Elenco Electronics M-1007K, Assembly And Instruction Manual

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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

IN

/ V

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.