12 Bit ADC?

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Hi

I was wondering if there are any PICs with built in 12 Bit ADC?

Thank you
a.

[FvM]

Many dsPIC30 and PIC24H devices have 12 Bit ADC.

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Hi FvM

Thank you for your reply.
Does the PICC PCWH program these chips?

Regards
a.

[FvM]

You need PCD respectively PCWHD compiler.

[PCM programmer]

Microchip makes many 18F-series PICs that have a 12-bit A/D.
See the Microchip Advanced Part Selector at the link below:
http://www.microchip.com/maps/microcontroller.aspx
How to use it:

1. Near the top, you will see 3 tickboxes. Uncheck the boxes for
16 and 32 bits, so only the 8 bit box is selected.

2. Go to the left-hand column, about 1/3 of the way down, and in the
row where it says "A/D Bits", set it to "12". The screen will automatically
update.

3. Now look in the top center at the scroll box called "Search Results".
It lists a lot of 18F parts that have a 12-bit A/D, for example the
18F4523.

4. You can click on the part number in the scroll box and the screen
will update with the specs of the PIC shown below the scroll box.

It would be much better to use an 18F part. You already have the
compiler. The PCH compiler is currently much more stable than PCD.
Do a search on all of FvM's recent posts to confirm this. I strongly
suggest using PCH at this time.

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Hi PCM Programmer and FvM

Thank you for your replies- they have been most helpful.

I am currently using the PIC18F2550 with 10bit A/D.

I see that the PIC18F2553 is exactly pin compatible to the 2550, with 12 bit A/D. That is GREAT!

So is it just a matter of changing the code to:
#include <18F2553.H>
#device ADC=12

or are there other issues that I have to worry about when doing 12bit A/D?

Thank you
a.

[PCM programmer]

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Hi PCM Programmer

The compiler version is 4.067.

The compiler does not complain if I put in ADC=12.
It does not complain if I put in ADC=16 either.

Is 12bit A/D as noisy as the 10bit i.e. I usually get approximately +/-1bit noise. Will the 12bit give +/-1bit or many more to match the noise of the 10bit A/D?

Thank you
a.

[Ttelmah]

You need to work out where most of your 'noise' is coming from.
The PIC ADC, has a huge number of different noise sources.
Largest for most people, is the Vref. Using the supply as Vref, you would be 'lucky' to get down to one bit of noise. Switching to an external Vref, and ensuring that this is stable, and well smoothed helps a lot. You gain even more, if you use Vref+, and Vref-, and connect the Vref- to the ground used for your analog circuitry. Great care is needed in design. For example, you can use a separate 'analog' ground plane, but then you potentially introduce new problems, depending on where the plains join, and how signals are routed round them. However using a separate plane for analog, is probably necessary if you have any high power circuitry involved. Then, with the analog gound connected to Vref-, and using an external Vref, use the INT_AD, and sleep. What you do, is to actually stop the processor while the ADC conversion is performed, and have it awaken automatically when the conversion is complete. Without this, the noise from the processor clocks, and any I/O being performed, probably accounts for a large part of your +/- one bit.
The minimum Vref span, increases to 3V, for the 12bit converter to give 12bit accuracies.
Unfortunately, getting a 'true' 12bit, or more of repeatable linear ADC accuracy is not as easy as some people may think. Most of the higher bit count ADC's, unless very expensive, are _audio_ ADC's, and do not give true repeatable linear bit counts. For instumentation work, you need to be using very careful analog design and component selection all the way to your ADC, and at the ADC itself. Component costs rise significantly, you may be surprised at how much noise simple components like resistors actually introduce. 12bits is still reasonably 'easy', with but even here, care is needed, and going beyond this, becomes increasingly difficult. It is quite common to see units being sold offering '16bit resolution', yet do a full analysis, and find that the real useable resoution, is only in the order of perhaps 13bits....

Best Wishes

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Hi Ttelmah

Thank you for your thorough reply. I will get the relevant PIC chips and start working on the 12 bit A/D.

I appreciate your help
a.

[Guest]

So does CCS support the ADC=12 or do you have to use the ADC=16?
Also when setting up the Vref+ and Vref- do you connect the pins to vcc and vss respectfully or do you also have to setup in software?

[FvM]

If the manual or the device file comments don't make it clear, check the register settings and data processing in the list file. Everything is at your fingertips.