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Is ADC inaccuracy 0.1V normal for STR71X?

hg-chen
Associate II
Posted on September 04, 2008 at 11:59

Is ADC inaccuracy 0.1V normal for STR71X?

4 REPLIES 4
hg-chen
Associate II
Posted on July 18, 2008 at 09:16

STR711FR2H6 is used.

the ADC inaccuracy is about 0.1V,is this normal,

any hardware and software information for improving STR71X ADC accuracy?

avelez
Associate II
Posted on July 29, 2008 at 05:01

I don't know what have you tried before, but did you try to calibrate the ADC before taking samples of the voltage? There's a section in the ADC section of the reference manual that explains how to do it.

Posted on September 04, 2008 at 07:13

I'm personnaly using a STR710FZ2, and I have quite the same question.

In the Reference Manual (Rev1, July 2007), there is a figure (figure 74) which gives the ADC output value versus the input voltage. There are 2 constant mentionned, ADC_DATA(0v) and ADC_DATA(2.5v), which are the results for 0v and 2.5v inputs respectively.

On this figure we can see that thoses values are not 0x800 and 0x000, as I first expected. They don't even seem to be close to it (but the figure isn't probably to scale).

For now, I assume those values are 0x800 and 0x000, which leads me to a 0.1v measurement when I ground the pin, and a 2.1v measurement when I put 2.5v on it.

On the contrary, when I left the pin open, I read 1.25v almost exactly.

Does all this mean we have to apply external voltages on the ADC pin, read the values, and use them for calibration ?

I don''t have needs for a very high precision on this ADC. Are there maybe some ''average'' values for ADC_Data(0v) and ADC_Data(2.5v) available ?

If there isn't a way to go without calibration, could the input/output buffer structure be used to apply a 0v (throught a weak pull-down maybe) ?

Thanks !

kleshov
Associate II
Posted on September 04, 2008 at 11:59

Well, as the manual says, you have to calibrate the ADC. The manual also explains that as many as two different bandgap reference voltages within the ADC are inaccurate (+-5% variation). Assuming the two bandgap references are independent, you'll have to make at least two measurements to calibrate the ADC.

There is another cause for concern. The forum topic here

http://www.st.com/mcu/forums-cat-4734-17.html

suggests that you have to re-calibrate the ADC after each power-up.

Well, I'm not putting up with this nonsense. I'm using an external ADC.