noise question

Using Arduino Sensors
1

Hi everybody,

I've been playing with a piezo element, my arduino, and an oscilloscope, and it seems like when I use Arduino gnd, the noise is way higher than when I use the oscilloscope gnd? Thoughts? I can be more specific if needed, but right now I've got the piezo (+) connected to the oscilloscope and to gnd through a 10mohm resistor. piezo (-) goes to gnd

I'm just wondering about noisy Arduino gnd, and whether the analog port should be noisy too, and what people do about that.

fyi I am also a super newbie. Have posted to this forum before about this project - I'm really bad at this / kind of lost.

thanks,

2

The Arduino and scope grounds should be connected together. There can only be one ground.

3

Also, why the 10 meg ohm resistor?

4

And how are you comparing the noise levels? You need to measure or compute RMS noise voltages into identical load resistors.

5

Good questions. So

why the 10 meg ohm resistor?

Because http://www.arduino.cc/en/Tutorial/KnockSensor says to put a parallel resistor. I actually don't know why. Is it just to control the voltage and protect the analog port? Beats me. I've gotten rid of it before.

how are you measuring noise levels?

A good question, too. I'm just eyeballing it on the oscilloscope.

How does one

compute RMS noise voltages onto identical load resistors

?

I'm kind of shooting in all directions here. If you look at my other postings, I'm doing a project with a piezo disk to measure raindrops. The voltage from the drops is usually not high enough for the signal to be noticed by the A/D converter. It's usually in the mV range, but low mV; when I tried to use a number of op-amps, I got results that really only amplified my 'big' signals - basically, signals that I could already see got bigger, but signals that I couldn't see didn't move.

6

Because http://www.arduino.cc/en/Tutorial/KnockSensor says to put a parallel resistor. I actually don't know why. Is it just to control the voltage and protect the analog port? Beats me. I've gotten rid of it before.

That page doesn't say what the resistor is actually doing. I assume it's to discharge the piezo capacitance
between knocks. I don't see how a 1Mohm R in parallel could possibly protect the port. You would need
a clamping diode or zener for that.

7

A piezo sensor probably acts as a fairly pure current noise source, so the load resistor is what determines the noise voltage - hence the need to measure with identical load resistances.

Also noise amplititude depends on bandwidth, and we are probably talking about two setups with wildly different bandwidths...