View Full Version : noise problem in ADC(atmega32)

09-16-2005, 06:04 AM
I am having too much problems with noise in the in-built ADC of atmega32

Instead of external voltage I used internal 2.56 Vref and applied the voltage, then I am getting 2.55V for 2.24V. I used a capacitor of 150pf,22pf b/w AREF and ground.

I removed the multimeter probes from the breadboard, then a small difference came. But the noise is too much. My aim is to interface LM35 with atmega32. So I am just checking with a potentiometer.

Is there any way to avoid this noise

wht should be the value of capacitor b/w AREF and ground

My code

.org 0x000
rjmp RESET


ldi r16,0xff
out $11,r16 ;DDRD as output
out $12,r16 ; for checking if ADC is not good
out $17,r16
out $18,r16


ldi r16,0b11100000 ; setting AD1,AD0 in ADCL
;internal 2.56 reference,Single
;channel A0
out $07,r16 ;setting ADMUX

ldi r16,0b10000101 ;setting control reg. ck/32
out $06,r16 ;setting ADCSRA



sbi $06,6

sbis $06,4
rjmp WAIT

in r16,$04;ADCL
in r17,$05;ADCH

out $12,r17
out $18,r16

Bibin John

09-16-2005, 11:58 AM
150pf and 22pf is way small....try putting a .1uf cap between AREF and GND....this should solve your probelm.

09-16-2005, 04:39 PM
i put 10uF and 1uF but it become more error

tell me the exact value I should put

can u explain the reason for it

09-16-2005, 08:08 PM
Hey Yogi....I told you to put a *0.1uf* cap, not 1uf or 10uf; 0.1uf and 0.01uf are most commonly used bypass capacitor to bypass unwanted noises to ground.

Hope this helps

09-16-2005, 08:14 PM
try running your ckt on a dry cell....or a SMPS
dont use a regular battery eliminator

09-17-2005, 12:39 AM
can't we do it with adapter

ok now I will use battery

wht is the value of capacitance

i am getting too many errors

error increases with time, if the value is high, error is also high

09-19-2005, 05:55 PM
Use a proper power supply regulation for the adapter ............ might help. And breadboards could be the cause of the problem ... try PCB ... they are better in terms of noise elimination !

10-20-2005, 11:48 AM
Since you are using internal voltage for Reference your ground should be common. What i mean to say is that the reference voltage of your Micro and the reference voltage of your signal should have the same ground.

Secondly, when using a capacitor to bypass noise use a 0.1uF or a 0.01 uF BOX capacitors.between signal and ground. This might help

When you are using a multimeter , the probs have resistance and so is the internal resistance of the meter. These affect the signal as the meter is acting as cap/resistor in parallel to your signal. Use a osciloscope if possible. or take indirect readings.


Nimesh S Chinoy