Q: Pc Causing noise on a circuit

I have a few op amps driving a ph probe inital value .059mv/unit boosted
to 4 and offset by 2 volts ... anyhow the valies i read are correct, but
once i plug the serial from my pc to a db9 / max232 std config the
readings go off totally, the max232 is using 10uf/50v from power and to
ground taking pins 2/3 from db9 grounding shield and 5 to common, 7+8
jumped 4+6 jumped

Anyone have any ideas? it runs perfectly fine when the serial is
unplugged...

Thanks

 

Hi Chris,

....And the PC still running ?
Try ferrite beads on the leads from pins 2 and 3 close to the PC, it
might cure the problem.
A ferrite "tube" around the leads (like the bumps on eg. a monitor lead)
after the MAX232 might rid you of any noise not caught by the beads.

 

Optoisolators on the serial port pins?

 

Here's the weird thing came home last night my circit has been unplugged
all day powewd off the pc readings were ok, booted up everything was
fine, left over night, this morning readings are scrambled, any time i
plug in the serial pc on or not .... maybee a bad max232 ?

 

What uC are you using ? perhaps its an oscilator problem.
If your using a PIC16f628 for example, the internal osc. is not
reliable for serial communications, and will sometimes be okay,
sometimes not.
Make sure your using an external Xtal.

Alex.