Measuring Low Current Consumption with a DMM

Measuring Low Current Consumption with a DMM
Keysight Technologies
Measuring Low Current Consumption
with a Digital Multimeter
Application Brief
Test Challenges:
–– Characterizing the power consumption of a battery
powered device
–– Testing the current draw of a low leakage diode
02 | Keysight | Measuring Low Current Consumption with a Digital Multimeter – Application Brief
Overview
DMMs measure current. It’s one of the basic functions of a digital multimeter (DMM). These instruments are the go-to tool for most
engineers and technicians when they need to make current or voltage measurements. Keysight has added additional current ranges to
the Truevolt DMM series that allow you to measure a wider range of currents than other 6.5 digit DMMs. Learn some of the details on
how expanded current ranges can help your measurements.
Characterizing the power consumption of a
battery powered device
You are tasked with characterizing the power consumption of a
battery powered device. Designers have optimized the device’s
current draw, but need complete measurements of the current
from sleep mode to full operating load. To fully characterize the
device’s different current cycles, you decide to use a 34465A DMM.
Its’ 1-µA DC current range provides the pico-amp resolution
needed to accurately characterize the sleep mode. It also has a
high range, 10 A, which allows you to measure a wide range of
currents for your battery powered device when operating in full
load conditions.
Testing the current draw of a low leakage diode
You are testing the current draw of a low leakage diode. To do so,
you need pico-amp resolution, but with the ability to measure
forward currents with dozens of mA’s. While you can perform
these measurements with a more expensive electrometer, you
might also be able to make them using a stable DC power supply
and a new Truevolt DMM. The 34465A/34470A Truevolt DMMs
feature a low DC current range of 1 µA, which allows for pico-amp
resolution with less than 100 pA accuracy (1 year specs, depending
on current reading) to accurately characterize leakage current.
With a no-open circuit range change up to 3 A, you can characterize
your device’s backward and forward current without breaking the
circuit.
03 | Keysight | Measuring Low Current Consumption with a Digital Multimeter – Application Brief
DMM current readings
Measurement tip
Many 6½ digit DMMs are limited to a 1-mA low current range when measuring DC. This
offers a best case resolution of 1 nA. Other DMMs offer a 100-µA or 200-µA range,
which in turn offers 100-pA resolution.
When measuring very small currents
be sure to null your readings before
your crucial measurement. Doing
so removes any system related
offsets, allowing the measurement
to be more accurate.
The new 34465A and 34470A Truevolt Series of DMMs feature a low current range of 1 µA,
which in turn offers an exceptional 1-pA resolution. The new Truevolt DMMs also offer
extended current ranges that are simply not available with other DMMs in this class. The
new current ranges include 10 µA, 1 µA on the low end, and a 10-A range on the higher
end (Figure 1). With pA resolution on the 1-µA range, the potential current measurements
range from 1 pA to 10 A, resulting in 13 orders of magnitude to be measured by these DMMs.
Accuracy specifications: DC current and other DC functions
Specification ± (% of reading + % of range)
Range/frequency
24 hours TCAL ± 1 °C
90 days TCAL ± 5 °C
1 year TCAL ± 5 °C
2 years TCAL ± 5 °C
Temperature
coefficient/°C
1 μA
0.007 + 0.005
0.030 + 0.005
0.050 + 0.005
0.060 + 0.005
0.020 + 0.010
10 μA
0.007 + 0.002
0.030 + 0.002
0.050 + 0.002
0.060 + 0.002
0.015 + 0.006
100 μA
0.007 + 0.001
0.030 + 0.001
0.050 + 0.001
0.060 + 0.001
0.015 + 0.004
1 mA
0.007 + 0.003
0.030 + 0.005
0.050 + 0.005
0.060 + 0.005
0.015 + 0.005
10 mA
0.007 + 0.020
0.030 + 0.020
0.050 + 0.020
0.060 + 0.020
0.020 + 0.020
100 mA
0.010 + 0.004
0.030 + 0.005
0.050 + 0.005
0.060 + 0.005
0.020 + 0.005
1A
0.050 + 0.006
0.070 + 0.010
0.080 + 0.010
0.100 + 0.010
0.050 + 0.010
3A
0.180 + 0.0020
0.200 + 0.020
0.200 + 0.020
0.230 + 0.020
0.050 + 0.020
10 A
0.050 + 0.0010
0.120 + 0.010
0.120 + 0.010
0.150 + 0.010
0.050 + 0.010
DC current
Figure 1. Shown here are all of the new current ranges available with the 34465A and 34470A Truevolt Series of DMMs.
Please refer to the Truevolt Series DMM data sheet, publication number 5991-1983EN, for additional specification details.
04 | Keysight | Measuring Low Current Consumption with a Digital Multimeter – Application Brief
Burden voltage
When measuring low levels of current, burden voltage can be a concern. Burden voltage
is the change in potential created when current flows through the shunt resistor of a
DMM. While this is not normally a concern, very sensitive components must be aware
of the voltage, especially if the DMM is placed in series in the return path as shown in
Figure 2. By adding a small voltage above the low of the power supply, the low of your
device might well be above your design tolerance.
