[PIC] 12 bit ADC measurement problem

[khaleel.arya]

MPLAB8.8
dsPIC30F5011

Hi,
I am using a 12bit SAR [succesive approximation converter] adc for measurement and I am getting a variation of 15~20 mv difference with out sampling w.r.t DC Voltages.

i have used a crystal,primary oscillator of 8MHZ with 4 PLL
*************************************************************************
Fcy = Fosc/4 = (Source Osc freq * PLL mul)/(program post scalar *4)
Tcy = 1/ Fcy
A minimum of 333.33 ns is required for TAD,
TAD = (Tcy(ADCS + 1))/2
from this
ADCS = (2*TAD/Tcy) - 1

these are my initialisations fr adc convertion clock
---------------------------------------------------------------------------

ADCON3bits.SAMC	= 14;		// fr 12 bit ADc 14TAD
ADCON3bits.ADRC	=  0;		// Clock derived from system clock
ADCON3bits.ADCS	=  5;

-----------------------------------------------------------------------------------
is there any wrong with this calculation or any other suggestions to get accurate values


Regards
khaleel

 

[Easyrider83]

Use independed ground and reference voltage for ADC_Ref+/-.

 

[khaleel.arya]

independent ground in the sense?
can you explain it briefly

 

[Easyrider83]

I didn't seen your PCB. For reference voltages reasonable to use intependend ground and supply, connected via low pass filter, based on inductor and capacitor.

 

[Aussie Susan]

What are the voltages you are expecting to measure - in other words what is the relative error of the 15-20mV.
12-bit ADCs mean that the last bit is 1/4096 of the maximum range. If you are trying to measure (say) 0 to 5V then the bottom bit is +/- 1mV.
However, unless you are dealing with a *VERY* (electrical) noise-free environment, the best you can typically expect to measure with an ADC is something like 8 (+/-0.5%) or perhaps 10 bits (+/- 0.1%). That is just the nature of electrical signals .
Susan

 

[khaleel.arya]

sorry for late reply,

I have connected a keypad which is made with different resistors and provides different voltages when i press the different buttons.when i compare the voltages im getting with voltages shown by multi meter it showing a difference of 15-20mv.

 

[Easyrider83]

Increase to 100-200mV and it will work.