![\"ADC](\"http:\/\/embedded-lab.com\/blog\/wp-content\/uploads\/2014\/10\/ADC-internal.png\")
<\/a><\/p>\nTo begin with, there are some prerequisites. The very first thing is to realize the inputs of the ADC block (VINP and VINN) as like the two inputs of a regular op-amp. The ADC is differential in nature. Secondly there are several areas that need more than a few lines of \u201cATMEL\u201d explanation and several assumptions. We can\u2019t see the internal circuitry after all. Take things initially as if they are in black boxes and then slowly but systematically probe them until fully unveiled. This is somewhat a my-kind of learning technique. Thirdly a word of caution, an ADC channel doesn\u2019t necessarily mean a physical ADC pin as in traditional MCUs. In the XMega32A4U there\u2019s one physical ADC – ADCA<\/i><\/b> comprising of four virtual channels that can be connected to a number of signal sources both internal and external (physical pins). Lastly we won\u2019t be using 8 bit ADC conversions in our discussion anywhere as it is less precise and seldom used. However 8 bit conversions are faster than 12 bit ones.<\/p>\n Cons:<\/i><\/b><\/p>\n People who have dealt with instrumentations or analog electronics will certainly know what a differential amplifier is. Well basically it just amplifies\/passes the difference in signal between its two input terminals. Simply Vout <\/i><\/b>= m<\/i><\/b> x (Vinp<\/i><\/b> \u2013 Vinn<\/i><\/b>), where m<\/i><\/b> is the gain, Vinp<\/i><\/b> is the positive input and Vinn<\/i><\/b> is the negative input. In the XMega ADC block this differential mode is technically the same stuff. You have two ADC pins as inputs in differential mode and an optional gain stage. Typically use of\u00a0 differential mode is that for measuring current flowing through a shunt. However a few things are to be strictly followed:<\/p>\n <\/p>\n <\/p>\n <\/p>\n <\/p>\n This is what people use mostly. Unlike the differential mode it has both signed and unsigned conversion mode of operation. The only difference between these modes is to what the negative terminal of the ADC block is connected to. Apparently single-ended mode seems to be the easiest and simple mode of operation but trust me this mode is where people get their head blown off. It took several days for me to understand this mode completely. Before going on further please note all of the figures, tables and formulae are taken from Atmel\u2019s official docs and so nothing is based on imagination.<\/p>\n The key region of confusion of unsigned single-ended conversion mode is the \u2206V<\/i><\/b> offset. Its physical value is about 5% of the reference and so if the reference is 1.0V then its value is 0.05V. Remember what I said about differential amp earlier? If so then check the block diagram shown below.<\/p>\n <\/p>\n The negative input is connected to some sort of reference. This is the first piece of confusion and things get more complicated to realize when you go through the formula for unsigned single-ended ADC conversion, which is:<\/p>\n <\/p>\n Note that the formula and the block diagram don\u2019t technically agree with each other. Throughout all Atmel\u2019s technical docs I found the same stuff over and over again with no elaborate explanation. However what I found during my research is that the formula is consistent. I didn\u2019t find any answer to the missing VREF \/ 2<\/i><\/b> part. My first question was why it is connected to some sort of offset rather than zero. The answer is somewhat both easy and complicated. By having the ADC configured internally this way an XMega ADC pin connected to ground (0.0V) does not give 0 ADC count but about 200 ADC counts. I thought it was a hardware bug and that the ADC is not truly a 12 bits ADC but somewhat less than that. The more I researched the more I found it to be useful. If 0V in an ADC pin results about 200 counts then definitely a small negative voltage at that ADC pin will result 0 ADC count. According to Atmel this offset helps to detect zero-crossing or true 0V rather than something near to 0V like \u00b10.001. I found that the XMega ADC can, in fact, detect a tiny quantity of negative potential. Usually we don\u2019t use this feature with common ADC uses but in certain precision instrumentations we need to know this and there this offset plays its part. If you check XMega datasheet then you\u2019ll notice that ADC input voltage range spans from -0.1V to (AVCC + 0.1) V. Though this feature has its advantage in some certain areas it comes at a cost of reduced top range which about 95% of the reference source\u2019s value. If, for example, the internal 1.0V reference is used then the ADC can measure voltages ranging from about -0.05V to about 0.95V. Thus it is still measuring a 1.0V span with 12 bits precision though apparently it may not look like that at first experience. Fortunately MikroC PRO for AVR compiler\u2019s ADC library is based on this mode of operation and so be aware not to curse yourself.<\/p>\n In the signed single-ended mode VINN is connected to ground and it is quite a simple story. In the signed mode the result of an ADC conversion is always 11bits. The MSB is used for sign. However you cannot apply negative voltage to an ADC pin with respect to ground under any circumstances except what your device\u2019s datasheet states. You\u2019ll get 0 \u2013 2047 counts for 0V \u2013 VREF range.<\/p>\n <\/p>\n
\nKey Pros and Cons of XMega ADCs<\/span><\/h4>\n
\nPros:<\/i><\/b><\/p>\n\n
![\"pipeline\"](\"http:\/\/embedded-lab.com\/blog\/wp-content\/uploads\/2014\/10\/pipeline.png\")
<\/a><\/p>\n\n
\n
![\"ADC](\"http:\/\/embedded-lab.com\/blog\/wp-content\/uploads\/2014\/10\/ADC-Characteristics1.png\")
\n<\/a><\/p>\nDifferential Mode<\/h4>\n
\n
![\"differential](\"http:\/\/embedded-lab.com\/blog\/wp-content\/uploads\/2014\/10\/differential-without-gain.png\")
<\/a><\/p>\n![\"differential](\"http:\/\/embedded-lab.com\/blog\/wp-content\/uploads\/2014\/10\/differential-with-gain.png\")
<\/a><\/p>\nSingle-Ended Mode<\/h4>\n
![\"single_ended_unsigned\"](\"http:\/\/embedded-lab.com\/blog\/wp-content\/uploads\/2014\/10\/single_ended_unsigned.png\")
<\/a><\/p>\n![\"Formula](\"http:\/\/embedded-lab.com\/blog\/wp-content\/uploads\/2014\/10\/Formula-1.png\")
<\/a><\/p>\n
![\"single_ended_signed\"](\"http:\/\/embedded-lab.com\/blog\/wp-content\/uploads\/2014\/10\/single_ended_signed.png\")
<\/a><\/p>\n![\"Schematic\"](\"http:\/\/embedded-lab.com\/blog\/wp-content\/uploads\/2014\/10\/Schematic.png\")
<\/a><\/p>\n