Hey everyone!

So, I have been taking a break from my endeavour of working on a buck converter controlled by a microcontroller and switched gears to another aspect f my project.

First, I have a DDS (AD983BRUZ) and the output of this is going to a non-inverting amplifier to bring the voltage up to the 5 V scale (min: 250 mV - max: 5 V). Then, this is going to a comparator, the AD790JNZ. I am incorporating hysteresis and I would like to get something less then 50 mV hysteresis. Which, I have gotten pretty close on the breadboard. Bandwidth wise, I need to be able to go up to at least 1 MHz.

Now, I am running into a snag because when I adjust the reference voltage and it gets to a certain point, the duty cycle jumps very quickly. This is when it gets close to 0%. Actually, probably closer to 10% when it does this. Very interesting. I as wondering if anyone else had that problem with this chip and if there was a way around it?

Alternativly, I am open to changing chips! If you feel that there is a better method for doing the same thing (or a better chip), please post here! I am attaching a copy of the circuit that I am building.

 

Does anyone have any idea? Or should I expound a little more?

 

So did I place this question in the wrong spot or something? Because I am not getting any hits. Is there something wrong with my post?

 

Do you have 0.1uF filter capacitors on the power pins?

 

Show the waveforms at the comparator inputs.

 

Does the comparator need a pull-up resistor on the output?

 

Here are screenshots of the waveform. The top waveform is the input, the bottom waveform is the output. Sadly, I do not know how to make a gif or I would completely make a little "video" about what is happening so you are going to have to take my word on this. I am adjusting the reference voltage via a single turn pot (If you look at my original circuit, this pot represents the DAC).

Now between the two pictures, I literally turned the pot only 1/64 of a turn if even that. And I am not yet even maxed out the pot. I still have more to go. Probably 1/8 of turn left.

@ Alec_t

Good thought but I call analog and they said that it is a push-pull output rather then an open collector

 

You don't show the voltage on the other input (I assume the DC from the pot). Without seeing that I can't determine what the problem might be.

 

You don't show the voltage on the other input (I assume the DC from the pot). Without seeing that I can't determine what the problem might be.

Here we go, the first picture is with a measurement (shot 3) and the next picture (shot 4) is with the cursors turned off. This is right at the point when the output voltage (Bottom) is low. The top measurement is the input to the inverting pin and the bottom is the output of the circuit.

 

Well, we're getting closer. What I would like to see is the two inputs shown simultaneously on the screen with the same zero volt position and vertical sensitivity for both, for each of your first two conditions (just before and just after the output pulse quits).

Also could you reduce the size (resolution) of the image before posting? I can only see about 1/8th of the image at a time on my screen.

 

Here we go, now 50% smaller

 

Darn it, give me one sec to post the other one

 

I believe I see the problem. Look at the data sheet for the comparator Input Characteristics common-mode voltage. The maximum is 2V below the plus supply voltage. It's not an input rail-rail type comparator. So the triangle wave must be no greater than 3V peak for proper operation.

 

Darn, that sucks!

Well what chip do you recommend for my application? I am going to be using a DAC to control the duty cycle and I need the frequency to go up to at least 1.5 MHz.

 

Well wait, let me try the function generator at a peak of 3 V

 

Hey look at that, it works!

 

Hey look at that, it works!

Good news. It's amazing how often things work better when the data sheet is followed.

 

Indeed, must have skipped that one