Arduino, Wire, and I2C Part 4: Noise and analysis

These weekend I spent some time recording sample data sets from the sparkfun SEN-10724 and looking at it. I'd say I was analyzing the data, except that might give the impression that I actually have a good idea what I'm doing.

The recording of data, I actually set up using the sparkfun "ethernet pro" board, which is basically an Arduino with built-in ethernet and seemingly poorly designed voltage regulation (it gets pretty hot if you power it with the intended voltage levels). I felt more comfortable using the ethernet rather than serial-over-USB since I would be sending more data than was reasonable for the 9600bps that the Arduino UNO is fixed at (over USB - the firmware for the atmega8u2 that handles the USB-to-serial interface is programmed such that it will only operate at 9600bps).

After finding a few errors with decoding the data (mostly in the PC side, the microcontroller was programmed to send the raw measurements as-is), I found that the ADXL345 accelerometer has noise levels way out of spec. I've plotted the measurements and attached images of said plots at the end of this post. The XYZ measurements of the ADXL345 all had significant levels of noise (RMSD 5-12 LSBs) and the magnetometer had noise on the X-axis only (RMSD 9 LSBs). During the test data recording, the sensor board was in a breadboard sitting on my desk with no significant sources of vibration.

One concern I had is that the schematic for the SEN-10724 had two .1μF capacitors "near" the ADXL345. The datasheet calls for a single .1μF capacitor between ground and V_DDI/O, and for a 1μF tantalum capacitor at V_S, with an optional 10μF tantalum capacitor in parallel.

If the circuit instead only has a .1μF capacitor at V_S, that might explain why the measurements look so noisy. In-circuit measurement of the capacitors is impossible, and I don't have a stockpile of SMD capacitors (strangely enough, after the 3 separate LED cube builds) to replace it with. Time to email customer support.