Specifications

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 volt-
age 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 com-
mand.
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 obvi-
ous. A system DMM has 0.01% to
0.1% accuracy, although this
doesn’t include other possible
errors that can affect the measure-
ment, such as cabling. In contrast,
the power supply accuracy figures
in the table include all applicable
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 measure-
ment 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
readback performs well between
full scale and 10% of full scale.
Newer power sources, such as the
HP 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 multiple range
readback. The HP 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 mea-
surement options, including rms,
newer dc sources, such as the
HP 66312B and 66332B, provide
rms peak measurements as well
(see Tip 8 ).
Tip
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