Design World – Power Transmission Reference Guide June 2019

(^46) DESIGN WORLD — EE NETWORK 6 • 2019 eeworldonline.com | designworldonline.com
TEST & MEASUREMENT HANDBOOK
rest of its time. We can estimate the energy
consumed by the target device per sensor
measurement as:
Energy = (3.3 V × 200 mA × 0.050 sec) + (3.3
V × 50 mA × 0.100 sec) + (3.3 V × 1 μA ×
(measurement interval - 0.15) sec)
If the device takes one sensor measurement
hourly, the total energy per sensor
measurement is then E = 0.033 J + 0.165 J +
0.0119 J
So, how accurate is this estimate? Start with
the first radio state energy estimate. The
Fluke 87 states that the burden voltage is
1.8 mV/mA in the 400-mA range. For our
measurement, we have
1.8 mV/mA × 200 mA = 360 mV drop
Due to the voltage drop across the multimeter,
the target only receives 2.94 V rather than the
supplied 3.3 V. This reduced voltage may have
caused unexpected behavior, such as brown
outs, on the target during the measurement.
If the target contains a dc-dc converter, the
measured current will be higher than in the
final product. We would reduce the burden
voltage by setting the multimeter to a larger
range at the expense of resolution.
Fortunately, the active state uses a
current range with the same burden voltage
specification as the radio state. If the active
state was only 6 mA and the multimeter was
on the 6-mA range, the burden voltage would
be 0.6 V! Because the multimeter switches
current-shunt resistors with the current range
setting, reducing current does NOT necessarily
reduce the burden voltage.
This multimeter is also not able to accurately
measure the sleep state current, which is
19.4% of the total energy! In the 0.6 mA
setting, the accuracy is:
Multimeters generally measure current via an
ammeter included in their functions. The most
common ammeter implementation places a
current shunt resistor in series with the load.
Typical ammeter test setup
Power
supply
Device
under
test
Ammeter

Joulescope typical connection setup

PC running

Joulescope app

USB

connection

Target

embedded

system

Joulescope

DC power supply

±(0.2% × 1 μA + 4 × 0.1 μA) = 0.402 μA

This amounts to 40% error on 19.4% of the

total budget, or 7.7% total error!

Bench multimeters usually provide additional

resolution and accuracy. However, the

burden voltage specifications are similar

and often in the range of 0.7 V for full-

scale measurements. You can sacrifice the

additional resolution to provide a reasonable

burden voltage.

A multimeter is a vital tool, but the

operator must be constantly aware of the

burden voltage and resolution. Developers

must be diligent in performing this tedious

task regularly. In practice, developers using

multimeters to measure power rarely perform

testing frequently. Infrequent testing allows

the product to collect “surprises” that are

not discovered until late in the design cycle.

OSCILLOSCOPES

Oscilloscopes sample voltages at regular

intervals, often over a million times per

second, to construct a voltage waveform.

Oscilloscopes then display a graph showing

changes in voltage over time. By measuring

the voltage over an external shunt resistor,

oscilloscopes can effectively display changes in

current over time.

However, current measurements via

scope have two primary challenges. First, the

shunt resistor measurement technique has the

dynamic range issues associated with shunt

resistors. Oscilloscopes usually trade-off speed

for limited dynamic range and typically have

just 10 or 12 bits of dynamic range.

Second, oscilloscopes are usually

earth-ground referenced. The oscilloscope

measures the voltage difference between earth

ground and the signal. However, we want the

differential measurement across the shunt

resistor. Introducing shunt resistance into the

ground path often causes signal integrity issues.

We often want “high-side” shunt resistors on

the positive power supply. However, if the test

circuit is also earth-ground referenced, we

cannot use the oscilloscope’s standard probe to

measure the voltage difference across the shunt

resistors. We can either use two oscilloscope

probes and use a mathematical subtract feature,

which introduces additional measurement error,

or we can use differential oscilloscope probes,

which are often quite expensive. Either way, we

are still left with the dynamic range issue.

Jetperch — Test and Measurement HB 06-19 copy.indd 46 6/10/19 9:35 AM