Matched resistors are utilised in many applications including sensors for commercial, medical and military markets and pulse squaring elements. However, the most common applications is resistor divider circuits for amplifier applications. This article focuses on how to avoid amplifier output errors and discusses the merits of using resistor networks instead of discrete devices.
In amplifier circuits the gain is directly proportional to the reference voltage (Vout = R2/R1 * Vin). As many circuits use a resistor divider (R1 and R2) to set the reference voltage any shift in resistor parameters will cause a corresponding error in the gain. A mismatch in the performance of the resistors in the divider circuit must therefore be avoided. Low resistor TCRs, tight ratio tolerances, and excellent load-life stability are required for peak performance.
Resistor Tolerance Issues
If R1 and R2 vary proportionally, the gain will not change. However, If one resistor changes relative to the other, then the ratio of R1 to R2 and therefore the gain will change. Matching resistor tolerance is therefore critical to ensure any change is proportional.
In precision applications something better than a 1% resistor tolerance will be required. If we assume both resistors have a 0.1% tolerance then the worst case matching for two resistors would be ±0.2% or 2000ppm at room temperature.
Selecting discrete resistors with the same tolerance can be resource intensive and time consuming and there is always the chance resistors may be mixed up when they reach the customer.
Thermal Issues And Resistor Network Performance
Temperature will impact on resistor value. If the temperature affects both resistors to the same extent there is not an issue but differential temperature effects should be avoided. Issue to consider include
· Proximity of external heat sources
· Temperature Co-efficient of Resistance
If the resistors are not in the closest practical physical contact then the proximity of external heat sources may impact on the resistance of one resistor in relation to the other. A further mismatch may be introduced by manufacturing variables in discrete resistors that result in differential self-heating caused by uneven power dissipation.
Differences in resistor manufacturing and materials also impact on the variations in temperature coefficient of resistance of two discrete devices. The same chance in temperature will result in a different change in resistance over two discrete devices
Using network resistors in matched resistor applications significantly reduces the impact of temperature effects as resistors are in close physical contact and a closely matched in both manufacturing processes and materials. The resistors may be also trimmed if necessary for optimal resistance matching.
Resistor networks therefore offers a number of advantages over discrete, conventionally mounted, devices in precision amplifier applications. This construction is also easier to cool than conventionally mounted devices.
Various substrate materials are available but for many applications thick film technology is preferred due to the high packaging density, its mechanical properties and excellent thermal performance.
Contact TSEC today to discuss your network resistors design and manufacturing issues