OP AMP input error (LM7121)

[Jaunutis]

Hey,
I'm making transient load device, and my op amp is not working properly.
Why my + input is 300mV, when it should be 0mV? 300mV comes from the op amp.

Thanks

 

[JonSnell Electronic]

Did you use a resistor to tie the +input to ground? If so, what value did you choose.
The LM7121 is a high speed video driver.

 

[Jaunutis]

This is my op amp circuit. Op amp + and - is opposite. Why do you think there is 300mV on 3 pin?

 

[abraxalito]

Very likely the reason is you have oscillation. A wide bandwidth opamp driving a MOSFET gate means there's significant capacitive load. Ah and now I notice your snubber has a 10uF capacitor in series with 2R4 so you can be fairly sure you have oscillation even without the MOSFET in circuit. How did you arrive at those values for your snubber network (R8,C8)?

 

[JonSnell Electronic]

Pins 2 and 3 are reversed in your drawing. Pin 2 is -ve input not +ve

 

[Jaunutis]

http://cds.linear.com/docs/en/application-note/an104f.pdf
I used fig 6. from this article.

 

[Jaunutis]

Pins 2 and 3 are reversed in your drawing. Pin 2 is -ve input not +ve

I know it's reversed. It doesn't change anything.

 

[AndrewT]

http://cds.linear.com/docs/en/application-note/an104f.pdf
I used fig 6. from this article.

Fig6 and none of the others have pin numbers associated with the inputs.

You appear to have invented some pin numbering.

Did you look at the chip datasheet?

 

[Jaunutis]

I looked at the datasheet and it's OK. all pins are correct.

 

[abraxalito]

http://cds.linear.com/docs/en/application-note/an104f.pdf
I used fig 6. from this article.

That article shows an LT1210 whereas you've changed that. What makes you sure your substitution isn't going to break the circuit? The specified part has provision to improve stability into large C loads (and the capability is being used, along with its substantial drive capability) which yours does not.

 

[Ketje]

I looked at the datasheet and it's OK. all pins are correct.

There are two datasheets involved giving diff. nbrs.
Can't you adjust R11 ?
Mona