adc input

[rino]

hi,
I'm very newbie with pics. I need to make a conversion from an analog input; i've easily done it with a test signal whose range was 0-5 volts, but for the real application the input will be around 100 mV. I guess i'll need to change some settings,but i don't know what i should do and how.
Hope somebody could help me,
tanx
rino

[dbotkin]

Well, partly it depends on the resolution you need. You could just multiply the ADC reading by, say, 50 if you don't need precise readings.

With Vref = Vdd = 5.0V, you will get ADC readings from 0 to 20 or 21.

With Vref = Vdd = 3.0V, you will get 0 to 34 or so.

With Vref = 2.5V, you'll get 0 to 41 or so.

You can use an op-amp to amplify your input signal for better resolution. You'll need to watch out for noise, proper grounding and all that, and may need some software filtering if the input is noisy or unstable.

It would help some to know what your requirements are - resolution, accuracy, that sort of thing.

[SherpaDoug]

You need an amplifier to bring your 100mV signal up to 5V, that is a gain of fifty. You could try to do the gain in software by just multiplying by 50 but you would loose about 6 bits of resolution. It is better done in hardware.

Look for information on "op amps" and "instrumentation amps"
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[Guest]

if the only way to avoid the amp means losing 6 bits, i think i'll buy the amp and check the noise.
thank you very much
rino