eeworldonline.com | designworldonline.com 6 • 2019 DESIGN WORLD — EE NETWORK 45CURRENT MEASUREMENTS
The magnetic field caused by current flow can also be measured
using Hall-effect sensors. Changing magnetic fields caused by ac
currents can use a sense coil inductance that will measure the rate of
change of coil current, which can then be processed to yield a figure
for ac current flow.
A point to note is that with all these techniques, it becomes
more difficult to measure the magnetic field with sufficient precision
as the current becomes smaller (under 1 mA).
That brings us to shunt resistors. A resistor placed in the path
of current produces a voltage according to Ohm’s Law: V = I × R,
or I = V /R when solved for current. If the resistance is known and
we measure voltage across the resistor, we can compute current.
Resistors used for current measurements are called shunt resistors.
Most modern ammeters and DVMs measuring current use shunt
resistors. The best part about this approach is that we can select a
shunt resistor value that gives us a suitable voltage range!
A shunt resistor is also called a “current-sense resistor,” or
simply “sense resistor.” By design, shunt resistors cause a voltage
drop, also called burden voltage or insertion loss. If this voltage is
too large, it affects the load. The additional resistance also changes
the source impedance as seen by the load, which can cause some
load circuits to behave differently. Ideally, the shunt resistance would
be so small that it would not affect the target circuit. Practically, the
shunt resistance has to create a measurable voltage.
It’s difficult to measure a large current range with a single shunt
resistor. The voltmeter has a fixed range. To expand the range, most
ammeters use multiple shunt resistors, each with different resistances.
However, if the current changes over time, a shunt resistor that is too
large can cause an excessive voltage drop that affects the behavior of
the target circuit. If the shunt resistor is too small, it cannot accurately
measure the current.
Multimeters are well-suited for measuring currents that are
constant, either as direct current or “constant” RMS alternating current. Multimeters cannot easily measure currents that vary rapidly
or that change dramatically over time.
Most ammeters, including those in multimeters, have significant
limitations including:Burden voltage: The voltage drop (also called insertion loss) across
the ammeter which results in a lower voltage being delivered to the
device under test.Leakage current: The amount of current diverted through the
ammeter and not delivered to the device under test.Bandwidth: The response of the measurement in the presence of a
time-varying signal. For target devices that use a positive dc supply,
the bandwidth relates to the change in load presented by the target
device.Dynamic range: The variation between the minimum current and the
maximum current used by the device under test.Consider the specifications for a well-known, quality hand-held
multimeter, the Fluke 87 (See Table 1). The specification is silent on
leakage current. The dc bandwidth is on the order of 1 Hz. The ac
bandwidth has much worse performance (±1%) and the bandwidth is
45 Hz to 2 kHz.
Now suppose we connect the multimeter to estimate the energy
consumed by a target device. Further suppose the target device
periodically takes sensor measurements and reports them over RF.
The target device must take the measurement from the sensor,
send the measurement over RF and then go back to sleep, a typical
sequence for IoT devices. In our simple example, the target device
has three states: radio, active and sleep.
To estimate the total energy consumption, recall energy is the
integral of power over time (P = I × V, E = ∫ P dt). For constant power,
the integral can be simplified to just the term for power multiplied
by the time duration, E = P × t. The classic way to estimate energy
is to first measure the duration of each state, often either via an
oscilloscope inspecting the voltage across a fixed shunt resistor or with
a logic analyzer inspecting bits set by the microcontroller. You can then
force the system into each state and directly measure the current using
the multimeter.Suppose the device uses a 3.3-V supply and we found the device
drew 200 mA during its 50 msec radio state, 50 mA during its 100
msec active state, and 1 μA during its sleep state where it spends the The recently released Joulescope is
designed to automatically handle wide
current ranges and rapid changes in energy
consumption, while allowing the target
device to run normally. This instrument
displays data via a connection to a PC
and accurately measures electrical current
over nine orders of magnitude from ampsdown to nanoamps. This wide range allows
accurate and precise current measurement
for modern devices where sleep modes
are often just nanoamps (nA) or microamps
(μA). The Joulescope also has a total
voltage drop of 25 mV at 1 A, allowing the
target device to run correctly. Joulescope’s
extremely fast current range switchingmaintains a low voltage drop even under
rapidly varying current demands. Via a
connection to a PC, Joulescope reports
cumulative energy consumption along with
real-time current, voltage, and power. The
multimeter view shows the most recent
value while an oscilloscope view allows you
to explore changes over time. Range Resolution Accuracy Burden voltage10 A 10 mA ±(0.2% + 2) 30 mV/A6 A 1 mA ±(0.2% + 4) 30 mV/A400 mA 0.1 mA ±(0.2% + 2) 1.8 mV/mA60 mA 0.01 mA ±(0.2% + 4) 1.8 mV/mA6 mA 1 μA ±(0.2% + 2) 100 μV/μA0.6 mA 0.1 μA ±(0.2% + 4) 100 μV/μARange Resolution Accuracy Burden voltageTable 1Selected Fluke 87 specsJetperch — Test and Measurement HB 06-19 copy.indd 45 6/7/19 2:05 PM