Theory of Operation 4
Keysight 34420A Service Guide 117
Ohms Current Source
This discussion refers to the Chapter 8, "Ohms Current Source Schematic" and the
simplified schematic on the next page. The Ohms current is generated by applying
a stable voltage across a stable resistance. The exact value of the current
becomes part of the range gain constant stored during calibration.
The +7 V reference voltage is applied to the 40 kΩ resistor in RP1B using U451A
and Q451. This current flows through Q451 and either the 28.57 kΩ resistor in
U102D or the 11.43 kΩ resistor in RP1C. Switch U101D determines which resistor
is used.
The current through Q451 produces a 5 V drop across the 28.57 kΩ resistor or a 2
V drop across the 11.43 kΩ resistor. Depending upon the range set, the 5 V signal
across the 28.57 kΩ resistor is applied to either the 5 kΩ, 50 kΩ, 0.5 MΩ, or 1 MΩ
resistor in U102D using U451B, Q455, and Q456. The resulting current is the
current source output for the 1 kΩ to 1 MΩ ranges. The 1Ω, 10 Ω, and 100 Ω
ranges use Q452, Q453, and Q454 to select the 10 mA current source in response
to programming from the CPU. When the REG46 bit is set to O V, Q454 is turned
off which turns on switches Q452 and Q453. The 2 V signal across the 11.43 kΩ
resistor is applied to the 200 Ω range resistor in RP1C. This results in a 10 mA
current flowing through Q452, Q455, and Q456, which then flows through over
voltage protection transistors Q459 through Q462 to relay K101 where it is
switched to the Channel 1 HI input terminal during ohms measurements.
Table 4-2 below shows the range, current source, and references used for 2-wire
or 4-wire resistance measurements.
Table 4-2 2-wire or 4-wire resistance measurements
Ohms range Current
Reference
resistor
Range resistor
1 Ω 10 ma 11.43 kΩ 200 Ω
10 Ω 10ma 11.43 kΩ 200 Ω
100 Ω 10 ma 11.43 kΩ 200 Ω
1 kΩ 1 ma 28.57 kΩ 5 kΩ
10 kΩ .1 ma 28.57 kΩ 50 kΩ
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