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For resistance measurements, an internal voltage source drives the test resistor in series with a known resistor.
The ratio of the test resistor voltage to the known resistor voltage is used to determine the value of the test
resistor.
The input of the 7106 IC is fed to an A/D converter. Here the DC voltage is changed to a digital format. The
resulting signals are processed in the decoders to light the appropriate LCD segments.
Timing for the overall operation of the A/D converter is derived from an external oscillator whose frequency is
selected to be 25kHz. In the IC, this frequency is divided by four before it clocks the decade counters. It is then
further divided to form the three convert-cycles phases. The final readout is clocked at about two readings per
second.
The digitized measurements are presented to the display as four decoded digits (seven segments) plus polarity.
The decimal point position on the display is determined by the selector switch setting.
A/D CONVERTER
A simplified circuit diagram of the analog portion of the A/D converter is shown in Figure 7-3. Each of the
switches shown represent analog gates which are operated by the digital section of the A/D converter. The
basic timing for switch operation is keyed by the external oscillator. The conversion process is continuously
repeated. A complete cycle is shown in Figure 7-3.
Any given measurement cycle performed by the A/D converter can be divided into three consecutive time
periods, autozero (AZ), integrate (INTEG) and read. A counter determines the length of the time periods. The
integrate period is fixed at 1000 clock pulses. The read period is a variable time that is proportional to the
unknown input voltage. It can vary from zero counts for zero input voltage to 2000 counts for a full scale input
voltage. The autozero period varies from 1000 to 3000 counts. For an input voltage less than full scale autozero
gets the unused portion of the read period. The value of the voltage is determined by counting the number of
clock pulses that occur during the read period.
During autozero a ground reference is applied as an input to the A/D converter. Under ideal conditions, the
output of the comparator would also go to zero. However, input-offset-voltage errors accumulate in the amplifier
loop and appear at the comparator output as an error voltage. This error is impressed across the AZ capacitor
where it is stored for the remainder of the measurement cycle. The stored level is used to provide offset voltage
correction during the integrate and read periods.
The integrate period begins at the end of the autozero period. As the period begins, the AZ switch opens and
the INTEG switch closes. This applies the unknown input voltage to the input of the A/D converter. The voltage
is buffered and passed on to the integrator to determine the charge rate (slope) on the INTEG capacitor At the
end of the fixed integrate period, the capacitor is charged to a level proportional to the unknown input voltage.
During the read period, this voltage is translated to a digital indication by discharging the capacitor at a fixed
rate and counting the number of clock pulses that occur before it returns to the original autozero level.
As the read period begins, the INTEG switch opens and the read switch closes. This applies a known reference
voltage to the input to the A/D converter. The polarity of this voltage is automatically selected to be opposite
that of the unknown input voltage, thus causing the INTEG capacitor to discharge at a fixed rate (slope). This
rate is determined by the known reference voltage. When the charge is equal to the initial starting point
(autozero level), the read period is ended. Since the discharge slope is fixed during the read period, the time
required for discharge is proportional to the unknown input voltage. Specifically, the digital reading displayed is
1000 (V
/ V
).
IN
REF
The autozero period and thus a new measurement cycle begins at the end of the read period. At the same time
the counter is released for operation by transferring its contents (the previous measurement value) to a series of
latches. This stored data is then decoded and buffered before being used to drive the LCD display.
-12-
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