Keysight Technologies 34972A Service Guide

Theory of Operation 5
Keysight 34970A/34972A Service Guide 141
Ohms Current Source
Unless otherwise noted, components in this discussion are located on the A4
circuit assembly (34970-66504).
The ohms current source flows from the Input HI terminal to the Input LO terminal
for both the 2-wire and 4-wire ohms functions. Each current value is generated by
forcing a stable, precise voltage across a stable resistance. The value of the
current becomes part of the range gain constant stored during calibration.
The +7 V reference voltage is used to generate a stable reference current with
U201A. R201 and R202 are the resistance references for the current sources as
shown in the table below. The IREF current is used to produce a precise voltage
drop across the 28.57 kW resistor in U102D-4. The IREF generated using R202
produces an approximate 5 V drop across the 28.57 kW resistor. The IREF
generated using R201 produces an approximate 0.5 V drop. This voltage is used
to force a reference voltage across the selected current source range resistor (5
kW, 50 kW, 500 kW, 1MW) by U201B. The resulting precision current flows through
JFET Q202 and protection circuit Q203 to Q211, and CR202 to relay K102 where it
is switched to the Input HI terminal for ohms measurements.
The protection circuits are designed to protect the ohms current source from
inadvertently applied voltages in excess of ±1000 V. Protection from large positive
voltages is provided by the reverse breakdown voltage of CR202. Protection from
large negative voltages is provided by the sum of the collector to base breakdown
voltages of Q203, Q205, Q207, and Q209. Bias for these transistors is provided by
Q211 and R203 to R206 while negative over voltages are applied.
Ohms Range Current
Open Circuit
Voltage
Compliance
Limit
Reference
Isource R
U102D
100 W
1 kW
10 kW
100 kW
1 MW
10 MW
100 MW
[1]
1 ma
1 mA
100 mA
10 mA
5 mA
500 nA
500 nA
[a]
[a] Measured in parallel with the internal 10 MW resistor.
9 V
9 V
9 V
9 V
9 V
14 V
5 V
2.5 V
2.5 V
4 V
4V
8 V
10 V
R202
R202
R202
R202
R202
R201
R201
5 kW
5 kW
50 kW
500 kW
1 MW
1 MW
1 MW

Keysight Technologies 34972A Specifications

General

Brand	Keysight Technologies
Model	34972A
Category	Switch
Language	English