How to imprrove adc reading stability?

Hi Bruno

Bruno Duarte28 said:

The problem it that the values are not stable even when the voltage and current are. I've added a RC low pass filter to avoid noise from the power supply and a sliding window digital filter but the adc still have spikes on the readings.

You should not experience the above until ADC input itself is fluctuating! So you need to check the same and need to be very sure that input is very stable. Another way to stabilize the ADC is to increase the Acquisition period to full ie the ACQPS register. Also, try f=implementing a moving average filter.

Regards,

Gautam

Hello Gautam,

I've already applied a moving average filter, with 300 position and increased the ACQPS to 199.

With this, the average values is more stable but if i check the input values from the adc, for a medium 250 value some readings get higher than 300 or lower than 200.

A low Pass with 0,1uF and 47R gives a 15kHz curring frequency ...

I'm trying a much lower cutting frequency in the 150Hz Range or bellow.

With the osciloscope i get a periodic spike. I think thats what messing with my readings.

Those spikes have 20ms period ... so I guess that is what is messing my ADC.

I'll have to try to solve that. Thank you for the help =)

Best Regards,

Bruno Duarte

Hello Devin,

I've been doing further tests and the resuts are not good.

This are the waveforms ( from the TI compiler ) of the AdcAResult1 variable for 3 different voltages ( 2,98V ; 1.5V and 0.75V)

As you can see, the value swings too much on stable voltage readings ( i was using a Pot to change the value and measuring the voltage with a digital multimeter )

i dont know if it is a incorrect confguration or what can be the cause of this instability.

Attached is the ADC configuration function. 

Thanks.

void AdcConfig(void)
{

	EALLOW;
	AdcaRegs.ADCCTL2.bit.PRESCALE 		= 6; 		//set ADCCLK divider to /4
	AdcbRegs.ADCCTL2.bit.PRESCALE 		= 6; 		//set ADCCLK divider to /4

	// Sequencia de PowerUp
	//power up the ADCs
	AdcaRegs.ADCCTL1.bit.ADCPWDNZ  		= 1;      	// Power up ADC
	AdcbRegs.ADCCTL1.bit.ADCPWDNZ  		= 1;      	// Power up ADC
	//Set pulse positions to late
	AdcaRegs.ADCCTL1.bit.INTPULSEPOS 	= 1; 		// INT pulse generation occurs 1 cycle prior to ADC result latching into its result register
	AdcbRegs.ADCCTL1.bit.INTPULSEPOS 	= 1; 		// INT pulse generation occurs 1 cycle prior to ADC result latching into its result register

	DELAY_US(500);

	//ADC_A
	AdcaRegs.ADCSOC2CTL.bit.CHSEL 		= 15; 		// SOC2 will convert pin A15
	AdcaRegs.ADCSOC2CTL.bit.ACQPS 		= 19; 		// ADCSOC1CTL = 0x0A86, CHSEL 		-> sample window 7 cycles
	AdcaRegs.ADCSOC2CTL.bit.TRIGSEL		= 1;

	//ADC_B
	AdcbRegs.ADCSOC2CTL.bit.CHSEL 		= 15; 		// SOC2 will convert pin b2  		-> ADCINB2 || J3.26
	AdcbRegs.ADCSOC2CTL.bit.ACQPS 		= 19; 		// ADCSOC1CTL = 0x0A86, CHSEL 		-> sample window 7 cycles
	AdcbRegs.ADCSOC2CTL.bit.TRIGSEL		= 1;

	AdcaRegs.ADCINTSEL1N2.bit.INT1E 	= 1;   			//enable INT1 flag

	EDIS;
}