analogue port in 16f877!!!

guest!! · Guest

hi everyone,

i wanted to ask two things.

1) i want the reference voltage of the A/D in 16f877 to be from 0-->1 volts. so i applied 1v to port A3 and included the command

kender · Joined: 09 Aug 2004 Posts: 768 Location: Silicon Valley

If your A/D is configured for 10bits, does 255 seem like a correct value - about 1/4 of the range.

Just in case, could you post your *.h file and the code where you make a call to read_adc()?

You can not configure the A/D to do 16 bits, but you can over-sample and average, which will effectively increase the resolution of the A/D. If I remember correctly, you need to oversample by a factor of 4 for each extra bit. So, to do 16 bits, you'll need to oversample by a factor of 64.

Ttelmah · Guest

Two comments.
First, oversampling, only 'works', if the noise on the reading is normally distributed, and the values actually 'read' have an exact relationship between the probability of taking the different values each 'side' of the real reading, and the actual voltage present. While, given normal noise sources it can work suprisingly well, to give an extra bit or two, it will not give an extra 6 bits of useable resolution, especially given the nature of some of the noise sources round the processor.
Second, one thing that may not be helping, is that the ADC reference voltage is below the minimum needed for the ADC to actually give it's design resolution. If you look at the AD converter specification, you will find that there is a minimum voltage required between Vref low, and Vref high, to give the quoted 10bit resolution. This is normally in the order of 2.5 to 3v. With 1v in, you will be lucky to actually be getting even 9bits of useable resolution.
True 16bit resolution, over a 1v range, will require a much more upmarket ADC, than the one present in a PIC, and some careful design. The 'high bit count' ADC's present in units like audio systems, do not actually give this sort of resolution. There will need to be less than 15uV, of total error, between the Vref, signal input etc., to genuinely offer this sort of accuracy...
One other obvious command is required, which is to select the channel to read. I might be inclined to suspect that the channel is not selected, and is perhaps reading an input that is high, and the ADC is in 8bit mode...

Best Wishes