If the burden voltage gets sufficiently large, the offset voltage on the return path might
cause a problem; depending on your device. The burden voltages of the 34465A and
34470A DMMs are shown in Figure 3.
Figure 2. Shown here is the DMM in series on the return path.
DC and AC current burden voltage at full scale
DC current range
Burden voltage
1 μA
< 0.0011 V
10 μA
< 0.011 V
100 μA
< 0.11 V
1 mA
< 0.11 V
10 mA
< 0.027 V
100 mA
< 0.27 V
1A
< 0.7 V/0.05 V1
3A
< 2.0 V/0.15 V1
10 A
< 0.5 V
1. The second burden voltage can be obtained by using the 10 A range input.
Table 2. This table represents the burden voltages of the different current ranges of the 34465A and 34470A.
Measurement tip
Consider moving the DMM in
series to the positive side of your
power supply. If you can increase
the voltage to accommodate for
the burden voltage, you can still
supply the correct voltage to your
device and measure current.
05 | Keysight | Measuring Low Current Consumption with a Digital Multimeter – Application Brief
Dynamic current measurements
Dynamic current measurements can be quite complex due to the DMM range change
based on the level of current you are measuring. Keysight offers specialized instruments
(e.g., the Keysight N6782A SMU) that can digitize power, voltage and current without
ranging issues. While this may be a great solution for current characterization, the
solution is a bit more expensive than using a Truevolt DMM. In situations where budget
or flexibility is a concern, the Truevolt DMM allows you to accomplish many of the same
measurements.
Whether you are measuring the reverse and forward bias current of an LED or the
sleep/operating mode current of a battery powered device, you will have a very large
difference between the two modes of operation.
Figure 4 illustrates a simple block diagram for battery drain analysis. You can effectively
monitor the power drain from your battery powered device using one DMM to monitor
the voltage and another DMM to monitor the current. On the Truevolt DMMs, the 10-A
input is separate from the low current input. This setup uses the low current input.
Figure 4. This block diagram depicts a battery drain analysis using two DMMs.
Measurement tip
Using the Truevolt DMM digitizing
capability and High Speed (HS)
option, you can measure fast
changing characteristics while
measuring with a time resolution
with a time resolution of 20 µs. To
effectively digitize with accurate
timing, ensure that auto zero and
auto ranging is turned off. This
eliminates the timing variation
that occurs when the DMM makes
additional readings for the auto
zero or switching to a different
range.
06 | Keysight | Measuring Low Current Consumption with a Digital Multimeter – Application Brief
Figure 5 shows a typical current profile of a portable radio transceiver. As you can see,
the current draw is complicated with a wide range of sleep, standby and active modes.
The dynamic range of the current is large because the operating currents are drawing
approximately 30 to 40 mA, while the standby currents are only 1 to 10 µA.
In order to get accurate readings for both ranges with a DMM, you need to take multiple
reading sweeps with different ranges. One method for capturing the current profile
would be to run the Device-Under-Test (DUT) multiple times to capture the sleep and
standby modes separately and then the operating mode currents. In the first capture,
set the DMM to the 100-mA range and 0.001 plc (20 µS per sample). This will capture
the complete current signal, including the active mode values between 30 to 40 mA,
but provides less resolution on the lower current measurements.
Figure 5. Shown here is a current profile from a portable radio.
Figure 6 shows a trend chart view of the currents read using a 34465A DMM. Notice
that the lower current measurements seem to be very uniform, which is due a loss of
resolution. To measure the low current values, you need to take another measurement
at a lower current range. Once you have captured the readings, the data can be saved
to memory and analyzed on a PC.
Figure 6. This operating current was captured with the 34465A DMM. Shown here is the DMM front panel.
Measurement tip
You can use Keysight’s BenchVue
software to control and trigger
both DMMs simultaneously. Use
the digitizing mode of the DMM
to ensure 20 µS/sample timing.
Note that the datalogging mode in
BenchVue includes PC overhead
that can interfere when precise
timing is required.
07 | Keysight | Measuring Low Current Consumption with a Digital Multimeter – Application Brief
Next, you can set the DMM to a lower current range for the standby and/or sleep
currents. Figure 7 shows a graphical data capture, using BenchVue, of the standby
currents measured by the top graph. The bottom graph shows the DCV readings. Figure 8
shows the statistics and actual readings from the data set. The currents captured range
from approximately 2 to 10 µA. They were captured with the DUT in standby mode, thus
allowing for a single range current capture. Anything above 120 percent of the range will
result in an overload condition.
Figure 7. These two graphs illustrate the digitized DCI (top image) and DCV (bottom image) consumption.
Summary
With extended current ranges and
digitizing capabilities, the 34465A and
34470A Truevolt DMMs can effectively
characterize dynamic currents. Whether
your DUT is drawing high current at
10 A or you need to measure lower
current in the pico-amp region, the
new Truevolt DMMs have the features
and measurement capabilities to be a
workhorse instrument on your bench.
Figure 8. Shown here are the statistics and actual readings from the data set captured in Figure 7.
08 | Keysight | Measuring Low Current Consumption with a Digital Multimeter – Application Brief
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© Keysight Technologies, 2015
Published in the USA January 29, 2015
5992-0418EN
www.keysight.com
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