E-3.
Circuit Descriptions
The circuit descriptions which follow are
referenced to Figures E-1, E-2, E-3 and the
schematic diagrams at the back of this manual.
In the following descriptions, references to
integrated circuits are given in the form "IC201-
1", which refers to Integrated Circuit 201, pin 1.
E-4.
Analog to Digital Conversion
The A to D conversion is done with a ICL8068
/ICL71C03 chip set. The ICL8068 takes care of
the analog part and the ICL71C03 takes care of
the digital part of the 4 ½ digit 20,000 count
dual slope conversion.
COUNTERS
DISPLAY
COMPARATOR
BUFFER
MULTIPLEXER
ZERO
CROSSING
DETECTOR
INTEG
UUT
CONTROL
LOGIC
INPUT
SWITCH
DC CONSTANT
CURRENT SOURCE
RANGE
SWITCH
MODEL 620A-4R OHMMETER BLOCK DIAGRAM
NUMBER OF COUNTS TO ZERO CROSSING
PROPORTIONAL TO V
IN
AFTER ZERO CROSSING,
ANALOG SECTION WILL BE
IN AUTOZERO CONFIGURATION
BUSY
OUTPUT
INTEGRATOR
OUTPUT
CLOCK
AZ PHASE l INT PHASE ll DEINT PHASE ll AZ
INTERNAL
LATCH
POLARITY
DETECTED
ZERO CROSSING
OCCURS
ZERO CROSSING
DETECTED
Figure E1 - 620 Conversion Timing Diagram
Summary of Contents for 620A 4R
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