Image Number 4 for United States Patent #5225784.
A DC current-comparator-based circuit generates an adjustable output proportional to an input signal, i.e. an input voltage or current. One use of the circuit is in the formation of a DC resistance bridge that can be controlled automatically by a microprocessor. The ends of a pair of test resistors (the resistances of which are to be compared) are connected to respective ratio windings of the current comparator. The same potential is applied across these resistors by a master power supply. A microprocessor is alternately supplied with two voltage signals, a first being proportional to the current in a variable one of the ratio windings of the comparator, and the second being proportional to any inequality between the current in the other ratio winding and the test resistor to which it is connected. The microprocessor controls a slave power supply that receives both the first signal and a third signal that is indicative of any unbalance in the bridge. Balance is achieved by adjusting the variable ratio winding. Another use of the basic circuit is in the formation of an improved digital-to-analog converter, in which case the digital input controls the number of turns on the variable winding and the third signal provides the equivalent analog output.