Simpson 460 4 Series - Page 43

Theory of Operation
current through Rx, regardless of the value of Rx. The magni-
tude of the current is determined by the resistance (precision
resistors selected by the ohms range switch) in series with Rx.
5.3.23 The A/D Converter measures the voltage developed across
Rx, and the value indicated on the numerical display is equal
to the resistance of Rx.
5.3.24. When the Instrument is set for LO OHMS measurements,
the full scale voltage across Rx is set to 200mV. The maximum
open circuit voltage for this function is 1 volt. This condition
is accomplished by the “ON” state of the Active Voltage Clamp.
5.3.25 When the Instrument is set for HI OHMS measurements,
the full scale voltage across Rx is set to 2 volts. The maximum
open circuit voltage for this function is 7 volts.
5.4 A/D CONVERTER
5.4.1 The basic block diagram of the A/D Converter is shown
in Figure 5-4. The circuit utilizes the “Up-Down” integration tech-
nique for the analog-to-digital conversion. This technique is based
on the conversion of a DC voltage into a proportional time period.
The converter initially accumulates a charge proportional to the
input signal on an integrating capacitor for a fixed period of time.
In the second integration period, the capacitor discharges back
to the “zero” voltage starting point, when the input signal is re-
moved and a reference voltage opposite in polarity is placed at
the input of the converter. The ratio of the polarity is placed at
the input of the converter. The ratio of the time required to remove
the integrator charge to the initial fixed time period is propor-
tional to the amplitude of the input DC voltage.
5.4.2 The "Up-Down" A/D Converter requires two periods of
integration for each measurement: The input signal (Ein) is inte-
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