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Frequency Response of Filter

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garimella

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I have an MDAC IC (AD7533), Vref is a sine wave of amplitude 5V/50Hz. 10 bit Counter interfaces to MDAC and the output of MDAC is connected to a 1st order LPF (with a cut off frequency of 50Hz). I expect a 3dB attenuation at output, which is observed. My questions are related to choosing clock frequeuncy for the counter. For example, If i choose 1KHz clock, then we expect MDAC output to change in the instances of 1 ms. But LPF is letting through the input instataneous change without filtering. Why is that so? Does LPF also have small signal bandwidth specs.

Connection scheme

CLK source----Counter --- MDAC----LPF---->output
 

If i choose 1KHz clock, then we expect MDAC output to change in the instances of 1 ms. But LPF is letting through the input instantaneous change without filtering. Why is that so? Does LPF also have small signal bandwidth specs.
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If designed with a cut-off frequency of 50 Hz, that's the small signal bandwidth. 50 Hz first order low pass corresponds to a RC time constant of 3 ms, you'll see a respective exponential step response. If it's not present, check your design.
 

A 1ms clock pulse on a counter-DAC duo results in a 5mV step size change on a 10bit DAC (5V amplitude). Does this mean 50Hz LPF lets through small step size signals without much of attenuation (based on my observations)? I am surprised about this. Can you add light to this
 

The individual steps should be smoothed in so far that you see a continuous ramp instead of a step curve. Like below

filter.png
 

I agree but still I am not so clear because a 1KHz pulse updates MDAC in 1ms. A 50 Hz LPF should attenuate 1 KHz step (5mV) by factor of 30dB or more. So I should not see any thing at the output. How can you explain that. By the way I do see the output!! but why?
 

A 50 Hz LPF should attenuate 1 KHz step (5mV) by factor of 30dB or more.
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The statement is simply wrong. The LPF attenuates an 1 kHz sine by 30 dB. I don't know what an "1 kHz step" is meaned to be, I think it doesn't exist at all. You can either have a single 5 mV step or a step sequence with 1 kHz update rate as in my previous example. Both waveforms have frequency components down to 0 Hz that pass a LPF.
 

Hi FvM

Yes I agree, I am questioning this because I do not see frequency components (down to 0 Hz) with appreciable magnitude in spectrum analyzer
 

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