LSM303DLHC low-power and high precision modes

Question

The LSM303DLHC is a 3-axis accelerometer + 3-axis magnetometer chip from ST. The datasheet is a bit light on details however. I'm trying to work out

• what is different about the low-power mode and how much power does it save, and
• what is the high precision mode?

The only info I can glean from the datasheet is on page 15:

LSM303DLHC provides two different acceleration operating modes, respectively reported as “normal mode” and “low-power mode”. While normal mode guarantees high resolution, low-power mode reduces further the current consumption.

followed by a table with the following info:

Low-power mode - BW(Hz) = ODR/2, Turn on time = 1ms

Normal mode - BW(Hz) = ODR/9, Turn on time = 7/ODR

ODR means output data rate, and I assume BW means bandwidth, but bandwidth of what? The term bandwidth or BW doesn't appear in the rest of the datasheet. In the electrical characteristic there is no info on the low-power mode consumption.

Can anyone shed some light on this?

Answer 1

"High precision" is used as a close synonym with "normal" mode, and is the opposite of "low power" mode. From the datasheet, it appears that the high precision mode samples the raw sensor 4 times for each output sensor value.

Output Data Rate (ODR) is the fastest rate that the sensor produces data (and hence that you should read that data). Freescale has this to say (source):

Q: Once the previous measurement has been read, how long does it take to get the next one? What is the duration between two DRDY (data ready) flags? Does the 80-Hz output data rate mean 12.5 ms between two DR flags?

A: The 80-Hz output data rate means 12.5 ms between two DR flags.

So, for the LSM303DLHC you can read the accelerometer at up to 1.344 kHz in normal mode or 5.376kHz in low-power mode. Notice how 5.376kHz is 4x as fast as 1.344kHz.

Bandwidth refers to the maximum frequency signal that you can reliably measure. Analog Devices has a nice quote on the subject:

Output Data Rate, in digital-output accelerometers, defines the rate at which data is sampled. Bandwidth is the highest frequency signal that can be sampled without aliasing by the specified Output Data Rate. Per the Nyquist sampling criterion, bandwidth is half the Output Data Rate.

For the LSM303DLHC the fastest signal that the accelerometer can measure in normal (high-precision) mode is 1344Hz/9 = 149.3Hz. In reality you'll need to give yourself some margin since the signal will likely not be repeating infinitely.

Answer 2

It probably means that it is able to detect accelerations up to the stated BW frequency.

Answer 3

As an additional detail, it probably should be presumed that the "low power" mode is "dirtier"- with the bonus that while you're likely to get the occasional outlier that can be compensated for or a bit of LSB noise, but have a sample rate that is 2.688 kHz. Some designs can manage with the precision and an assured output BW of 149.4 Hz (Station keeping might be one of those few applications...) but most will need the higher sample rate with lowered "precision" of readings.

One of the things you need to know is that things like airmice and the like don't like the low sample rates at full power. >:-D