Analog input to a PIC with built ADC

[fuseshut]

WHen hooking up a 0-5VDC analog control input to a PIC uC is it advisable to connect the signal input to a buffer stage and the a 10Hz low pass filter, then connect to A0 of the PIC?

 

[bill318]

If the analog control input is coming from outside the PICs own circuit board and may even use a different power supply, then yes it is a good idea to provide signal conditioning prior to applying it to any microcontroller. This can take the form of a buffer amp, differential amp, opto-coupler ...

Any filtering really depends on what the signal is used for and the environment the circuit must live in. Without more details, it is difficult to properly answer this question.

 

[itsmoked]

Generally you do not need buffer stages. But often a resistor with a cap (filter) makes sense. Remember that if you use a cap larger than about 2.4K you MUST use a cap.

 

[IRstuff]

The only reason you would normally use a filter is to prevent aliasing.

TTFN

 

[itsmoked]

Well that made a hellofah lotta sense!

Meant:

Remember that if you use a resistor larger than about 2.4K you MUST use a cap.

 

[coconutalley]

If your goal is to make a robust design, I say YES. At the very least I would consider putting a clamping device: zener or diodes and some resistance at the input to protect the PIC from spikes [surges & ESD] etc and possible overcurrent. Under normal conditions, if the 5V is well regulated, then you don't need the protection but what happens when something goes wrong with the 5V line?
The other thing: the 0-5V is probably not the real effective range of the ADC, so you'll need to scale it to below 5V.
An analog front end filter also helps a lot by rejecting frequencies that you'd need to sort out digitally.

If your goal is make the cheapest PIC controller, I'd say don't put anything except may be a resistor but remember that you might get a call later from some unhappy people.

 

[Madcow]

http://techtrain.microchip.com/webseminars/WebSemCListArch.aspx

Try this link and watch the seminar on "Do I Filter Before, After, or Never?"

I usually get away with just an rc.

Maybe a diode clamp to vcc because the analog pins seem
to be more susseptable to over voltage.

 

[itsmoked]

Let me point out that PICs have monster diode clamps built into them. You can hook 120Vac to them if you use the appropriate dropping resistor.