help with oscillator driving comparator

[chopcooey]

hello there, i am currently making an analog function generator for a project; i know my design could be improved, but i have a problem from which i have no idea the source is.

Basically, i am using an op amp(opa602) wien bridge oscillator to generate sine waves, and varying frequency with a potentiometer. At this point i have no problem, im obtaining clean sine waves at up to higher than 1.5 MHz, as long as my closed loop gain of 3 is well adjusted.

Then, im using the sine wave output to drive a comparator(tried with and without hysteresis) circuit (with op amp lm7171), and i generate decent square waves of the same frequencies. But my problem is that, when i plug the output of the sinewave wien oscillator to the comparator, my sine wave becomes more and more distorted as frequency increases.

I could probably use an extra switch to stop powering the comparator for when i want a sine wave but that's not really what i wanna do. I could probably also make a square wave oscillator seperatly instead, but i would rather have my frequency control in just one spot. Has anyone any idea as to what creates that distortion and how to deal with it?

 

[duke37]

The oscillator will be somewhat sensitive to load and the comparator load will change through the cycle. You could add a voltage follower as a buffer between the oscillator and the comparator.

 

[chopcooey]

The oscillator will be somewhat sensitive to load and the comparator load will change through the cycle. You could add a voltage follower as a buffer between the oscillator and the comparator.

thanks for reply. I forgot to mention, but i did try adding an extra buffer and i had the same problem still. Also, i don't think it should make much of a difference since op amps have high input impedance when used as a comparator.

 

[KrisBlueNZ]

When you added a buffer, what problem did you have? Did adding the buffer cause distortion at the output of the oscillator at high frequencies, or did the buffer work OK until you connected the comparator to it?

The LM7171 isn't a comparator; it's an op-amp - specifically a "voltage feedback" amplifier. I don't know how, or even whether, it's any different from a conventional op-amp, but the "simplified schematic diagram" on page 16 of the data sheet does not look like a standard op-amp. There are plenty of high-speed comparator ICs available. Why did you choose the LM7171?

Can you post a complete schematic diagram please.