how to determine the dynamic range of an active RC filter?

[[email protected]]

hi,
I' ve now designed an active RC filter and I also designed the opamp
used in the filter.
Now I' ve got a question that how can I deter mine the dynamic range of
the filter?
First, how do I know which is the largest signal that the filter can
process with?
Second, how do I know which is the smallest signal that the filter can
process with? Is it the same as the noise of the filter? If yes, then
what is the difference between dynamic range and S/N?
Looking forward to reply!
Thanks!!!

 

[Fred Bartoli]

hi,
I' ve now designed an active RC filter and I also designed the opamp
used in the filter.
Now I' ve got a question that how can I deter mine the dynamic range of
the filter?
First, how do I know which is the largest signal that the filter can
process with?
Second, how do I know which is the smallest signal that the filter can
process with? Is it the same as the noise of the filter? If yes, then
what is the difference between dynamic range and S/N?
Looking forward to reply!
Thanks!!!

That's funny how people are able to _design_ such things and can't answer
such elementary questions.
Wondering what definition of designing they have.

 

[Jim Thompson]

That's funny how people are able to _design_ such things and can't answer
such elementary questions.
Wondering what definition of designing they have.

Bwahahahaha! They don't _design_, they "plug-in" ;-)

But this one sounds like homework.

...Jim Thompson

 

[Tim Shoppa]

That's funny how people are able to _design_ such things and can't answer
such elementary questions.
Wondering what definition of designing they have.

My reading was that he picked a cookbook-style op-amp active filter,
plugged in some numbers, and got some component values and randomly
picked an op-amp off the shelf.

Yeah, some people call that "designing", but realistically "design by
cookbook" happens much more often. I rarely design my own op-amps out
of grains of silicon myself anymore, and whatever filter topologies
I've invented, I can guarantee you they all oscillated or motorboated
whether I wanted them to or not!

Tim.

 

[Rene Tschaggelar]

hi,
I' ve now designed an active RC filter and I also designed the opamp
used in the filter.
Now I' ve got a question that how can I deter mine the dynamic range of
the filter?
First, how do I know which is the largest signal that the filter can
process with?
Second, how do I know which is the smallest signal that the filter can
process with? Is it the same as the noise of the filter? If yes, then
what is the difference between dynamic range and S/N?

The biggest signal is somewhat smaller than the
rails, limited by the rails. The smallest signal
is indeed the noise.

Rene

 

[Jon]

The worst case situation for an an active filter is the response to a
step input. Calculate (or measure) the step response. The overshoot
value must be less than the output swing capability of the op amp.
This is what determines the maximum input voltage Vinmax. If you have
designed a multi-stage filter, put the lowest Q sections first. This
will minimize the step response overshoot of the first stage. The
lowest Q section is the one in which the ratio of the real to imaginary
components of the poles is the largest.
~
The smallest signal that the filter can process depends on the minimum
acceptable S/N ratio. Let the minimum acceptable S/N ratio = N dB.
Let the maximum possible output = Vmax. Then the minimum signal you
can process is calculated by Vinmin = Vmax/[10^(N/20)].
~
The dynamic range is then 20log(Vinmax/Vinmin)
Regards,
Jon

 

[Jim Thompson]

The worst case situation for an an active filter is the response to a
step input. Calculate (or measure) the step response. The overshoot
value must be less than the output swing capability of the op amp.
This is what determines the maximum input voltage Vinmax. If you have
designed a multi-stage filter, put the lowest Q sections first. This
will minimize the step response overshoot of the first stage. The
lowest Q section is the one in which the ratio of the real to imaginary
components of the poles is the largest.
~
The smallest signal that the filter can process depends on the minimum
acceptable S/N ratio. Let the minimum acceptable S/N ratio = N dB.
Let the maximum possible output = Vmax. Then the minimum signal you
can process is calculated by Vinmin = Vmax/[10^(N/20)].
~
The dynamic range is then 20log(Vinmax/Vinmin)
Regards,
Jon

It will depend on the filter architecture. For example, a
state-variable filter will generally have internal nodes which are
sq-rt(2) times higher than the input signal at fo.

My "p+1" architecture never exceeds the size of input signal.

...Jim Thompson

 

[[email protected]]

Thanks for your detailed explanation,Jon,
I would try what you' ve told me!
sorry for my poor knowledge in filter and opamp, I only did this
reseach for 4 months. But I really designed the opamp my self for a
structure of folded cascode.
Would you be so kind to reconmend me some books that teach dynamic
range and THD of filter
Thanks very much!

 

[[email protected]]

I wonder what determines the "minimum acceptable S/N ratio"
The filter is a continuous-time filter, so if the smallest signal that
the filter can process is equal to the noise signal?
looking forward to reply!
thanks!