Zeroing out temperature variations on a production line is a challenging task when the device to be compensated has a temperature coefficient that varies in both magnitude and sign from unit to unit.
This simple circuit can meet the challenge (see the Figure 1). It’s inexpensive and delivers an output voltage with a tempco that can be adjusted from –10 to +10 mV/°C for the values shown in the diagram.
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| Figure 1. | Almost any tempco, whether it be positive, negative, or zero, can be set with this inexpensive, simple circuit. For the values shown, the output is zero at 25°C and exhibits a tempco of between –10 and +10 mV/°C depending on the position of the wiper of RSET. |
The heart of the compensator is the Analog Devices AD590 temperature transducer, which may be located remotely if desired. The device produces a current proportional to the absolute temperature with a scale factor of 1 µA/Kelvin. Resistor ROS and the –15-V supply to which it’s connected sink a current equal to the room-temperature AD590 current of 298.2 µA.
This allows the circuit’s operation to be centered at about 25 °C (298.2 K). Other room-temperature values can be accommodated by simply changing ROS.
The maximum magnitude of the circuit’s temperature coefficient (VX) equals the parallel combination of ROS, RSET, and optional resistor RX multiplied by the AD590 scale factor of 1 µA/K. For the values shown in the diagram, including an RX value of 14.3 kΩ, that maximum magnitude is 10 mV/°C.
Potentiometer RSET, resistors R1 and R2, and the two op amps form an amplifier with a gain that varies linearly from –1 to +1 as the potentiometer wiper moves from bottom to top. That amplifier buffers and scales VX so that the compensator’s output, VTC, can be adjusted over the range of –10 to +10 mV/°C (if RX is omitted, the range is approximately –33 to +33 mV/°C).
