measuring instrument with a sensor whose electrical resistance is changed in dependence on the parameter

摘要

In a measurement system having a sensor the electrical resistance R. sub.s of which varies as a function of a parameter being measured, and a bridge circuit having the sensor and a long transmission wire in a first arm connected to a first control node, a second long transmission wire in a second arm connected to a second control node, and third and fourth arms respectively connected to the first and second nodes, the wires having equal resistances R.sub.c, the wires being connected to the sensor and being arranged so that any resistance- changing conditions affect both wires equally during operation of the system, that improvement wherein the third bridge arm is connected between the first control node and a first current injection node, the fourth bridge arm is connected between the second control node and a second current injection node, a reference voltage source is provided, first active control circuitry is connected to receive inputs from the first and second control nodes and from the voltage source, and to provide to the first injection node a current controlled so as to maintain the control nodes at a predetermined voltage difference related to the voltage at the source, second active control circuitry is connected to the bridge to provide to the second injection node a current controlled so as to maintain the injection nodes at a voltage difference related to the predetermined difference, and output circuitry is connected to the bridge to provide an output related to one of the controlled currents.