Hi Yalda,
The short answer is, you can't. You can make it seem like more bits by oversampling, but that will introduce errors as well. The best thing to do would be to add an external ADC, like LTC1298 which interfaces with SPI (3 lines from your micro.) Hope this answers your question, but if you have more, just ask.
Best wishes,
Robert
Hi Yalda,
You asked for more info, and I'm giving it here, in the hope it will help others, as well. Basically, using the LTC1298 involves this:
(1) Activate CS by taking it low.
(2) Send (shift out) configuration bits to the LTC1298.
(3) Read (shift in) the 12-bit measurement from the LTC1298.
(4) Deactivate CS by taking it high.
The data sheet show these processes, and how to wire it to your controller. You should download the data sheet for the LTC1298 and look through it. You can find it here: https://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1155,C1001,C1158,P1445,D2760
What language are you using to program your micro? I'm not too familliar with the AVR, I'm more into PICs, and it has been very easy to use this chip with PIC.
Hope this helps, but if you have more questions, just ask.
Regards,
Robert
yalda_1749,
You can get more resolution by adding noise to the analog input, and taking the average of multiple samples of the A/D output. Keep in mind, however, that this technique improves the resolution only, not the accuracy.
Regards,
Kral