Not getting the FFT of COMPARATOR

[sagar.dhange]

Hello everyone,
I designed the hysteresis type of comparator using internal positive feedback......for 0 to 1.8v supply....
I am getting the proper output.....
But when I am doing the FFT of that, the harmonic at the fundamental frequency(i.e. at input frequency) is coming out to be +1.18dB (positive) but it should be negative......
So will any one please help me how to achieve this....??

I am attaching the screen shot of my design.....please go through it..........

Waiting for your reply's.......

 

[godfreyl]

Can you show input and output waveforms so we can see what the problem is?

 

[sagar.dhange]

Hi......
I am attaching the input and output screen shot to it......
1) upperone is +ve input to comparator
middle one is -ve input to comparator i.e. reference at 900m
lower is output of the comparator...
2) 8192 point FFT of the output at 10KHz of input freq.

 

[godfreyl]

The waveform and FFT both look correct. The fundamental should be about +1dB. Remember, the amplitude of the fundamental is greater than the amplitude of the square wave.

 

[sagar.dhange]

Thanx a lot........
One more problem........
I am using this comparator in Sigma Delta ADC for Audio range (upto 20KHz)..
But the higher harmonics(2nd, 3rd, ....) are not suppressing as required.....
Any suggestions for that..........

 

[godfreyl]

What do you mean? Comparators and ADCs aren't supposed to suppress harmonics. An ADC should have an anti-aliasing filter to remove anything in the input that is above the Nyquist frequency.

 

[sagar.dhange]

I designed a two stage class A/AB amplifier whose lower 3dB Band Width is coming out to be 21KHz......
Also I designed the above comparator..........
And with the help of these 2 blocks I designed Sigma Delta ADC with 1 bit DAC in feedback path.......
Amplifier is used as an integrator(using R and C).......
But at the output the FFT of this entire system is showing many harmonics and also I am unable to get the required Band Width in PSD vs Freq graph..........
Please help me to solve this problem.........
In this case also I have to use anti aliasing or what....???

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Please go through the following link........I attached the required graphs there regarding the above problem of Sigma Delta ADC........


https://www.edaboard.com/threads/289963/#post1239716

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