TMS320F28069: ADC Digital Count not matching Expected Value

Siddharth Gupta84

Intellectual 450 points

Part Number: TMS320F28069

Hi,

I am using TMS320F28069. Following are my observation :

Schematic Points

Vref hi = Open (No voltage impressed)- internal reference is used.

Vref lo = 0 Volts

VDDA = 3.3V

S/n	Voltage (Volts)	Digital Count Received in ADC	Expected count	Difference
1	0	4	0	-4.00
2	0.5	671	620.45	-50.55
3	1	1256	1240.91	-15.09
4	1.5	1830	1861.36	31.36
5	2	2459	2481.82	22.82
6	2.5	3012	3102.27	90.27
7	3	3533	3722.73	189.73

Digital Count which I am expecting for a particular voltage = (Voltage*4095)/3.3

But as you can see in the table the difference in count exceeds as the voltage is increased. This is causing error in my readings.

Kindly suggest what can I do to reduce this difference in readings.

Regards,

Siddharth Gupta

over 9 years ago

0 Todd Anderson78572 over 9 years ago

Guru 14640 points

Somebody can comment further, but your expected counts need to be truncated - there is no way the ADC will give a "fractional count." Also, there is usually some amount of noise in the value - a value of 4 for 0 volts is actually pretty good. Not sure about the values with higher count differences...

Hopefully, that helps.

0 Todd Anderson78572 over 9 years ago in reply to Todd Anderson78572

Guru 14640 points

You may want to check out SPRAAP6A - "An Overview of Designing Analog Interface With TMS320F28xx/28xxx DSCs"
very good stuff...

0 Devin Cottier over 9 years ago in reply to Todd Anderson78572

TI__Guru 60865 points

Hi Siddharth,

Some things you may want to look at:
-How are you forcing the voltage? What is the impedance of your signal source? Are you measuring the voltage at the pin with a DMM?
-Are you taking a single sample, or an average of many samples?
-Are you sure the ADCCLK is running at 45MHz or less?
-Are you doing offset self-calibration? (if you are using the example code, have you made sure all changes made by the example function have been reversed before doing your sampling?)

0 Siddharth Gupta84 over 9 years ago in reply to Todd Anderson78572

Intellectual 450 points

Hi Todd,

Not expecting fractional counts at ADC. Concerned about the difference, which is increasing as the input voltage is increased. Leading to non uniform offset\error.

Regards,

Siddharth

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TMS320F28069: ADC Digital Count not matching Expected Value