Specifications

11
Use a Power Supply to Measure DUT
Supply Current
Accurately measuring DUT supply
currents above 10 A is beyond the
range of the typical DMM in ammeter
mode. You could use an external
shunt and the DMM’s voltage mode,
but using the power supply itself is a
better solution. Many supplies
include an accurate measurement
system, including a shunt. Using the
power source’s shunt to take current
or voltage measurements at the DUT
can be as simple as sending a MEAS
command.
The following table shows the level of
measurement accuracy you can
expect with a good-quality supply:
Output level Typical accuracy
Full 0.1% to 0.5%
10% of full output 0.5% to 1%
1% of full output near 10%
While the advantages of using the
power source to measure high cur-
rents is clear, using it to measure low
currents may not be as obvious. A
system DMM has 0.01% to 0.1% accu-
racy, although this doesn’t include
other possible errors that can affect
the measurement, such as cabling. In
contrast, the power supply accuracy
figures in the table include all applic-
able factors.
A good system DMM can measure
current down to the picoamp level,
but you rarely need to measure DUT
supply currents this low. In most
cases, the toughest measurement will
involve current draw by a battery-
powered device in sleep mode (such
as a cellular phone), where measuring
1-10 mA with reasonable accuracy is
usually all you need.
Most power supplies’ current read-
back performs well between full scale
and 10% of full scale. Newer power
sources, such as the Agilent 66000A
modular power system, offer full
scale accuracy of 0.06% at 16 A and
3.8% accuracy at 160 mA. You can
also choose a power supply with mul-
tiple range readback. The 663XA
series can measure as low as 2.5 µA
and offers accuracy of 5.1% at 50 µA
(with full-scale accuracy of 0.2%).
Also, keep in mind that while ac
sources offer many current measure-
ment options, including rms, newer
dc sources, such as the 66312A and
66332A, provide rms peak measure-
ments as well (see Tip 8).
Tip
10