User`s guide

280
Chapter 7 Tutorial
Measurement Fundamentals
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 M or > 10 G)
C
i
= DMM input capacitance
Error (V) = I
b
X R
s
V
s
DMM
R
s
HI
LO
R
i
C
i
I
b
34970A Refresh UG.book Page 280 Wednesday, February 17, 2010 12:34 PM