Wednesday, June 8, 2011

Tools for Low Power Design of Wireless Sensors

Agilent’s N6781A and N6782A SMU modules, for the N6700 and N6705 modular power systems, provide cutting edge technology for battery drain analysis and low power design. These SMUs provide low current measurement capability, fast transient response, and digitized measurements for characterizing dynamic current. But what sets these solutions apart from anything else on the market is their seamless current ranging capability. The seamless current ranging capability allows the N6781A and N6782A to seamlessly change current measurement ranges without any discontinuities in the output. See my 6/27/10 post for more details on the patented seamless current ranging capability.

The N6781A and N6782A were specifically targeted at the mobile phone and smart device markets, but they are also proving an ideal solution for other battery powered devices such as wireless sensors. In this post I want to share some details on how a designer of wireless temperature / humidity sensors used the N6782A SMU module as a tool for optimizing their design for low power consumption. Below is a screen shot from the 14585A software using digitized measurements from the N6782A SMU module (click to enlarge).

The 14585A software, among other things, provides a scope like display of digitized current and voltage measurements from the N6705B mainframe. In the screen shot above you can see two current pulses representing the data transmit cycle of the sensor under test. The pulses peak around 14 mA with a sleep current around 4 uA. In the screen shot below we zoomed in on the start of one of the pulses. As you can see the ~400 ms pulse is really a group of pulses representing transmitted bits. Circled in red is a 2 mA step representing an LED turn-on. As you can see the LED turns on about 15 ms before the transmit cycle actually begins. By delaying the LED turn-on by ~15 ms the designer could lower the sensor’s power consumption and increase battery life.

When we used the N6782A to capture the current profile of the T/H sensor we used the seamless current ranging. When the T/H sensor was in a sleep state (current in uA region) the N6782A was using its 1 mA current measurement range as the current consumption began to shoot up because of the start of a data transmission the N6782A seamlessly switched to its 100 mA current measurement range. This means we captured 18 bits of measurement resolution throughout the current capture. Let’s use that resolution to zoom in on the sleep current as shown in the screen shot below.

The N6782A is sampling at 5 us period and the screen shot above is showing about 1.5 s worth of data. Since there are too many data points to view the 14585A software decimates the data and shows three traces: a min, max, and average trace. Notice the current anomalies circled in red. They occurred on regular intervals of about 0.9 s. In the screen shot below we take a closer look at the anomalies in the sleep current.

Here we zoomed on the sleep current anomaly and placed markers around it. The average current around the anomaly is ~6 uA while the average current during the anomaly is ~11 uA. Since the T/H sensor runs for >1 year before a battery change, eliminating the anomaly will significantly add to the device’s battery life. The whole point being that the anomaly was easy to spot with the N6782A low current measurement capability and its high speed current digitizer. And remember all the current measurement points we looked at in this post, from the pulses to the sleep current, were captured with a single data log measurement. This was made possible by the seamless current ranging capability which ensures we got 18 bits of measurement resolution from the sleep current to the transmit pulse peak current. Without the seamless ranging we would have to make multiple data log measurements of the current, each at a different measurement range, and try and superimpose the data together. The designer of the sensor plans to use the N6782A, the N6705B mainframe, and the 14585A current analysis software to help them reduce the size and increase the battery life of their sensor design.