HP 34970A - Page 354

Loading Errors Due to Input Bias Current The semiconductor
devices used in the input circuits of the internal
DMM have slight
leakage currents called bias currents. The effect of the input bias
current is a loading error at the internal
DMM’s input terminals.
The leakage current will approximately double for every 10
°C
temperature rise, thus making the problem much more apparent at
higher temperatures.
Where:
I
b
= DMM bias current
R
s
= DUT source resistance
R
i
= Input resistance (10 MΩ or >10 GΩ)
C
i
= DMM input capacitance
Error (V) = I
b
x R
s
V
S
R
S
HI
LO
I
b
R
i
DMM
C
i
Chapter 8 Tutorial
Measurement Fundamentals
358