-case RTCR between independent (not co-packaged) Rejustors of 200ppm/ºC.

From a resistive perspective Rejustors and laser trimming are approximately equal. Laser-trimmed thin film resistors can achieve precision in the range of 0.01%, Rejustors can exceed 0.01%. Laser-trimming requires time to measure the resistor, align the laser, trim and re-measure. Total trim time is proportional to the required precision, just as it is for Rejustors.

The resistance of Rejustors is initially adjusted down from their as-manufactured value. This is similar to laser trimming thin-film resistors which only increase their resistance. One principal difference is that the Rejustor can be adjusted up or down within the active range. Rejustors can be adjusted at probe or during calibration, just like laser-trimming. However, the Rejustor can also be adjusted at ATE or in the field. The final difference relates to cost overhead. Rejustors require an adjustment algorithm that can be implemented in a low-cost microcontroller for factory, field or even remote trimming of the Rejustor. Laser-trimming machines require a large capital outlay and skilled operators to operate and maintain the equipment. In addition alignment marks and test-structures are generally recommended for laser trimming, which has an impact on area required for the trim process.

The Rejustor is capable of achieving better precision, in an equivalent amount of time. The Rejustor offers more flexibility regarding where the operation is performed with several orders-of-magnitude less capital costs.

Digital pots have several negative characteristics which are not present in the Rejustor:

• Output error due to the voltage coefficient of the wiper. As the voltage on the wiper varies, either due to a change in wiper position or from an applied AC signal, the resistance of the wiper itself varies nonlinearly.
• Temperature coefficient of wiper resistance. A rough estimate of about 300ppm/ºC can often be assumed for its temperature coefficient. Since the wiper resistance is usually small in comparison to the overall pot resistance, this error is minor but may still be noticeable in some applications if high wiper currents exist.
• Absolute TCR is much higher than the RTCR. The resistance between the high side of the pot and the wiper will vary by the value of the absolute TCR over rated temperature.

The precision of a digital pot is related to the number of selectable bits in the device. A device with eight-bit precision is adjustable in 256 increments (the minimum precision is 0.4%), which is less than the precision capability of the Rejustor. Digital Pots require memory to "remember" their resistance, Rejustors do not.

In summary the resistance of digital pots varies with voltage and temperature (non-linearly) and it provides less precision than the Rejustor for a higher cost.

Digital compensation techniques are overkill for most sensor applications, requiring more die area, power-dissipation, non-volatile components (EEPROMS) to hold trim with various limitations on accuracy.

First-order affects like offset and output voltage are analog conditions with high variability from one sensor to another. Being able to normalize offset and span in a production environment, using analog solutions redefines the architectures for sensor systems. With Rejustors, all analog sensor assemblies can be configured with identical zero-offset, output voltage range and temperature response. Just as Eli Whitney discovered that interchangeable parts increase production efficiency, so too can the Rejustor improve production for analog sensor assemblies.

Higher-order affects, such as non-linear response have a greater impact on high-precision applications. Instead of dedication DSP resources to solving first-order affects with high-variability, digital processing can be used to correct second- and higher-order affects which are more generic to a sensor family.

Creating equal characteristics for all analog sensors also has a down-stream advantage. Previously sensor assemblies using digital compensation techniques would be programmed specifically for the offset and span of each sensor individually. If a sensor failed in the field, a technician would either have to replace the sensor and re-program the digital look-up tables, or replace both the analog sensor assembly and the digital processing unit. Failing that, a new sensor operating with the previous sensor's offset and span correction tables would produce erroneous results. Subsequently, a sensor change that requires a lookup table change, a digital processor change or the potential for erroneous results has a negative impact on cost and system integrity.

Finally, digital compensation is a non-starter for precision voltage regulator and voltage reference applications. Regulators need to power-up in a fixed state, whereas digital pots need to be programmed after power-up.

Rejustors and EPADS have similarities and differences. Both are semiconductor based, both provide high-precision resistance trimming, they can store their resistance almost indefinitely and are readily integrated with op amps and other analog technologies.

• EPADS are an active semiconductor device, while Rejustors are pure passive resistors in operation — simple slabs of resistive material.
• EPADS are more ESD sensitive than Rejustors.
• EPADS are not cost-competitive with thin-film resistors or Rejustors.