Measure resistances easily, without reference resistor or current source

25 October 2000 Analogue, Mixed Signal, LSI

The circuit of Figure 1 shows how to use the new LT1168 instrumentation amplifier to achieve a precision resistance-to-voltage conversion as simply as possible. Normally, the resistor across pins 1 and 8 is the gain-set resistor and the voltage across pins 3 and 2 is the variable to be measured. In this case, however, the 1,25 V reference establishes a fixed input voltage so that the variable to be measured is now the resistance. The equation for VOUT vs RT is VOUT = 1,25 V . 49,4 kOhm/RT.

Given the limitation on output swing (with the supply voltages shown), the smallest measurable resistance is about 4,5K. The highest resolvable resistance is limited to about 200M by the 300 µV output offset voltage of the LT1168.

The 0,05% accuracy of the LT1634 is not an issue here because it is subtracted at the Ref pin of the LT1168 and only contributes to gain error.

Figure 2 shows output voltage vs temperature for 10 and 100K (at 25°C) thermistors from two manufacturers. Thermistors are difficult to linearise, so although the output is still not linear with temperature, it can, at least, be read directly by an ADC and compared against a lookup table. The circuit has good noise immunity but does not tolerate capacitance at pin 1 or 8 and so is not ideal for resistive devices placed remotely from the LT1168.

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