skip ADC calibration

per239955_st · ‎2006-01-03

Posted on January 03, 2006 at 12:03

per239955_st · ‎2006-01-02

Posted on January 02, 2006 at 12:16

Hi,

I don't want our production department to need calibrate the ADC since we don't require very good accuracy.

The refernce manual mentions 5% accuracy for the internal 1.25V reference signal and the 1.85v feedback.

What accuracy does that translate to for the actual ADC value when/if not calibrating?

What ''theoretical'' values should be used for the constants....

#define off 0xB3B /* digital value for the 0 volt */

#define ref 0x4b9 /* digital value for the 2.5 volt */

.... that are used in AN1809 when not calibrating?

My measured values are

#define off 0x9B1

#define ref 0x58B

which seems to be 17% and 32% off the values in AN1809.

per239955_st · ‎2006-01-02

Posted on January 02, 2006 at 13:32

Hi again,

Here's how I have derived the theoretical 'vref' and 'off' values.

It seems that the full range (0x000 - 0xfff) for the ADC is 3.3v.

Calculating 1.25v and 2.5v from that gives.

0.00v => 0x000

1.25v => 0x60f

2.50v => 0xC1e

3.30v => 0xfff

Transforming it into signed two's complement with 1.25v as the midpoint gives

0.00v => 0x9f0

1.25v => 0x000

2.50v => 0x60f

3.30v => 0x9ef

I know that ADC does not support values above 2.5v but up to 3.1v the

conversion values are correct (linear). :p (but should of course not be used)

So, what do you think, are 0x9f0 and 0x60f the 'correct' values to use when not calibrating?

hichem2 · ‎2006-01-03

Posted on January 03, 2006 at 12:03

Hello Beach,

the offset values used in AN1809 was measured using an STR712FR2T6, normally you have to measure at the begening the new value according to the used device.

regards,

Hich