Energy Audit Circuit
One of the most challenging requirements of wireless sensor devices is reduce their power consumption in order to achieve a desired lifetime. In order to do this, it is important to measure the current consumption. Estimates of current consumption based only on figures obtained from the component manufacturer's datasheets provide general indications, but should not be relied upon for accurate device lifetime calculations. Optimisation of current consumption and accurate lifetime calculations require an in-depth knowledge of the characteristics of the components used, their inter-operation in a particular device and the effect of software implementations.
A suitable multi-meter gives high accuracy, but does not give the required resolution in time to accurately profile the current drawn by a mote by viewing it on an Oscilloscope. For this reason an energy audit circuit was designed by the WiSAR Lab to allow the system designer to quantify the time spent and current consumed within each state of the system.
The WiSAR team has used this energy audit circuit to profile the EZ430-RF2500 from Texas Instruments and the WiSAR Lab Development mote, with a set of tests including measuring the current consumption during standard and low power modes, during sensing operations and during wireless transmission. These tests allow us to optimise device drivers to minimise current consumption.
This energy auditing circuit consists of an amplifier board with a low value precision resistor placed in series to the current path, which in turn produces a small voltage drop which is amplified as an output signal proportional to the current. The amplifier used is the AD621 instrumentation amplifier with gain set to 100.
To eliminate the need for an external power supply a micro power DC/DC converter was incorporated to power the instrumentation amplifier from a mini USB connection. The LT1610 linear DC/DC converter was used because of its fixed switching frequency of 1.7 MHz allowing easy filtering, feedback resistor values were chosen to allow an output swing between -15V to +15V which corresponds to a dynamic range up to 30mA.
