changing output voltage

Guest · Posted: Mon Mar 12, 2007 3:20 pm

I'm working with the 16f877 and am looking for a quick way to set the output voltage level between 0 and 5 volts. Does anyone know how to do this or point me to some example code?

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

See AN538 from Microchip: Using PWM to generate an Analog Output.
http://ww1.microchip.com/downloads/en/AppNotes/00538c.pdf
The schematic on page 2 shows an Opamp powered by +/- 10v.
I don't think you need that. I think a single-supply Opamp running
from +5v and Ground should work.

Another article:
http://www.tigoe.net/pcomp/analogout.shtml

Ttelmah · Guest

Directly, you cannot do this. The PIC, only has _logic_ outputs. However you can get close, using one of the PWM pins, feeding into a simple resistor/capacitor integrator. Provided your load is quite a high impedance, and the resistor chosen is large, good results from perhaps 0.4 to 4.6v can be obtained.

Best Wishes

Humberto · Posted: Mon Mar 12, 2007 7:30 pm

You can drive a digital potentiometer (connected between Gnd and +V) using a PIC.
In the potentiometer cursor tap you will get an analog output in the range of 0 to 5V,
if you an need an output with more current and lower impedance, it is a matter of
connecting a transistor or an unity gain op amp in voltage follower configuration.

Humberto

jecottrell · Joined: 16 Jan 2005 Posts: 559 Location: Tucson, AZ

If you do use a Digipot read the spec sheet for the current limits on the taps. Microchip parts a 1mA max on the center tap. That's bitten me several times. Now I remember to use a buffer.

John