Table of Contents
Mode1 3456A
resistance is then applied through the Input Terminals
•
and the FRONT/REAR switch to the Input Circuitry.
This dc voltage is then applied to the Input Amplifier.
g. For a ratio measurement, two complete measure
ment cycles are taken. The first cycle is either a DC, AC,
or AC + DC measurement. The second cycle is the
measurement of the DC reference.
h. The Input Amplifier is used as a buffer and also
does voltage amplification. This is to provide the same
input level to the A/D Converter for full scale inputs in
all functions and ranges. The amplifiers gain is 1 in the
ACV function and on the 1 0 V range in the DCV func
tion. With the 1 V and 1 00 mV ranges the gain is 10 and
1 00 respectively and in the high Ohms ranges the gain is
2. In the 100 V and the 1000 V ranges, the gain is XIO
amd X I respectively, with the input signal divided by
the 100:1 Divider. The different gain configurations
provide an A/D Converter full scale input of 1 0 V dc.
i. The A/D Converter converts its input from a dc
voltage to digital information by using a new, improved
multi-slope conversion technique (Multi-Slope
information is sent to the Inguard Controller where the
Autozero reading is subtracted from the measured
reading. The compensated reading is then sent to the
Outguard Controller via the Isolation Logic circuitry.
The Multi-Slope technique allows for slow as well as
c
fast circuit operaiton while ensuring high accuracy.
stability. and resolution.
j. The information is then processed by the Main
Controller and is displayed on the front panel and/or
sent to the HP-lB. If a math feature is selected, the
math calculation is done before the recalculated reading
is displayed.
1.113 (351 --<
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TO A30.
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NO CONNECTION
Figure 8·2. Simplfied Front}Re.r Switch Circuit
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or
10 INPUTS
INPUT SWITCHIN'
�NO AC CONVERTER
14"-1"
8·22. Another circuit
Reference Supply. Its purpose is to supply an accurate
reference voltage for the Ohms Current Source and the
A/D Converter.
8·23. Front}Rear Switch.
8-24. Refer to Figure 8·2 Simplified Front/Rear Switch
Circuit for the following discussion. The input signal
applied to the HI and LO VOLTS input terminals (solid
border--front terminals, dashed border--rear terminals)
passes through the Front/Rear switch to the inputs to
the Input Switching and the AC Converter. With the
Front/Rear switch in the front position, a low logic
state via a ground connection is sensed by the Inguard
Logic microprocessor. This feature is used to remotely
determine over the HP-IB the position of the
Front/Rear switch.
8·25. Input Protection.
8-26. Refer to Figure 8-3 High Voltage Protection Cir
11).
cuits for the following discussion. A 2.5 kV surge
voltage protector
This
(combination connected between HI and gnd) provides
high voltage protection for the input circuits. E l will arc
over at 2.5 kV ( ± 20070) peak levels.
static discharge protection (up to 1 5 kV) from very·
short duration transients by clipping off the peaks to a
safe level. Guard is protected from excessive excursions
by a 430 volt varistor (RVl).
Figure 8-3. High Voltage Protection Circuill.
8-27. Additional high voltage protection is provided in
the ohms, the dc, and the ac circuit measurement paths.
These circuits are discussed in the sections covering
ohms, dc. and ac measurements.
8·28. DC Volts Measurement
8·29.
A20
Board Compon�nt NumberiDg.
board. which contains the majority of the circuits that
will be discussed, is made up of six separate circuits with
ID
the Inguard Section is the
(El)
in series with a R-C network
E l
also provides
TO INPUT SWJTCNIHG
�NO AC CONV{RlfR
Service
The A20
8- 3
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