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Subject: Re: Circuit that combines 2 x 8-bit ADCs to get 1 higher resolution ADC

Date: 04/27/00 at 7:45 AM
Posted by: Ray
E-mail: rerogers@gte.net
Message Posted:

In Reply to: Circuit that combines 2 x 8-bit ADCs to get 1 higher resolution ADC posted by Yeu Sheuan Lee on 04/27/00 at 4:45 AM:

Sorry but the internals are such that you won't get higher accuracy, but you can improve
the precision. This technique is called dithering.
To be more explicit:
Accuracy means that the absolute reading that can be refered to the NIST (NBS) or standards in
Paris.
Precision refers to the ability to have stable readings after some initial unkown errors are
allowed.
Dithering is the process of adding a signal (or noise) to the input of the A/D and then removing
it after the conversion. I prefer using a ramp on the input and then averaging over one ramp
period. In order to be succesful you must have a "autozeroing" function that resets the system
whenever the ambient temperature or aging moves the A/D converter thresholds beyond
acceptable levels. I have done it and the process is totally amenable to engineering
analysis but does require a lot of carefull work to get it correct.
Also there is an intergated dual slope converter (minus integrating cap) somewhere that
provides variable resolution and speed. If you are interested I will look it up.
I hope these comments are not to far off of the intent of your question. There is another
technique that will offset and amplify the conversion window; providing a virtual high
resolution A/D. This also requires carefull design but will work.
Have you look at the new Linear Technology converters? They are great.

Ray


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