ADC problem

[sfouques9]

Posted on March 11, 2005 at 03:20

ADC problem

[sfouques9]

Posted on March 07, 2005 at 06:22

Hello,

I have a pb with my adc conversion. Here is attached the schematic.

When Vin is under 4V I have a good conversion. But when Vin is more than 4V, there is a saturation at 4.2V. Here are the measurements : Vin= 1V ADC measurement = 1V

3V 3V

4V 4V

4.3V 4.1V

4.5V 4.13V

5V 4.2V

I checked Varef and I have 4.998V. I've also tried with minimum ADC frequency and maximum sample time so the condensator can charge itself during the maximum time possible.

So I don't understand why I can't measure a voltage more than 4V :-W.

I have the same pb with all my boards, so I don't think it's a component (ST10F168) failure.

Does anyone has an idea??

Thanks,

Steph

________________

Attachments :

ADC_in.doc : https://st--c.eu10.content.force.com/sfc/dist/version/download/?oid=00Db0000000YtG6&ids=0680X000006I0Nc&d=%2Fa%2F0X0000000bYo%2FJfWet_rTbQDNp5ZIv2WndTFgPt3DB44mlK7mlJaTnws&asPdf=false

[najoua]

Posted on March 09, 2005 at 06:01

Hello,

Try to remove the Zener diode and verify if the problem persists.

[sfouques9]

Posted on March 09, 2005 at 07:00

thanks for your reply... but I tried without no Zener and no Capacitor, and I have the same problem.... The fact is, I measure 5V at the AN in!!!!!!! So I was thinking about a bit to change in a register..........

The pb still to be solved......

@++,

Steph

[sfouques9]

Posted on March 11, 2005 at 03:20

Hello,

I finally found where the pb came from : the op amp I was using to generate the signal to convert wasn't a rail to rail and had a 4.3V saturation.

So, nothing to see with the ST10!!!

Bye,

Steph.