Low pass filter

[tiger66]

Hi All
I need to connect a low pass filter to the output of a fully
differential amplifier.

I read a book and the first order low pass filter seem to only have
one input. Does that mean that I need to create two LPF, one for Vout+
and one for Vout-?

Thanks

 

[Anthony Fremont]

Hi All
I need to connect a low pass filter to the output of a fully
differential amplifier.

I read a book and the first order low pass filter seem to only have
one input. Does that mean that I need to create two LPF, one for Vout+
and one for Vout-?

You need to be more descriptive of the power levels and frequencies
involved. If you're talking about line level audio, then download TI's
FilterPro software. It does differential input filters too.
http://focus.ti.com/docs/toolsw/folders/print/filterpro.html

 

[Jamie]

Hi All
I need to connect a low pass filter to the output of a fully
differential amplifier.

I read a book and the first order low pass filter seem to only have
one input. Does that mean that I need to create two LPF, one for Vout+
and one for Vout-?

Thanks

I assume that you must not have access to the internals of this
amp or you need to remove noise from the other end of the leads?
And, if you don't have access to the input to where this is going?
then you must make an inline filter.
you didn't specify what spectrum you're trying to filter out?
if this is spike noise ? a common mode choke will work, you simply
pass both outputs through it..

 

[Tam/WB2TT]

Hi All
I need to connect a low pass filter to the output of a fully
differential amplifier.

I read a book and the first order low pass filter seem to only have
one input. Does that mean that I need to create two LPF, one for Vout+
and one for Vout-?

Thanks

No, you need a balanced filter. That assumes you are going to feed a
balanced load. However, common mode problems might make the two filter
approach better.

Tam

 

[tiger66]

I assume that you must not have access to the internals of this
amp or you need to remove noise from the other end of the leads?
And, if you don't have access to the input to where this is going?
then you must make an inline filter.
you didn't specify what spectrum you're trying to filter out?
if this is spike noise ? a common mode choke will work, you simply
pass both outputs through it..

Thanks for the response

I am trying to design a chopper stabilized amplifier and the last
stage is a LPF. The stage before LPF is a modulator and its output is
a modulated signal. So I try to use a LPF to keep the first harmonic
and filter the other harmonics of the signal coming out of the
modulator.

My input is a 1khz, 5V DC signal, so I think I should have a LPF with
cutoff frequency at 1kHz.

Can you example what do you mean by pass both outputs through it.

From my understanding LFP consists of a opamp, 2 resistors and 1

capacitor. The -ve pin of the opamp connects to one output of my
modulator and the +ve pin of the opamp connects to ground.

So, do you suggest that I should connect the other output from
modulator to the +ve pin of the opamp instead of the ground?

Thanks

 

[tiger66]

No, you need a balanced filter. That assumes you are going to feed a
balanced load. However, common mode problems might make the two filter
approach better.

Tam

Hi Tam
Can you example how I can implement a two filter approach?

Do I need to make additional connection beside connecting one LPF to
each modulator output?

Thanks

 

[Eeyore]

Hi All
I need to connect a low pass filter to the output of a fully
differential amplifier.

I read a book and the first order low pass filter seem to only have
one input. Does that mean that I need to create two LPF, one for Vout+
and one for Vout-?

How about a differential filter ?

Graham

 

[Tam/WB2TT]

Hi Tam
Can you example how I can implement a two filter approach?

Do I need to make additional connection beside connecting one LPF to
each modulator output?

Thanks

I have always used a balanced filter, but if your amplifier can drive the
signals with respect to ground you might be able to use two filters . Can't
think of any other connections you would have to make. Unless the two
filters are absolutely identical, the phase shift and amplitude will be
slightly different for the + and - signals, and things that should cancel
out won't.

Can't you use another op amp, and convert to a single ended signal? This
would take care of the common mode, and give you a clean signal with respect
to ground. From your original posting I couldn't tell if this was at
microwave , or what.

You really should simulate this in SWCad, or the like. For two filters, see
what happens if the components are slightly different.

Tam

 

[Ban]

Thanks for the response

I am trying to design a chopper stabilized amplifier and the last
stage is a LPF. The stage before LPF is a modulator and its output is
a modulated signal. So I try to use a LPF to keep the first harmonic
and filter the other harmonics of the signal coming out of the
modulator.

My input is a 1khz, 5V DC signal, so I think I should have a LPF with
cutoff frequency at 1kHz.

Can you example what do you mean by pass both outputs through it.

capacitor. The -ve pin of the opamp connects to one output of my
modulator and the +ve pin of the opamp connects to ground.

So, do you suggest that I should connect the other output from
modulator to the +ve pin of the opamp instead of the ground?

Thanks

You can simply take two equal resistors from the outputs into a common
capacitor with half the computed (f=RC*2pi) value. An Instrumentation Amp
would then give you the single ended output.
IMHO if you do the switching right, you actually don't need a filter.

 

[Jamie]

Thanks for the response

I am trying to design a chopper stabilized amplifier and the last
stage is a LPF. The stage before LPF is a modulator and its output is
a modulated signal. So I try to use a LPF to keep the first harmonic
and filter the other harmonics of the signal coming out of the
modulator.

My input is a 1khz, 5V DC signal, so I think I should have a LPF with
cutoff frequency at 1kHz.

Can you example what do you mean by pass both outputs through it.

capacitor. The -ve pin of the opamp connects to one output of my
modulator and the +ve pin of the opamp connects to ground.

So, do you suggest that I should connect the other output from
modulator to the +ve pin of the opamp instead of the ground?

Thanks

Hmm, i just replaced a Chopper Op-amp on a voltage sensing board
at work, it just used a RC network to smooth off the edges.
In any case, i would try a simple common mode filter and check it
on the scope ..
I'm assuming that you must be using a very square wave signal that
you're trying to remove the harmonics that are getting generated from
it ?
Most likely, any filter you use is going to curve the left and right
shades.. if that is fine with you, i think maybe just having a non
inductive type resistor on each output would fix your problem.
a little by pass capacitance should be applied on the output side of
the R's
I'm thinking 100 Ohm R should do it with like a .001 uf by pass, but
that is just off the top of my head.

filters like Active types may tend to take your nice square wave and
truncate it. That has been my experience on that subject, it only makes
sense that it would do that. But then again, the idea i gave you above
may also give you some deforms thus giving you what i call a hook tooth
wave, in which case an inductor added to the scene can handle that.
I think you need to experiment a little.