Section 7-Theory Of Operation; Introduction; Overall Functional Description - Keithley 192 Instruction Manual

SECTION 7
THEORY OF OPERATION
7.1 INTRODUCTION
This section contains circuit descriptions for the Model 192.
Descriptions are also included for the Model 1910 AC
Option. A separate instruction manual is available for the
Model 1920 TRMS option. The information is arranged
to
provide a description of individual functional circuit blocks.
To facilitate understanding, the descriptions are keyed to
accompany simplified block diagrams and schematics.
Detailed schematics of the Model 192 and Model 1910 are
located at the end of this manual.
7.2 OVERALL FUNCTIONAL DESCRIPTION
The Model 192 isa 6X digit, ~2.000.000 counts DMM with
five DC voltage and six resistance ranges Istandard). The
unit has l@ and 1mR sensitivity with 0.00005% resolution.
When the Model 1910 AC Voltage Option is installed, AC
voltages from lo@ to 1OOOV can be measured. The most
unique feature of the Model 192 is its' hybrid Analog-to-
Digital converter which uses both charge balance and single
slope conversion techniques. A microcomputer located
within the Model 192 controls the Analog-to-Digital conver-
sion process. The charge balance/single slope convsrter
feature, along with the internal microcomputer, provides
many enhancements to the DMM's performance. These
enhanced features include:
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Extremely High Accuracy
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High Conversion Speeds
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Quick Settling Times
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Reduction in the number of component parts
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Non-Linear Digital Filtering
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Push button nulling of any on-scale input signal
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Automatic 2/4 Terminal Ohms Measurements
These items will be described in more detail in the latter part
of this section. Refer to Figure 7-1 for a simplified signal
flow block diagram of the Model 192.
The Model 192 is partitioned into two electrically isolated
sections, digital and analog. These sections are isolated by
opt0 isolators and by isolated secondary windings on the
power transformer. Partitioning and isolating the unit allows
the LO or common input of the DMM's analog section to be
placed at any potential (between + 12OOV1. Simultaneous-
ly, the LO or common side of the digital circuitry can be
maintained at voltage potentials within 30V of chaws
ground. This isolation simplifies interfacing of the Model
192 with external digital equipment.
Operation of the Model 192 is centered around the AID
converter. The converter requires a conditioned analog
input for the wide range of DC inputs used, and the conver-
sion of ohms and AC voltages to DC voltages. The A/D
Figure 7-1. Basic Block Diagram
7-l