Isolating serial noise

You can't draw enough current from a PC serial port to power a MAX232 - typical MAX232 power consumption is 8mA.

MAX232 datasheet:

Good article about taking power from the serial port:

Why don't you forget about the opto-isolators and just power the MAX232 from the same supply as the PIC?

The PIC should not be disturbed by noise from the PC; and any noise-sensitive circuitry will likely pickup noise from the PIC too!

MAX232 -

MAX232

too!

ic's

from

see

How many milli-volts are we talking about? I wonder if you have a ground-loop problem? Check your earthing.

circuit

Could you post a schematic and/or photo of your layout?

You might be able to do this without the MAX232: drive the opto LED, via a series current-limiting resistor, directly from the PC serial port. Going the other way, if you can independently control DTR and RTS, you could do this:

DTR (+12V)------[1K]------[opto-transistor]-----RTS (-12V) Connect RxD to the junction of the 1k resistor and the transistor.

yep thats the article i followd, the pic a/d does pickup noise when plugged into the serial port, verified by readings and a scope, all ic's have filtering caps ad is also filtered, if i unplug the serial gnd from the pic board the readings are clean as soon as i plug it back in i see a wave......

unless is there a way to split off my pic power supply for that circuit instead of the serail bu that would not cause noise??

yep i do have a ground loop problem, this was the solution i was thinking about to solve it.....

the max setup is currently like

is there another way to get rid of the ground loop?

Chris

Andrew Holme wrote:

Google for "ground loop."

You could possibly place a "star point earth" at the PIC GND terminal.

Do you get noise if you only connect the GND wire and leave the other RS232 signals disconnected?

You mentioned them in your original post, but you don't show any power supply decoupling caps in that circuit.

A photo might be more helpful since a schematic doesn't show if you have a layout problem. What is the amplitude of the noise at the PIC A/D input?