The universal thermal-compensation module in Fig 1 can neutralize the temperature coefficient of both signs within a ñ0.6%/°C range. The circuit can compensate the temperature coefficient of offset voltage, gain, sensitivity, or, in general, the linear terms of a device's overall temperature behavior. The circuit consists of a pair of negative-temperature-coefficient (NTC) thermistors, labeled R_T, and of fixed resistors with calculated values that reduce the inherent nonlinearity of the thermistors. Because of this linearization, the overall nonlinearity stays within ±1.2% for a 0 to 50°C temperature range.

When the 1-Mohm potentiometer is in the maximum left position, the TC has a maximum positive value of 0.6%/°C. In the maximum right position, the circuit produces maximum NTC of the same absolute value. You can continuously adjust the TC between these two extremes. For proper operation, good thermal contact is necessary between the module (the thermistors, primarily) and the device that you're compensating. The TC of the circuit depends only on the wiper position-not on the input voltage. So, you can adjust the output voltage and TC independently. The module has a gain of 0.5 at 25°C.

The resistor values in Fig 1 are the exact calculated numbers that provide the best linearization. These values are feasible in film technology with laser trimming. For discrete components, you simply have to choose the closest values. The circuit uses inexpensive, chip-NTC thermistors with nominal values of 1k at 25°C and constant B=3100, but any other types are applicable. However, differences in constant B may cause slightly higher nonlinearity. The power consumption of a thermistor is ó0.1xV_IN2 mW. This power has to be small enough to avoid the self-heating of the thermistor body. In most practical cases, V_IN varies from 1 to 10V, so the power varies from 0.1 to 10 mW.

Performing the compensation requires that you first measure the output of the device at room temperature with the wiper electrode close to the center. Then, heat or cool the device to either a minus or plus temperature close to the margins, such as 0 and 50°C. Make the TC adjustment by rotating the wiper so that the error decreases. The criterion of compensation is simple: The output of the device at temperature extremes should be the same as the room-temperature output.

Strictly speaking, this module can compensate only the linear part of the initial TC of a device. Residual TC of the device is 90% less than initial. The circuit can actually operate over a -25 to +85°C temperature range, but was optimized for a 0 to 50°C operating range. You may see increasing nonlinearity outside of this range. A special program can estimate the nonlinearity in any temperature range and can calculate the proper values of the resistors. (The program is available from the author.)

For precise compensation, a reduced range of TC is preferable. Choosing R_PAR800ohm, R_SER2.5 kohm, and R_BR3 kohm reduces the full range of TC to ±0.1%°C. (DI #1703)