Everyone wants longer battery life for their cell phone or other mobile devices. This typically
leads designers to think about obtaining better batteries. But battery technology improves
very slowly. Often, it makes more sense to focus on getting the most from your present
battery by understanding how your device consumes energy from it. This can be done by analyzing
the dynamic battery current drain required by the mobile device. Some wireless devices
spend most of their time in a power-savings mode or sleep state in order to conserve
battery power. Although the current drain may be very small, perhaps less than ten micro-amps,
the large amount of time spent in a sleep state represents a significant amount of the total
power consumed by the device. In fact, it may be larger than the infrequent transmit
state when the device sends data. This pulsed current profile has these characteristics:
Very long periods - up to tens of seconds Low duty cycles - from tenths of a percent
to several percent Extremely high crest factors – as much as
one hundred or more The key challenges in this current profile
measurement are being able to: 1) Accurately measure the very low level sleep
state current in the presence of high level peaks of transmit state current, and
2) Accurately measure the timing relationships and durations of the two states.
Conventional test equipment is available with fast digitizing to capture the durations of
the two types of operating states. However, when set to a fixed range high enough to measure
the high current state, the measurement accuracy for low-level measurement is severely degraded.
This is because the offset error and noise on a high measurement range is often larger
than the low-level current to be measured. When used in an auto-range mode, the low-level
accuracy is better, but now the slow reaction of the auto-range capability means that many
of the data points after transitions from high to low or low to high will be missed.
Agilent Technologies has a DC source designed specifically to solve this difficult measurement task.
The N6781A Source Measure Unit is tuned for battery drain analysis. When combined
with an Agilent N6705B series mainframe and 14585A software, it gives you superior insight
into the nature of the dynamic current drain being demanded by the mobile device.
In the following measurement example, the battery drain current of a wireless temperature sensor
has the following characteristics: Period of 4 seconds
Duty cycle of 0.17% Crest factor of 400
The N6781A SMU was used both to power the temperature sensor and to measure the sensor’s
current consumption. The internal zero-ohm ammeter is key to precise measurements of
low-level current. Initially, the current was measured with the N6781A set to its fixed
100 milli-amp measurement range. As shown here, this is comparable to the way conventional
test equipment would be forced to make this measurement because of the 21 milli-amp transmit
state current pulses. With this configuration, the measurement error of the 10 micro-amp
sleep-state current is 115%. In other words, we’re measuring a 10 micro-amp current with
an uncertainty of plus or minus 11.5 micro-amps – clearly not sufficient to understand the
energy being consumed from the battery. Then we changed the N6781A SMU to use its innovative
measurement system that seamlessly auto-ranges, continuously digitizing this current from
nano-amps to amps at over 195 kilo-samples per second. This provides accurate measurements
over a greatly extended dynamic range. With seamless ranging, we can now measure the
10 micro-amp sleep-state current with an error of only 1.15%. And with seamless ranging,
the measurement error of the overall average current improved from 18.9% to 0.245%.
This is because the noise floor decreased from 47 micro-amps to 10 micro-amps peak to peak.
This gives you much greater insight into the power-saving operation of the sensor.
The Agilent N6781A provides unmatched accuracy because it has high-speed seamless auto-ranging
resulting in a wide dynamic measurement range from nano-amps to amps. For more information,
go to www.agilent.com/find/N6781. And look for more videos in this series
on optimizing battery life in mobile devices.