Hello guys,
I Have 2 equal circuits (image) one circuit input is 1Mhz square wave and the other 20MHz.
I measuring between points A and B.
At 1Mhz output i get sware wave but with little spike.
On the 20Mhz output i get weird and big spikes
On the rising edge and the falling edge.
How can i "clean" the signal?to get clean square 20MHz and 1Mhz wave.

 

What part number is the opamp? It could be just too slow for the task. Are you using a 10x probe? What is the bandwidth of your scope and the probe?
You can also try removing the ground lead from the probe and use a spring ground lead instead, or a similar piece of wire like this http://feichti.net/impy/img/spring_ground.jpg to get lower inductance on the probe.
A 20MHz square wave has lot of harmonic frequencies, you should typically have at lest 10 times the bandwidth on your oscilloscope i.e. 200MHz to be able to see a sort of clean square wave.

 

Also what is the amplitude of that square wave on the output? You might try getting a screen print to a usb disk from that oscilloscope, instead of using a camera.

 

What part number is the opamp? It could be just too slow for the task. Are you using a 10x probe? What is the bandwidth of your scope and the probe?
You can also try removing the ground lead from the probe and use a spring ground lead instead, or a similar piece of wire like this http://feichti.net/impy/img/spring_ground.jpg to get lower inductance on the probe.
A 20MHz square wave has lot of harmonic frequencies, you should typically have at lest 10 times the bandwidth on your oscilloscope i.e. 200MHz to be able to see a sort of clean square wave.

Hi,
Thank you for your replay.
My scope b.w 100mhz and probe is x10
I have tested the wave directly from fpga without connecting to opamp and it looks good.
The opaamp is opa650 it's have 500mhz b.w for unity gain amp.

 

From the datasheet it looks that it is not slew rate-limited at that amplitude. Check that you have proper bypassing on the power supply of the opamp, with as short traces as possible, but I think the main issue is the way you have the probe connected, try the ground spring and see how it changes. Also you could check if the supply rails are stable, just to be sure.

 

Probing techniques -

http://www.tek.com/dl/Probing%20Tec...ge%20Measurements%2051W_60161_1_HR_Letter.pdf

https://www.keysight.com/upload/cmc_upload/All/Probing-6-Hints-Scopes-Dec12-2007-webcast.pdf

https://www.analog.com/media/en/technical-documentation/application-notes/an47fa.pdf


Regards, Dana.