"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

  • 0
    •  Analog Employees 
    on Sep 24, 2013 9:46 AM

    Hi Marcos,

    The input voltage range of the OP42 looks like it can only accept voltages within 4V from the rails. Another thing is that since you're using an inverting configuration, you would expect your output to go from 0 to -1.5V. The output can't get there because your supplies are limited to ground. Can you possibly use dual supplies? This would solve both the input voltage range and the output problem. It should work with +/-5V supply.

    If you can't provide a negative supply and you just want to learn how the op amp works, you could do a non-inverting configuration -- that is, have the same feedback circuit but tie one end of the 10k resistor to ground and apply the input to Vin+ instead, as in the figure below. You'll need to offset your input to about 3V for the amplifier to give you an output, and use a higher power supply, maybe about 10V.

    Try this out and let me know if it works!

    Kris

  • 0
    •  Analog Employees 
    on Sep 24, 2013 9:51 AM

    Hi Marcos,

    Harry's got it right, I stand corrected on the supply voltage. The minimum you can use is a +/-8V dual or a 16V single supply. Otherwise, it should work as you intended!

    Thanks,
    Kris

  • 0
    •  Analog Employees 
    on Sep 25, 2013 5:55 AM

    Hi Marcos,

    The part should be able to handle +/-15V. If your part is getting hot, you might have a short in the circuit. What power supply are you using? It's always a good idea to check the current on the power supply to see that you're not drawing too much - about 6mA is expected out of the OP42. Try double-checking your connections just to make sure you're not pulling any power pins or the output to ground. Also check that you have the feedback circuit at the Vin- pin.

    If you have these things checked out, with either the inverting or the non-inverting configuration and using dual supplies (stick with +/-12V or +/-15V as Harry suggested), the best way to start out would be to input a DC voltage and see if it's being amplified correctly.

    Let me know how it goes!

    Kris

  • Hello Harry and Kris,

    Thank you very much for your quick answers. I have tried as you suggested, but there is still something that I am doing wrong.

    My connections now are (to the best of my knowledge):

    1 (NULL) -> Not connected

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

    3 (+IN)    -> signal

    4 (V-)      -> 0

    5 (NULL) -> Not connected

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

    7 (V+)     -> ~10V

    8 (N.C.)  -> Not connected

    This is the configuration that you suggested as a non-inverting amplifier. When I input a square signal which oscillates from 0.1V to 0.9V I obtain another square signal which oscillates from 2.8V to 7.3V (I was expecting 0.2V to 1.8V).

    I have also tried using V- = -15V and V+=15V (properly grounded) since the datasheet said maximum difference 40V (and this is the only negative voltage that I have), and I think I have fried the opamp: the output was constantly ~-13V and it got really hot. I have other one but I wanted to ask you before trying anything else.

    Thank you for your help,

    Marcos

  • 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