...the world's most energy friendly microcontrollers
2010-11-12 - t0008_1.01
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www.energymicro.com
7 Advanced Energy Monitor
7.1 Usage
The AEM data is collected by the board controller and can be displayed by a PC application. For instance,
the "Gecko commander" utility contains a "power" command which can dump power data to file. A GUI
application for displaying power usage will be available for download from the Energy Micro download
section later.
7.2 AEM theory of operation
In order to be able to measure currents ranging from 0.1uA to 50mA (114dB dynamic range), two current
sense amplifiers are utilized. The amplifiers measure voltage drop over a small series resistor and
translates this into a current. Each amplifier is adjusted for current measurement in a specific range.
The ranges for the amplifiers overlap and a change between the two occurs when the current is 200uA.
To reduce noise, averaging of the samples is performed before the current measurement is presented
in the AEM GUI.
During startup of the kit, a calibration of the AEM is performed. This calibration compensates for the
offset error in the sense amplifiers.
7.3 AEM accuracy and performance
The Advanced Energy Monitor is capable of measuring currents in the range of 0.1uA to 50mA. For
currents above 200uA, the AEM is accurate within 0.1mA. When measuring currents below 200uA, the
accuracy increases to 1uA. Even though the absolute accuracy is 1uA in the sub 200uA range, the AEM
is able to detect changes in the current consumption as small as 100nA The measurement bandwidth
of the AEM is 60Hz when measuring currents below 200uA and 120Hz when measuring currents above
200uA. The table below summarizes accuracy of the two current sense amplifiers in different ranges.
Table 7.1. AEM accuracy
Current range
Low gain amplifier accuracy
High gain amplifier accuracy
50mA
0.1mA
-
1mA
0.1mA
-
200uA
0.01mA
1uA
10uA
-
0.1uA
1uA
-
0.1uA
Note
Having both USB and battery connected may increase the AEM readings.
