5-1.
INTRODUCTION
This section presents an overall functional description of
the 8840A, followed by a detailed circuit description. The
descriptions are supported by simplified schematics in text
and by the complete schematics in Section
10.
5-2.
OVERALL FUNCTIONAL DESCRIPTION
A
functional block
diagram
of the 8840A is shown in
Figure
5-1.
The basic signal
path
flows from left to right
across the center of the page. The input is sensed at the
input terminals,
scaled,
directed through the TracMHold
circuit, converted into digital representation by the
Analog-to-Digital
(AD)
Converter, processed by the Dig-
ital Controller, and sent to the display.
The
DC
Scaling circuit, which constitutes the "front end"
of the instrument, has two major hctions. First, it senses
the input and produces
an
equivalent
dc
voltage for all
functions except VAC and mA AC. (AC inputs are con-
vened to a dc voltage
by
the True
RMS
AC Option.)
Resistances are
sensed
as
a dc voltage using
a
known test
current from the Ohms Current Source. A
dc
current input
is converted to a
dc
voltage by a precision current shunt.
Second, the
DC
Scaling circuit scales the equivalent dc
voltages (for in-range inputs) to within the input range of
the
AID
Converter
(dV).
In addition, the DC Scaling
circuit provides input protection and provides analog filter-
ing for certain ranges and reading rates. (AC inputs are
scaled by the True
RMS
AC Option.)
The Track/Hold
(T/H)
circuit samples the scaled dc volt-
age and presents the
AD
Converter with a voltage that is
constant for the input portion of each
AID
conversion
cycle. The
T/H
circuit also provides additional scaling for
certain ranges.
The Digital Controller controls the operation of virtually
every part of the
8840A.
It
reads the front panel keyboard,
configures the instrument for each function and range,
triggers the
AD
Converter, calculates the result of each
AD
conversion cycle, averages
AID
samples, controls the
Theory
of
Operatjon
display, and communicates with the IEEE-488 Iaterface
Option via the Guard Crossing circuit. The heart of the
Digital Controller is the In-Guard Microcomputer
(G).
The Guard Crossing circuit permits serial asynchronous
communichtion between the Digital Controller +d the
IEEE-488 Interface Option, while isolating the two
Grcuits
electrically. Whereas the in-guard
power
supply floqts with
the voltage at the INPUT
LO
terminal, the IEEE-488
Interface Option operates with reference to earth ground.
I
The
"guard"
is the isolation between the in-guard and
out-guard circuits.
The Power Supply provides supply voltages to all
qarts
of
the instrument. The Precision Voltage Reference provides
precise reference voltages for the
AD
Converter zind the
Ohms Cument Source.
5-3.
DETAILED CIRCUIT DESCRIPTION
The following paragraphs give a detailed circuit
descrip-
tion of each of the functional blocks in Figure 5-1. For
clarity, measurement ranges are referred to
as
rl, r2, r3,
etc., where rl
is
the lowest possible range, 12 tWe next
higher range, and
so
on. Pins are designated by the iespec-
tive integrated circuit (e.g., U101-7 for UlOl pid 7).
54.
DC
SCAUNG
The DC Scaling circuit scales all in-range
dc
inputs so that
the output of the TracMHold
(T/H)
amplifier (U307) is
within =2V. In addition, the DC Scaling circuit Novides
input proteclion and analog filtering. Additional ding
is
provided by the the
T/H
Amplifier.
The following paragraphs describe the configuration of the
DC Scaling circuit in the DCV and
mA
DC functions
and
also
describe the analog filter. The ohms functi4ns are
described under a later heading because the
T/H
Axpplifier
provides additional input switching for these functions.
5-5.
VDC
Scaling
Scaling is performed in the VDC function by two preci-
sion resistors networks
(2301
and W02).
These
compo-
5-
1
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