User`s guide

290
Chapter 7 Tutorial
Measurement Fundamentals
Sources of Error in DC Current Measurements
When you connect the internal DMM in series with a test circuit to
measure current, a measurement error is introduced. The error is caused
by the DMM’s series burden voltage. A voltage is developed across the
wiring resistance and current shunt resistance of the internal DMM as
shown below.
V
s
= Source voltage
R
s
= Source resistance
V
b
= Burden voltage
R = Current shunt resistance
Error (%) =
Sources of Error in AC Current Measurements
Burden voltage errors, which apply to DC current, also apply to AC
current measurements. However, the burden voltage for AC current is
larger due to the internal DMM’s series inductance and the
measurement connections. The burden voltage increases as the input
frequency increases. Some circuits may oscillate when performing
current measurements due to the internal DMM’s series inductance and
the measurement connections.
V
s
R
s
V
b
R
DMM
100%– V
b

V
s
--------------------------------
34970A Refresh UG.book Page 290 Wednesday, February 17, 2010 12:34 PM