illustrated in Figure 5-12. Here the reference voltage is represented as a
battery. A calibrated voltage divider, R1, provides an adjustable output.
The unknown input voltage is connected to input terminals A and B. When the
voltage divider is adjusted so that the two voltages are equal, there will
be no current flow in the meter.
This condition is termed a null.
The
magnitude of the unknown voltage is equal to the product of the reference
voltage and voltage-divider ratio. In a practical differential voltmeter,
the voltage-divider controls are calibrated in terms of the input voltage.
Typical accuracies of this type of dc voltage measurement are of the order
of 0.005%.
Figure 5-12.
Simplified differential voltmeter
2.
The principal elements of the Model 887A Voltmeter are depicted in the
Functional Block Diagram of Figure 5-13.
In this diagram, the Reference
Supply battery of Figure 5-12 has been replaced by the Reference Power
Supply; the voltage divider has been replaced by the Kelvin-Varley Divider;
and the Meter has been replaced by the Null Detector.
Additionally, the
input circuit has been modified to include an AC to DC Converter and a DC
Attenuator.
Figure 5-13.
Functional block diagram, Model 887A/AB
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