Ohms Current Source
The ohms current source (schematic shown on page 9-10) 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 kΩ,
500 k
Ω, 1 MΩ) 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 overvoltages are applied.
Ohms Range Current
Open Circuit
Voltage
Compliance
Limit
Reference
Isource R
U102D
100 Ω
1 kΩ
10 kΩ
100 kΩ
1 MΩ
10 MΩ
100 MΩ [1]
1 mA
1 mA
100
µA
10
µA
5
µA
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 kΩ
50 kΩ
500 kΩ
1 MΩ
1 MΩ
1 MΩ
[1] Measured in parallel with the internal 10 M
Ω resistor.
Chapter 5 Theory of Operation
Ohms Current Source
98
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