Specifications

10
Tip
9
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. Current (with the
internal shunt) or voltage measure-
ments at the DUT can be as simple
as sending a MEAS command to the
power supply.
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 currents
is clear, using it to measure low
currents may not be as obvious.
A system DMM has 0.01 percent to
0.1 percent accuracy, 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 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 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 readback
performs well between full scale and
10% of full scale. You can also choose
a power supply with multiple range
readback. For example, power sources,
such as the Agilent N676xA precision
modules, offer full scale accuracy of
0.04% + 15 µA at low range (100 mA)
or 0.04% + 160 µA at high range (3 A).