Thanks to everyone that responded to this question with such clear explainations. I'm sure that it helped a lot of people on the piclist to also understand the technique a little better. Special thanks also to Hartono Darmawaskita, an applications engineer at Micochip who also took the trouble to write to me in person with an explaination, quoted below: > > Werner, > > I am an applications engineer at Microchip. This message was forwarded > to me. > > I am trying to explain what dithering is using an example. Let's say > you are reading a DC signal using an 8 bit A/D. The A/D input range in > this example is 0 to 2.5V, thus resolution is 9.77mV/bit. If your > signal is nice and quiet, the A/D is not going to change it's reading > unless the signal crosses the bit boundary, which is +/- 9.77mV away > from the current value. AC coupling the 100mV triangular waveform onto > the signal moves your signal up and down evenly from the actual value, > so the A/D can read different values around the actual signal in a > predictable manner, in this case 10 readings above and 10 readings > below the 8 bit current value. Averaging the A/D readings over an even > multiple period of the waveform will then give you more than 8 bit > result. Without the triangular waveform, the A/D will at the most give > you only 2 values for the signal. > > Regards, > Hartono > > > > ______________________________ Forward Header __________________________________ > Subject: Improving A/D resolution by dithering technique > Author: Werner Terreblanche at Internet_Exchange > Date: 12/9/96 9:44 AM > > > I received an application ideas book called "Dream Machine Reference > Book" from Phillips. In it they talk about a method of using > dithering to effectively get 12bit resolution out of an 8bit A/D. It > consist of using PWM configured as a DAC to feed a triangular 100mV > waveformthat is AC coupled, back to the original input. The input is then > oversampled and by averaging the number of readings and the combined > dither/input voltage, the technique achieves an effective 12-bit > resolution. Don't ask me how it works, I just wrote down what I > read in the description. > > It sounds like something which could be very usefull when using devices > like the PIC16C74. However, I can not quite understand how the > teqhnique actually works. Does it make any more sense to anyone else > out there? > > Regards > -- > Werner Terreblanche > wterreb@plessey.co.za (work) OR werner@aztec.co.za (home) > Plessey SA, PO Box 30451, Tokai 7966, Cape Town, South Africa > > Check out my Variometer Kit on: > http://www.aztec.co.za/users/werner/variokit.htm > ------------------------------------------------------------- > -- Werner Terreblanche wterreb@plessey.co.za (work) OR werner@aztec.co.za (home) Plessey SA, PO Box 30451, Tokai 7966, Cape Town, South Africa Check out my Variometer Kit on: http://www.aztec.co.za/users/werner/variokit.htm -------------------------------------------------------------