HP 34401A, User Manual

Page: 198 / 236

Loading Errors (dc volts)
Measurement loading errors occur when the resistance of the device-
under-test (
DUT
) is an appreciable percentage of the multimeter’s own
input resistance. The diagram below shows this error source.
To reduce the effects of loading errors, and to minimize noise pickup,
you can set the multimeter’s input resistance to greater than 10 G
Ω
for
the 100 mVdc, 1 Vdc, and 10 Vdc ranges. The input resistance is
maintained at 10 M
Ω
for the 100 Vdc and 1000 Vdc ranges.
Leakage Current Errors
The multimeter’s input capacitance will “charge up” due to input bias
currents when the terminals are open-circuited (if the input resistance
is 10 G
Ω
). The multimeter’s measuring circuitry exhibits approximately
30 pA of input bias current for ambient temperatures from 0
°
C to 30
°
C.
Bias current will double (
×
2) for every 8
°
C change in ambient temperature
above 30
°
C. This current generates small voltage offsets dependent upon
the source resistance of the device-under-test. This effect becomes
evident for a source resistance of greater than 100 k
Ω
, or when the
multimeter’s operating temperature is significantly greater than 30
°
C.
V
s
R
s
R
i
HI
LO
Ideal
Meter
V
s
= ideal DUT voltage
R
s
= DUT source resistance
R
i
= multimeter input resistance
( 10 M
Ω
or >10 G
Ω
)
Error
(
%
) =
100
x
R
s
R
s
+
R
i
Ideal
Meter
V
s
R
s
R
i
HI
LO
C
i
i
b
i
b
= multimeter bias current
R
s
= DUT source resistance
C
i
= multimeter input capacitance
For DCV ranges:
0.1V, 1V, 10V: C
i
< 700 pF
100V, 1000V: C
i
< 50 pF
For all ACV ranges: < 50 pF
Error (v)
≅
i
b
x R
s
7
Chapter 7 Measurement Tutorial
Loading Errors (dc volts)
199