Simpson 460 4 Series - Page 42

Theory of Operation
sion shunt resistance. The value of the shunt resistance depends
on the current range used and is selected to allow the voltage
developed across it to be proportional and numerically equal to
the current running through it. The A/D Converter measures
this voltage, and the value indicated on the numerical display
is equal to the current being measured.
5.3.15 The full range sensitivity of the digital voltmeter circuit
measurement is 200mV. Therefore, the internal resistance for each
current range equals 200mV divided by the full range current,
For example, if the full range current is 200A, then the internal
resistance would be 1000 ohms.
5.3.16 AC Current Measurements
5.3.17 The basic AC current measurement circuit is essentially
the same as the DC current measurement circuit (para. 5.3.12),
except that the voltage developed across the internal shunt resist-
ance is measured by the AC voltage measurement circuit.
5.3.18 Resistance Measurements
5.3.19 The basic resistance circuit is shown in Figure 5-3(b).
5.3.20 The resistance being measured, Rx, is connected to the
V-Q and COMMON jacks and a constant, current is applied
through it by the Instrument. The resulting voltage i is proportional
to the value of Rx. The value of the current is determined by the
resistance of the range selected.
5.3.21 The current through Rx is controlled by an Operational
Amplifier with inputs that "follow" Ex and an output which is
always Ex + Er (where Er is a reference voltage). This reference
voltage is developed by the current from a constant current source
through a feedback resistor.
5.3.22 The Ex + Er output of the amplifier maintains a constant
5-6