Ground noise from MAX232

[123jack]

I wasn't sure where to ask this - I hope this is ok.

I've not used a MAX232 before and was wondering about noise levels.
Obviously the charge pump is going to create some noise but I'm
seeing +200mV -50mV oscillation on the power/ground lines.

I was wondering if this is normal?
I have tried extra decoupling and keeping the caps close to the chip etc
but nothing seems to help. The 5V supply is clean offload and I've tried the chip
in isolation on breadboard with the same results.

I've tried 3 different versions of the chip - 232N 232CPE and 232NG

Does anyone have any experience or comment on this?
If it is normal is there a standard way people clean it up?
Are the 3v surface mount versions much cleaner?

cheers

jack

 

[ayhampic]

hi,
I have the same problem ..

I 'am using PC --> Serial Port --> Max232 --- > MicroController ---> Power Transistor ---> High Dc Motor Current .
the problem starts when I run the motor at its full power so I noticed that noise, First I solved it by putting A 100ohm before the GND of serial cable but the more current you drive the more resistance you need and that is not a convenient solution , Later I used an optocoupler and everything is ok since that .
I will appreciate your ideas if any .
regards .

 

[gabexhun]

Hi,
I had similar problems with TRS/MAX3221 (it's the single channel version of MAX232).
The charge pump was the problem. This particular type has a two stage arrangement, the first stage boosts up the Vcc to 5.5V, and the second stage inverts it to -5.5V.
The first stage is a simple charge pump booster, and that's where the problem was coming from. This booster stage is a fixed frequency, fixed (50%) duty cycle, burst mode converter. It means it runs in bursts to recharge the cap the at V+.
At my design, the burst were short with long intermittent waiting time, but during such a burst, the IC took over 100mA (if I remember correctly). These powerful burst load caused ringing in the supply lines.
I figured, that if I slow down the charging of the flying cap, I will have longer bursts and shorter waiting times, but with proportionally lower input current amplitude. I simply added a 47 ohm resistor in series with C1.
I checked however, if the IC can output enough power to drive a worst case load on the RS232 TX line, with the lowest Vcc that can occur. I found that even this load was not so severe to make the burst/waiting time ratio more than 70-80%. (I deliberately didn't use "burst duty cycle" to avoid confusing it with the charge pump's fixed 50% duty cycle).

Even with this trick I didn't fully succeed. I needed to increase the Vcc decoupling to 4.7uF and place a 22ohm resistor in series toward the rest of the power line.

Good luck, pls. report if it works for MAX232 too.

Gabor