Noise on 3-axis accelrometer board

I have just tried the Pololu 3 axis accelerometer board and noticed noise.

There is at least 50 mv of noise on each output.
The Vcc (3.3V) line has 15 mv of noise.
The 5V line has <2 mV of measured noise.
I am using a DVM to make measurements.
I am using a battery pack to power this test board.
There is a LM340T 5V regulator.
There are no other active component on this board.

Is this normal?


How are you measuring noise with a multimeter? We don’t really have the typical noise characterized, but it seems like that could be about right. The outputs already have 1kohm and 0.1uF RC filters on them, but you could add some more capacitance to both the 3.3V line and the outputs to smooth them out some more.

- Jan

A scope on pin 3 (3.3v) showed a 100 mv sawtooth waveform.
The outputs showed high frequency noise (approx 50 mv pp) riding a lower frequency 300mv pp wave.

A 15 uF tantalum cap from pin 3 to ground removed the sawtooth and reduced the noise on the outputs.

There is still a some noise ( approx 40 mv pp) on the outputs.

What is “normal” for the x, y and Z outputs.

We don’t have “normal” characterized. In general, though, voltages around what you’re describing can be difficult to measure. For instance, we can get 40 mV peak-to-peak just with the scope probe connected to its ground clip. Can you tell us more about how you are measuring the signals (e.g. what kind of equipment are you using, how are you making sure you don’t have mechanical vibrations, and so one). It would be great if you could post some screen shots from your scope.

- Jan

It turns out that the noise I was seeing was in my circuit. I am using a PIC and the analog supply line was shifting causing (apparent) noise on all the ADC inputs.

I have a sharp IR range finder in the circuit and it generates plenty of noise on the power line.
The sensor was connected to my analog power for my microprocessor.

After separating the sharp sensor and adding more bypass caps, the readings of the accelerometer output quieted down.

Thanks for the update. We’ve been looking into the problem more because of our recent introduction of the LISY300AL gyro carrier, in which we are using lower ESR capacitors to reduce the noise on the regulated 3.3V line. (The capacitor selection was more of a problem with the gyro, but moving to the X5R, 4.7 uF caps should help the accelerometer a bit, too.)

Those sharp distance sensors certainly put a ton of noise into everything every time they take a measurement!

- Jan