Parallel RC filter on a comparator output then into a CMOS logic gate

[lasteem1]

I'm analyzing a circuit with a comparator that has a parallel RC filter on its output then into a cmos logic gate. I understand the point of the Parallel RC filter from a current-source perspective, but I'm not sure I do in this case. Can anyone enlighten me?

 

[LvW]

Re: parallel RC filter

And what is the purpose of this filter?

 

[lasteem1]

Re: parallel RC filter

And what is the purpose of this filter?

Low Pass.

 

[LvW]

Re: parallel RC filter

I know that an RC parallel connection can act as a lowpass.
But my question was : For which PURPOSE ?
Why is a lowpass needed? Which part of the spectrum has to be attenuated?

 

[lasteem1]

Re: parallel RC filter

I know that an RC parallel connection can act as a lowpass.
But my question was : For which PURPOSE ?
Why is a lowpass needed? Which part of the spectrum has to be attenuated?

Sorry. Above 3Mhz.

 

[LvW]

Re: parallel RC filter

Is it correct that this RC combination is connected directly at the comparator output (voltage source with low output impedance)?

 

[lasteem1]

Re: parallel RC filter

Is it correct that this RC combination is connected directly at the comparator output (voltage source with low output impedance)?

Yes.

 

[LvW]

Re: parallel RC filter

In this case, if the active device works as a voltage source, the RC parallel combination acts as a load - without any influence on frequency behaviour.
Either the circuit is not correct or the ouput impedance must not be neglected (which means: it is no voltage source).

 

[lasteem1]

Re: parallel RC filter

Just so I'm not misrepresenting the circuit an image is attached.

 

[LvW]

Re: parallel RC filter

As the opamp has no negative feedback, I think its real output resistance Rout has to be taken into account. Together with the RC parallel load this forms a classical rc low pass - however, the corner frequency depends on Rout, which is not a fixed parameter. But perhaps, accuracy is not a main issue?

 

[lasteem1]

Re: parallel RC filter

I believe I see what you are saying. Considering Rout it really isn't a parallel RC filter? How much, in general, should I expect Rout to vary? And no accuracy isn't all that important.

 

[LvW]

Re: parallel RC filter

Details about Rout (value, tolerances) are to be found ion the data sheet.

What - according to your terminology - is a parallel RC filter?
In our case - it's just a series Rout and a shunt R||C. That's all.

 

[FvM]

parallel RC filter

There's nothing to analyse except the transient behaviour of the OP/comparator with RC load. It mainly depends on the output switching speed and transistor characteristic. It's a pulse circuit rather than a "filter".