7.3.1 AEM Details
The main differences between the AEM on the Wireless Pro Kit Mainboard (BRD4002A) and the Wireless STK Mainboard (BRD4001A)
is summarized in the table below with more in-depth information given in the text to follow.
Table 7.1. Advanced Energy Monitor Parameters
Parameter
BRD4002A
BRD4001A
Voltage
1.8 - 3.6 V
3.3 V
Sample Rate
100 kHz
10 kHz
Sense Resistor
10.5 Ω / 0.5 Ω
2.35 Ω
Measurement Range
1
0 - 495 mA
0 - 95 mA
Note:
1. The current sourcing capabilities of the LDO may be different than the measurement range.
Wireless Pro Kit Mainboard (BRD4002A) AEM Design Details
The AEM circuitry on the Wireless Pro Kit Mainboard is capable of measuring current signals in the range of approximately 0.1 µA to
495 mA. This is accomplished through a combination of a highly capable current sense amplifier, multiple sense resistors and gain
stages, and signal processing within the kit's board controller before the current sense signal is read by a host computer with 100 kHz
sample rate for display and/or storage. Averaging on the output data may be required to achieve sufficient accuracy in some situations,
such as low currents, which can be traded for lower bandwidth. High current applications require that the regulator is able to supply
enough current as described in Section
.
At low currents the current sense amplifier measures the voltage drop over a 10.5 Ω resistive path. The gain stage further amplifies this
voltage with two different parallel gain settings to obtain two current ranges. The transition between these two ranges occurs around
150 µA. When the current exceeds a threshold, which is typically between 10 and 30 mA, the AEM circuitry switches from the 10.5 Ω
resistive path to a 0.5 Ω sense resistor and is now capable of measuring currents up to approximately 495 mA. Should the current drop
below the threshold again, the sense resistor is changed back to the 10.5 Ω resistive path and the AEM is back to using two different
gain stages depending on whether the current is above or below 150 µA.
The expected typical accuracy of the AEM on the Wireless Pro Kit Mainboard is within 1 %, except for currents in the low tens of micro-
amps where offset errors start to dominate. In this low current region, the expected typical accuracy is some hundred nanoamps. At kit
power-up or on a power-cycle, an automatic AEM calibration is performed which compensates for offset errors in the current sense
amplifiers. To achieve the stated accuracy, averaging of the AEM output data is required in certain situations (typically at low currents
and close to the bottom of the measurement ranges) to reduce noise. Averaging can be applied in Energy Profiler to suit different re-
quirements during or after the acquisition. The analog bandwidth of the measurement circuit depends on multiple factors, such as out-
put current and capacitance on the VMCU net, and may be lower than the output data rate. Generally, higher output current and lower
capacitance on VMCU gives a higher analog bandwidth.
Wireless STK Mainboard (BRD4001A) AEM Design Details
The AEM circuitry on the Wireless STK Mainboard works conceptually in a similar way to the implementation on the Wireless Pro Kit
Mainboard except for two key differences: it uses only one 2.35 Ω sense resistor and the low-dropout regulator (LDO) is different. For
details about the two implementations, the reader is encouraged to see the schematics.
The AEM on the Wireless STK Mainboard is capable of measuring currents in the range of 0.1 µA to 95 mA. The second stage amplifier
amplifies the signal with two different gain settings with the transition occurring around 250 µA. For currents above 250 µA, the AEM is
accurate within 0.1 mA. When measuring currents below 250 µA, the accuracy increases to 1 µA. Even though the absolute accuracy is
1 µA in the sub 250 µA range, the AEM can detect changes in the current consumption as small as 0.1 µA. It is possible to source
currents above the measurement range as decribed in Section
. The board controller outputs the AEM data with 10 kHz
sample rate.
UG525: EFR32xG24 2.4 GHz 10 dBm Wireless Pro Kit User's Guide
Advanced Energy Monitor
silabs.com
| Building a more connected world.
Rev. 1.0 | 30
