Agilent Technologies 34970A, Service Manual, Page: 135 / 273

Ohms Current Source
Unless otherwise noted, components in this discussion are located
on the A4 circuit assembly (34970-66504). The schematics are
included in Chapter 8 starting on page 234.
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 k
Ω
resistor in
U102D-4. The IREF generated using R202 produces an approximate
5 V drop across the 28.57 k
Ω
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
Ω
, 50 k
Ω
, 500 k
Ω
, 1 M
Ω
) 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
Ω
1 k
Ω
10 k
Ω
100 k
Ω
1 M
Ω
10 M
Ω
100 M
Ω
[1]
1 mA
1 mA
100
µ
A
10
µ
A
5
µ
A
500 nA
500 nA
[1]
9 V
9 V
9 V
9 V
9 V
14 V
5 V
2.5 V
2.5 V
4 V
4 V
8 V
10 V
R202
R202
R202
R202
R202
R201
R201 5 k
Ω
5 k
Ω
50 k
Ω
500 k
Ω
1 M
Ω
1 M
Ω
1 M
Ω
[1] Measured in parallel with the internal 10 M
Ω
resistor.
5
Chapter 5 Theory of Operation
Internal DMM
133