OPA2675: Amplifier output noise

Hello SCH,

I am looking into this behavior for you; could you elaborate on your setup and use case for a high speed current feedback amp at 1Hz frequency input?  At such a low frequency the 1/f noise of the amplifier will dominate the overall noise.

Best,

Alec

Hi,

Our output requirement is to have a output of  5V, 1Hz square wave with less than 10ns rise/Fall time from 1Hz 3.3V square wave input. 

With calculation of Slew_rate = 2*pi*bandwidth*peakVoltage and Risetime= 0.35/bandwidth we concluded that we need an opamp with atleast  1100v/us SR.

Regards,

SCH

And load is 50Ohm

Hello Shradha,

Thank you for the updated information; it helps to know what features you require for your design.  I will do some analysis and simulation and report back to you with an update.  

Thank you for your insight. We will try with 402 ohms

Hello Shradha,

Please post an update with your results when you have them.  I or a member of my team will respond.

Best,

Alec

Hi,

Sorry we took long time to test. 

We changed the feed back network to 400ohms. 

Tested for 1hz, 1kHz, 1Mhz. Still we are getting same noise. Tested with 2 different load resistances of 400Ohms and 2Kohms. We are still getting noise in all the cases. Attaching images

1.  Output image for 1MHz input

2. Output image for 1kHz input

3. Output snap for 1Hz input

Hi Shradha,

this can happen when you directly touch the output pin or any other pin of OPAmp with the scope probe. Scope probes can show a nasty load capacitance and can fully destabilize the OPAmp. An isolation resistor is needed then as discussed in section 8.3.1.3 of datasheet.

Also use such a ground spring to avoid resonances caused by the ground wire (pig tailing !):

Another way to watch the output signal is to use a 50R cable in combination with a 50R termination resistance at the output of OPAmp and the 50R input impedance of scope.

Please show a photo of your setup and the layout.

Kai