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# "hello world" circuit with op42

Hello guys,

Sorry for this post, I am guessing it is extreamly basic but I just cannot get around it.

I am using an op42, The connections are:

1 (NULL) -> Not connected

2 (-IN)     -> through 10k to (6) and through 10k to signal

3 (+IN)    -> GROUND

4 (V-)      -> GROUND

5 (NULL) -> Not connected

6 (OUT)  -> through 10k to (2) and output

7 (V+)     -> ~5V

8 (N.C.)  -> Not connected

This is the simplest inverting cicuit that I could make. I was just playing with the op to learn how to use it. The signal is a square function from 0 to 1.5V.

The output of that configuration, I was expecting to be the inverted signal, nevertheless, what I obtain is a square signal with really small pk to pk and with an offset of 1.5V.

I have also tried with other resistors pairs (100R and 100R) and what I get is a DC voltage with the input signal on top.

Searching the web, they recomended to use a compensating resitor (with resistance equal to the parallel equivalent of the other two resistors) between (3) and GROUND, but this did not change anything at all.

Do you have any idea of what I am doing wrong?

Thank you

JMGL

Parents
• Hello,

I tried again the non-inverting circuit with V+ = +15V and V- = 0V as shown in the scheeme suggested by Kris. The results still were not as I expected: the square wave 0V to 1.5V turned into 3V to 10V (the expected gain was 2).

My DAq can only stand +10V and it was saturating. I was worried about the voltage that it was exposed to (I did not want to break it) so I decided to get another opamp which could work at 0 to +5V to be on the safe side with the DAq: instead of increasing the voltage, I went for the coward's option .

Using this new opamp with the same non-inverting circuit the results were as expected. I cannot understand what I kept doing wrong with the other one.

Thank you very much for your help, it definitely guided me in the right direction. I spent the whole day yesterday trying to use compensating resistors and other solutions which had nothing to do with the real problem: the supply voltage, which I would have not find if you had not pointed out.

Cheers

Marcos

Marcos

Reply
• Hello,

I tried again the non-inverting circuit with V+ = +15V and V- = 0V as shown in the scheeme suggested by Kris. The results still were not as I expected: the square wave 0V to 1.5V turned into 3V to 10V (the expected gain was 2).

My DAq can only stand +10V and it was saturating. I was worried about the voltage that it was exposed to (I did not want to break it) so I decided to get another opamp which could work at 0 to +5V to be on the safe side with the DAq: instead of increasing the voltage, I went for the coward's option .

Using this new opamp with the same non-inverting circuit the results were as expected. I cannot understand what I kept doing wrong with the other one.

Thank you very much for your help, it definitely guided me in the right direction. I spent the whole day yesterday trying to use compensating resistors and other solutions which had nothing to do with the real problem: the supply voltage, which I would have not find if you had not pointed out.

Cheers

Marcos

Marcos

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