[SOLVED] Voltage Follower For Max232 output

[kemalkemal]

Hi all.
As you can see from the circuit schematic i try to send some data from PIC to PC(serial USB converter).


I used a max232. When i made the connection i saw that PIC-UART signal levels attenuate. So i decided to use a voltage follower after max 232 before serial-USB converter.
Although i tried many opamps i got the same result. Without load circuit behaves OK. I could get output what i gave from input of follower. But when i connect the serial USB converter (as a load) the output of voltage follower decays. Interesting thing is it can follow the negative side of the signal but cannot the positive side. BTW i supplied the opamp with +-14 volts (at my all trials).

This is my signal (from the board i designed) without load.


And this is what i got from output of follower after i connected the serial USB converter.
You can see the negative side OK but there is problem at following the positive side.


In order to see different opamp behaviours i used a signal generator and same simple voltage follower circuit.
LT1112. It is apparent that load affects both input(blue) and output(yellow).

opa07.


At all trials i got noise at positive side. Is there any thing you can suggest?

 

[FvM]

Is there any thing you can suggest?

Abandon the useless OP buffers. All levels will be in required range without it.

 

[kemalkemal]

It seems LT1112 is giving the best result but not enough. There must be something i am missing or doing wrong.

 

[FvM]

May be you are just shorting the output, e.g. using the wrong cable and connecting TX to TX? PC or USB-to-RS232 bridge has TX on pin 3, need a cross cable to connect to your circuit.

 

[kemalkemal]

Thanx FvM for the reply. But TX pin; the one in the middle so it is unlikely to fool. But i checked it again:
T1OUT of max232 (pin14) ---> to the noninverting input of opamp. And output of opamp(also shorted to inverting input) --->to the 3rd pin(TX) of the DB9 connector. Besides the same abnormality happens in my demo circuit which i set on the breadboard. I am suspicious if USB-to-RS232 bridge is a complex load for the voltage follower. Or am i using totally wrong OA's for a voltage follower?

Edit: And once again edaboard saved my life thanks to FvM :-D After just a minute later sending the above reply i realized that TX of the sender and TX of the receiver is face to face :bang: