When a physical quantity measured has to be measured at a distance, it is normally necessary to provide the sensor proper with an amplifier. This amplifier needs a power supply, which means that a third wire has to be taken to the sensor. In the present circuit, the measurand and the power supply use the same two lines. To that end, the measured, buffered by IC1a, is converted into an additional supply current drawn from current source IClb. In other words, the magnitude of the input voltage may be gauged from the level of the supply current – (which, of course, also consists of the current drawn by the opamp and, possibly. the sensor).
When the specified sensor and opamp are used, the current is about 0.7 mA (0.65 mA with a supply voltage of 5 V: 0.7 mA at 10 V; and 0.77 mA at 15 V). With a full-scale deflection (FSD) of 10 mA, this is a deviation of about 7%, which is, however, easily compensated.
The circuit design is such that when the temperature at the sensor varies from 0 °C to 100 °C, the supply current changes from 0.7 mA to 10 7 mA. Thus, a moving coil ammeter in the supply line enables the temperature to be read quite simply; the supply current to the opamp (0.7 mA) is nullified with the adjustment screw on the meter.
Note that the voltage drop across long lines must be compensated by a higher supply voltage to prevent IC1 and IC2 from being underpowered (they must get at least 5 V).
