In the DC current function, a current is applied between the Input I
and LO terminals. Ranging is accomplished by relay K102 and
amplifier gain switching in U101. Since a known resistor (the shunt
resister) is connected between these terminals, a voltage proportional
to the unknown current is generated. The voltage sensed at R121 is
measured by the multimeter’s dc circuitry. The table below illustrates
the dc current measuring function configurations.
DCI Range
Shunt
Resistor
U101–10
Input
Amplifier
Gain
ADC Input
1A
100 mA
10 mA
0.1
Ω
5.1Ω
5.1Ω
100 mV
510 mV
51 mV
x100
x10
x100
10V
5.1V
5.1V
Resistance measurements are made by applying a known current
through an unknown resistance. The resulting voltage drop across the
unknown resistance is then measured by the multimeter’s dc circuitry.
The 100 M
Ω range is measured using the known internal 10 MΩ
resistance (U102A) in parallel with the unknown input resistance
while applying the 500 nA current source. The result is computed
from the measured data. The internal 10 M
Ω resistance is determined
whenever a zero calibration is performed.
In the 2-wire ohms function, the voltage drop is measured across
the
Input HI and Input LO terminals. In the 4-wire ohms function,
the voltage is measured across the
HI Sense and LO Sense terminals.
Lead resistances in series with the current source (
Input HI–LO) are
not part of the final measurement. However, they do reduce the
available current source compliance voltage for the resistor under
test. The ohms current source will become non-linear when the
compliance voltage limit is exceeded. The full scale voltage developed
across the unknown resistor and the dc amplifier gain for each
resistance range are tabulated below.
Ohms Range Voltage Across R Amplifier Gain ADC Input
100 Ω
1 kΩ to 100 kΩ
1 M
Ω
10 MΩ
100 MΩ
100 mV
1 V
5 V
5 V
4.5 V
x100
x10
x1
x1
x1
10 V
10 V
5 V
5 V
4.5 V
Chapter 5 Theory of Operation
Internal DMM
132
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