Chapter 4 Assembly Level Troubleshooting 71
Purpose Interface Bus (GPIB). The outguard circuitry is explained as
follows.
Main Controller The Main Controller consists of a microprocessor, program ROMs, storage
RAMs, calibration RAMs, and other associated circuitry. It controls the
measurement operation of the instrument, communicates with the front panel
(keyboard and display) and GPIB circuitry, performs the math operations,
and calculates for the correct measurements. The correct measurements are
calculated using the calibration constants in the calibration RAM which were
stored during instrument adjustment.
GPIB Circuitry The GPIB circuitry provides communication between the 3458A and other
instruments and controllers connected to the GPIB. Instrument control
commands are transferred to the main controller and measurement data is
transferred from the main controller through the GPIB circuitry.
Front Panel Circuitry The front panel circuitry consists of keyboard and display circuitry. The
main controller sends display data to the display circuitry to be displayed on
the front panel. The keyboard circuitry interrupts the main controller when
a key is pressed. The keyboard circuitry also sends data to the main controller
that determines which key is pressed to perform the appropriate action.
Isolation Logic Since both the outguard and inguard circuitry are referenced to two different
circuit grounds, they must be isolated from each other. To maintain this
isolation, direct connections for communications cannot be used. The
isolation logic is used to provide communication between the two circuits,
while keeping them isolated from each other. The isolation logic uses fiber
optic cables to transfer data between the inguard and outguard.
Inguard Section The inguard circuitry consists of all the analog, measurement circuitry (DC
circuitry, AC converter, etc.), analog-to-digital (A/D) converter, inguard
controller, and power supply. The inguard circuitry performs all instrument
measurements and converts the analog measurement data into digital
measurement data. This digital measurement data is transferred to the
outguard circuitry through the isolation logic. The inguard circuitry is
explained as follows.
Input Switching and
Signal Conditioning
This circuitry connects the signal path from the front/rear input terminals to
the appropriate DC, AC, or Ohms circuitry. The circuitry also provides the
means for an Autozero measurement. In an Autozero measurement, the input
of the DC and AC amplifiers is connected to ground for an offset
measurement. This measurement is stored in memory and subtracted from
the subsequent measurement taken at the front/rear terminals.
DC Ranging, DC
Amplifier, and Autocal
The A/D converter requires the same full-scale input voltage for full-scale
inputs on all ranges. For example, a 10 V full scale input is required for all
full scale inputs on the 0.1 V, 1 V, 100 V, and 1000 V ranges. This
requirement is met by the DC circuitry (DC amplifier in conjunction with
the DC ranging circuitry). For the lower 1 V and 0.1 V ranges, the circuitry
amplifies the input voltage by a gain of X10 and X100, respectively. For the
10 V range, the gain is 1. For the higher 100 V and 1000 V ranges, the
circuitry attenuates the inputs by 100. This results in full-scale voltages of
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