Agilent 34970A/34972A Data Acquisition / Switch Unit

Chapter 5 Theory of Operation
Internal DMM
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.
4
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 k resistor in U102D-4. The
IREF generated using R202 produces an approximate 5 V drop across
the 28.57 k 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 k, 50 k, 500 k,
1M) 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.
5
Ohms Range Current Open Circuit
Voltage
100 1 ma
9 V
100 M [1] 500 nA [1] 5 V
1 k 1 mA
9 V
10 k 100 A 9 V
100 k 10 A
9 V
1 M 5 A
9 V
10 M 500 nA 14 V
Compliance
Limit
2.5 V
2.5 V
4 V
4V
8 V
10 V
Reference
R202
R202
R202
R202
R202
R201
R201
Isource R
U102D
5 k
5 k
50 k
500 k
1 M
1 M
1 M
[1] Measured in parallel with the internal 10 M resistor.
143