ADXL354 adc noise question

hello, :o I'm using EVAL-ADXL354BZ on mega2560. Through its a analog acelerometer,i must convert singles. But the noise is quite big,its around 0.006g/hz average all axis.(it can reach 20ug/hz) I think its adc limit throgh(10bit),but maybe i can do something on my code but i dont know how :confused:

3 axis noise image (matlab): |500x328

code:

const int xpin = A2; const int ypin = A3; const int zpin = A4; double x,y,z,adc;

void setup() { Serial.begin(115200); analogReference(INTERNAL2V56);

}

void loop() { adc=2.4775; x=analogRead(xpin); y=analogRead(ypin); z=analogRead(zpin);

x=(x*adc/1023-0.9)/0.4; y=(y*adc/1023-0.9)/0.4; z=(z*adc/1023-0.9)/0.4;

Serial.print(x,5); Serial.print("\t"); Serial.print(y,5); Serial.print("\t"); Serial.print(z,5); Serial.println("\t"); // delay(5); }

The chip datasheet says 20ยตg/โˆšHz, and a bandwidth of 1.5kHz, so that's about 0.77mg rms

Given you give the units of noise wrongly, do you understand how to calculate noise? Where does this "0.006g/Hz" value come from? Do you mean 6mg/โˆšHz? What is your conversion factor from acceleration in g to voltage or ADC counts? Is it correct?

Noise is measured in power per unit bandwidth, or alternatively voltage per squareroot(bandwidth) in the amplitude domain. Noise adds as power, not as voltage.

What power supply are you using - for sensitive analog measurements you should avoid any switch-mode supplies and use linear regulator.

The graph is confusing - why not provide a standard graph without the confusing shading? I presume that the steps are ADC LSB steps?

If it's same board, ADXL354 evaluation has got 100nF output capacitors, so output bandwidth is limited to 49Hz. ADXL355 (digital version) works using 18 bit, while internal noise eats 4 bits, so you should get 14 bit noise free acceleration (you only read environmental noise). That's why you shouldn't have so much noise. Obviously, as MarkT said, you have to power ADXL354 with clean power supply (filters+LDO), at the same way for ADC and reference voltage. I don't know power supply schematics for Arduino ADC....

I'm working on ADXL354 array to reduce noise spectral density, keep us updated.