Ripples in square wave output of LM7171 high speed OpAmp.

Thread Starter

mva007

Joined Oct 26, 2021
6
I am trying to change the voltage level of a 10MHz clock signal. For this I am using LM7171 as a non-inverting summing amplifier, to DC shift the input signal, with some positive gain to reach the required voltage levels. The circuit has been soldered on a general purpose PCB and is operating at +/- 5V.

Though the simulations on LTSpice showed no problem, my output on the oscilloscope is being rippled and distorted. The distortion is not very prominent at low frequency of 100KHz, but still present.

Can someone tell me why is this happening? Do I have to make changes in my circuit design or take a new different IC? Please suggest the way out.
rippled_LM.jpegIMG_20211026_150756655_HDR.jpgIMG_20211026_150924095_HDR.jpg
 

Ian0

Joined Aug 7, 2020
3,760
You have a peak in the gain at 50MHz, probably due to the input capacitance.

What is the input voltage? What output voltage do you need? What power supply voltage do you have?
I think a simple Resistive attenuator would do a better job, or perhaps a comparator.
 

Thread Starter

mva007

Joined Oct 26, 2021
6
Hi mva,
Welcome to AAC.
What are the source impedances of the clock source and DC offset source.?

E
I am producing the ac signal from a function generator in High Impedance mode and giving the DC signal from a lab bench power supply whose source impedance I do not know.

FYI I am probing using 1x SMA to BNC cables.
 

Thread Starter

mva007

Joined Oct 26, 2021
6
You have a peak in the gain at 50MHz, probably due to the input capacitance.

What is the input voltage? What output voltage do you need? What power supply voltage do you have?
I think a simple Resistive attenuator would do a better job, or perhaps a comparator.
Please see the attached schematic for the input and output voltages. As mentioned, I am operating at +/- 5V.
Will the resistive network work at such high frequencies? What about loading?
 

ericgibbs

Joined Jan 29, 2010
14,183
Hi mva,
I can reproduce the waveform shown in your post by adding a low-value capacitance load on the OPA output.
What is the OPA output connected too?

E
 

Thread Starter

mva007

Joined Oct 26, 2021
6
Hi mva,
I can reproduce the waveform shown in your post by adding a low-value capacitance load on the OPA output.
What is the OPA output connected too?

E
The output is directly connected to the DSO via SMA to BNC cable. Is this problem somehow related to soldering of the board on general purpose PCB?
 

Thread Starter

mva007

Joined Oct 26, 2021
6
I see 0V to 2.45V for the output, but the input isn‘t too clear.
sorry for that!

The input clock signal is a 10MHz square wave varying between -0.84 to 0.91V voltage level. To compensate for the DC shift, I'm adding the positive DC of 0.84V at the other input of non-inverting summing amp.
 

DickCappels

Joined Aug 21, 2008
7,981
From what I have been reading here, the problem is probably that your amplifier is not set up to drive the capacitive load of the cable +the SMA input.

You might need to use a technique like the one below to isolate the feedback from the capacitive loading and possibly add a little capacitance across RF.

Edit: Gracious @atferrari , the third time is the charm (?)
1635257488294.png
 

Ian0

Joined Aug 7, 2020
3,760
From what I have been reading here, the problem is probably that your amplifier is not set up to drive the capacitive load of the cable +the SMA input.

You might need to use a technique like the one below to isolate the feedback from the capacitive loading and possibly add a little capacitance across RF.
View attachment 251163
I don’t know where you found that diagram, but the op-amp inputs are back-to-front!
 

DickCappels

Joined Aug 21, 2008
7,981
Back-to-front? I am anxious to know what that means.

Thank you for calling my attention to the circuit, I see they have the inverting and non-inverting inputs swapped. Now corrected.
 

ericgibbs

Joined Jan 29, 2010
14,183
hi mva.
A simple example of what the effect of a load capacitance will have.
The OPA is the nearest type i have.
E


Update:
A simple resistive fix is often used, 2nd image
 

Attachments

dl324

Joined Mar 30, 2015
13,527
Though the simulations on LTSpice showed no problem, my output on the oscilloscope is being rippled and distorted.
Try inserting a resistor in the ground lead to dampen ringing caused by ground lead inductance.

You can do a quick test by enabling the 20MHz bandwidth limit option.
 

Thread Starter

mva007

Joined Oct 26, 2021
6
hi mva.
A simple example of what the effect of a load capacitance will have.
The OPA is the nearest type i have.
E


Update:
A simple resistive fix is often used, 2nd image
It worked like a charm. I soldered an 81Ohm resistor at the output of my OpAmp.
IMG_20211029_115619919_HDR.jpgThanks for going above and beyond by simulating it and showing :)

I am bit confused on why this is working though, is this resistor matching my circuit impedance to the SMA cable? I didn't understand where many were pointing out that my OA is unable to drive capacitive load.
And how should one select this resistance value (I just tried with all resistors less than 100Ohms)?
 
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ericgibbs

Joined Jan 29, 2010
14,183
Last edited:
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