Agilent 34970A/34972A Data Acquisition / Switch Unit

Chapter 5 Theory of Operation
Internal DMM
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
1A
100 mA
10 mA
Shunt
Resistor
0.1
5.1
5.1
U101-10
Input
100 mV
510 mV
51 mV
Amplifier
Gain
x100
x10
x100
ADC Input
10 V
5.1 V
5.1 V
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 range is measured using the known internal 10 M
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 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
1 k to 100 k
1 M
10 M
100 M
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
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