Keysight 34401A, Service Manual

Page: 104 / 165

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.
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 M
Ω
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.
DCI Range Shunt Resistor U101–10 Input Amplifier Gain ADC Input
3A
1A
100 mA
10 mA 0.1 Ω
0.1
Ω
5.1 Ω
5.1 Ω
300 mV
100 mV
510 mV
51 mV x10
x100
x10
x100 3V
10V
5.1V
5.1V
Ohms Range Voltage Across R Amplifier Gain ADC Input
100
Ω
1 k Ω to 100 k Ω
1 M Ω
10 M Ω
100 M
Ω
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
5
Chapter 5 Theory of Operation
DC Amplifier
97