Analog to Digital conversion precision

ElectricalNoob · Joined: 17 May 2007 Posts: 15

I just wanted to confirm that if I get an analog signal that works from 75mV to 100mV, then I would not be able to capture that signal properly unless I used an opamp to amplify the signal to the 0 - 5V range. If anyone has worked with such low mV values without amplification, please let me know.

Simeon

inservi · Joined: 13 May 2007 Posts: 128

Hello,

All depending on the speed and resolution you need.

For example, the LTC1298 (12 bits resolution, 60ms Conversion Time, 11.1 ksps) can operate with reduced spans to 1V. Then the resolution will then be 244µV.

It is maybe enough ?

Best regards,
dro
_________________
in médio virtus

ElectricalNoob · Joined: 17 May 2007 Posts: 15

Sorry, I should have mentioned that I was trying to do the analog to digital conversion with a PIC16F877A. So would it be possible for the PIC16F877A to have high resolution?

Ttelmah · Guest

First, I wouldn't amplify it to 0-5v. 0-5v, tends to imply using the supply as a reference, and this is _not_ an accurate solution. I'd suggest using a dedicated 2.5v reference.
Then, why be afraid of an op-amp?. The odds are, that one would be needed anyway, on _impedance_ grounds. - The PIC, requires a reasonably low impedance drive on the ADC inputs, and few sensors will provide this without buffering. - The PIC's ADC, driven properly, can be used 'oversampled', to extend the accuracy, and it is very possible to get perhaps another 2 bits, if the input is reasonably stable. 25mV span, is not a particularly low range, and if you amplify by perhaps 100*, then subtract 7.5v from the output (an easy operation, with the 2.5v reference, and half a dual op-amp), you will have a signal giving a good resolution from the PIC's ADC.
Now, if the signal has got a low enough impedance, you could potentially manage possibly 40 levels without an amplifier, but much beyond this is getting unlikely. What sort of resolution do you want?.

Best Wishes

ElectricalNoob · Joined: 17 May 2007 Posts: 15

Was looking at something around 0.1mV accuracy. But doesn't look like a PIC16F877A can handle that. So I'll just use op amps, not a big deal.

Simeon