Figure E1 - 620 Conversion Timing Diagram
AFTER ZERO CROSSING,
ANALOG SECTION WILL BE
IN AUTOZERO CONFIGURATION
NUMBER OF COUNTS TO ZERO CROSSING
PROPORTIONAL TO V
IN
INTERNAL
LATCH
BUSY
OUTPUT
CLOCK
AZ
DEINT PHASE ll
INT PHASE ll
AZ PHASE l
ZERO CROSSING
DETECTED
INTEGRATOR
OUTPUT
ZERO CROSSING
OCCURS
POLARITY
DETECTED
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.
MODEL 620A-4 OHMMETER BLOCK DIAGRAM
COUNTS
PHASE I
PHASE II
PHASE III
4 ½ DIGIT
I0.00 I
I0.000
20.00 I
ZERO
CROSSING
DETECTOR
COMPARATOR
CONTROL
LOGIC
COUNTERS
INTEG
BUFFER
INPUT
SWITCH
MULTIPLEXER
UUT
DISPLAY
DC CONSTANT
CURRENT SOURCE
RANGE
SWITCH
