Ohms Measurements; Reference Voltage Section; Calibration Components; Current Regulator - HP 3456A Operating And Service Manual

Model 3456A
Pin Configuration Schematic
(Position
B
CD
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8-1 1 1 . JMPR 501 and 502 shown in the non-inverting
terminal of op amp U505 provide for fine adjustment of
the + 1 2 V. JMPR 503-507
troubleshooting aids if a power supply is loaded down.
8·112. OHMS MEASUREMENTS.
B·113. General.
8-114. The Ohm's Current Source in the 3456A supplies
a dc reference current to the unknown resistance during
resistance measurements. The resulting dc voltage drop
is proportional to the value of the current and unknown
resistance. This dc voltage is measured in a manner
similar to that used for the DCV function: by using the
Input Switching, the Input Amplifier, and the AID
Converter. Since the current source is a current sink, the
dc voltage drop is negative. The Outguard Logic
changes the reading polarity to obtain a positive front
panel ohms reading.
8-115. The FET switches in the Input Switching provide
for either the 2-wire or the 4-wire measurement method.
The front panel display shows a negative resistance if
the 4-wire connections are reversed or the circuit under
test has enough
offset voltage to give incorrect
resistance readings.
0)
BC
4C
2
,
T
Figure 8·25.
are provided for
u
r
th Ta
T
(BCD and BCD Complement)
- 1 2 V.
,
Switch
Positiun
8
0
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•
2
•
3
•
4
·
5
•
•
6
•
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7
B
•
•
A
9
•
•
•
•
B
•
•
C
0
• •
• ·
•
E
F
• • •
•
Connection
=
Data.
S601
Circuit Description.
B-116.
Refer
8-117. General.
Ohm's Current Source Schematic. and to schematic
Ohm's Current Source. for the following circuit descrip·
tion. The current source may be divided into three ma­
jor section: Reference Voltage, Current Regulator. and
High Voltage Protection.

8-118. Reference Voltage Section.

regulator is an inverting amplifier that has a near-zero
volt input. The voltage divider (fine-line resistors) bet­
ween the - 12 volt reference and ground provides a
stable - 5.5 voltage reference for U204's positive input
terminal. The feedback path for U204 maintains an out­
put voltage near - 13 volts.
8-119. Calibration components
amplifier's input slightly above ground. This provides a
range dependent fine adjustment for the - 1 3 V
reference voltage.

8-120. Current Regulator.

U205 senses via FET switch Q201 or Q202 the voltage
( - 5.5 V or - 9.25 V reference voltage respectively) pre­
sent on U204's feedback path. U205 controls a current
regulating FET to obtain the same voltage on its
negative terminal as that appearing its positive terminal.
Service
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- -
8
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to Figure 8-28, Simplified
The voltage
bias the inverting
The pOSlUve terminal of
8-17
2,