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26 HP E1312A/E1412A Multimeter Application Information Chapter 2
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 10G
Ω
for the
100mVdc, 1Vdc, and 10Vdc ranges. The input resistance is maintained at
10M
Ω
for the 100Vdc and 300Vdc 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
10GΩ). The multimeter's measuring circuitry exhibits approximately 30pA
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 100kΩ, or when the multimeter's operating
temperature is significantly greater than 30°C.
Table 2-1. Thermoelectric Voltages
Copper-to-… Approx. µV/°C
Copper <0.3
Gold 0.5
Silver 0.5
Brass 3
Beryllium Copper 5
Aluminum 5
Kovar or Alloy 42 40
Silicon 500
Copper-Oxide 1000
Cadmium-Tin Solder 0.2
Tin-Lead Solder 5
The HP E1312A
and HP E1412A
input terminals are
copper alloy.