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OpAmp is generating a lots of noise

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piotr_jan

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Hi Guys.

I am working on my hobby project - an oscilloscope based on ST Nucleo 446RE board. Everything is working fine and the project is progressing quickly. Unfortunately I am absolutely useless when I have to design something analogue. I need fast input buffer. In first prototype version I used 10MHz 10V/us OpAmp, and everything worked as I expected.
The Input signal 31.3KHz PWM signal generated by Atmega, divided by 2 (amplitude) using a resistor divider.
Here is the original signal without an input buffer:
https://youtu.be/EtVqZZmy5I0
Today I have received fast OpAmps with much higher slew rate (about 200v/us) and I hoped that the signal will have more square shape. And it actually
has, but I got a lots of noise form the opamp:
https://youtu.be/7ieETZmmcng
Inverting input is connected directly to the output. Single supply +5V.
What is the problem. Any help appreciated.

Cheers
Peter
 

My guess (without seeing the schematic) is that you are over-driving the op-amp and that it is ringing/oscillating when being driven into saturation.
Susan
 

I dont think so. Actually maximum signal voltage is 2.7V, Vcc is 5V. It is a Rail-To-Rail
 

Your fast opamp has no part number for us to see if it is allowed to have a gain of only 1. Maybe it is de-compensated and has a minimum allowed gain of 5.
Edit: Maybe you built the circuit on a solderless breadboard and there is messy long wires all over the place?
 

Oh I forgot : CLC4007ISO14
It was lete :)
 

Hi,

long wire in a breadboard while connecting can cause noise pickup. so try it in a pcb. the noise will get reduced.
 

It's not clear to my why your application requires a 100 MHz class OP. There are probably more convenient options to generate the intended output signal.

Although the CLC4007 datasheet mentions a G=+1 configuration, you can see from the "Non-Inverting Frequency Response" curve that the phase margin is rather small. Consider that the characterization measurements have been made with perfect power supply bypassing according to the datasheet. If you can't provide a similar low inductance bypass capacitor wiring, the OP is likely to oscillate in the 100 - 200 MHz region. Oscillations can be also brought up by connecting a capacitive load (e.g. one meter coax cable) to the amplifier output without isolation series resistor.
 
I have added a 5.6k resistor and now it looks much better.

it is actually soldered. The breadboard version works much better pajak.jpg

 

The resistor is in the loopback. I don't have the schematics. It is just simplest possible voltage follower.
When it is welded - it works much better
proto.jpg

And it works not too bad actually.
 

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