# Low pass filter

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Hi, I am using an ADS8345 and a OPA350 on the input as a buffer. I thought
about turning the pure buffer into low-pass filter instead, with the
buffer skills retained. My signals that are at the input are about 100Hz,
but I want to oversample, so the ADC will work at about 8kHz. For what
frequency will I design the capacitor, for 100Hz (i.e. 200Hz with Nyquist
criteria) or for 8kHz?? I put the resistors to 10ohms respetively 10kohms,
to keep the gain equals 1. But I design the low-pass filter with the
capacitor and the 10ohms-resistor.

Thank you in advance!

Re: Low pass filter

You have to design the filter so that it passes the highest
frequency component of interest, but keeps components at
the Nyquist frequnecy (here 4 kHz) well attenuated.

10 ohms is a very low impedance for a filter - are
you sure that it will be the determining resistance
in the filter time constant?

--

Tauno Voipio
tauno voipio (at) iki fi

Re: Low pass filter
Hi,

The reason for my resistance values are just that I want to keep the gain
at 1. But maybe is it bad choices, 10ohms and 10kohms....?? The gain is
calculated as G=(R1+R2)/R1, so I just wanted to keep R1>>R2. Any other
suggestions??

Thank you!

Re: Low pass filter
Hi,

The reason for my resistance values are just that I want to keep the gain
at 1. But maybe is it bad choices, 10ohms and 10kohms....?? The gain is
calculated as G=(R1+R2)/R1, so I just wanted to keep R1>>R2. Any other
suggestions??

Thank you!

Re: Low pass filter

why do you want the gain to be 1? You just add an amp, or adjust your ADC or
whatever. I think a good design requires that R1 and R2 are not too far
apart.

vax, 9000

Re: Low pass filter
In comp.arch.embedded,

If you input is really 100Hz only, you don't need a filter. If there's
unwanted higher frequency components (noise), then you can filter.

The Nyquist criterion shows you what the maximum allowable frequency
for your ADC input is. In the case of 8kHz sampling, this is 4kHz. So
you should design your filter so that it's attenuation at 4kHz is high
enough. If you really need 16-bit accuracy, the attenuation should
be around 100dB @ 4kHz. A simple first order RC filter has a roll-off
of 20dB/decade, so you'll need 5 decades. So you must design your
filter for 4kHz/10^5= 4mHz (0.004Hz). But that will attenuate your
100Hz signal significantly as well and will be unusable.

If you design for 12-bit, you'll need a decade less en can design for
0.04Hz, still unusable. So your options are to use a higher order
filter or to increase sampling rate (or both). Also take the pass
band attenuation into account

Microchip has a nice program to design filters like this, filterlab:
http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId14%06&dDocName=en010007

If you decide to stay with the first order filter, you need to decide
how much attenuation is allowable at your 100Hz and design your filter
for that. It will surpress high frequency spikes, but is far from
optimal.

--
Stef    (remove caps, dashes and .invalid from e-mail address to reply by mail)

What is mind?  No matter.  What is matter?  Never mind.
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Re: Low pass filter
Ok, maybe should I tell you about the card in total. First I have an ADC
(but absolutely first the buffers), then a microcontroller and then a
bluetooth module with an antenna (a TX-antenna). I thought that the
filter's job was to control the bandwidth of the buffer. Shouldn't I
design the filter for the input (the 100Hz) then?? Because there are
(unfortunately) a lot of different frequencies on the card, and in a wide
range (from 100Hz up to 2.4GHz). But on the analog part the range is from
100Hz to 8kHz.

So my qestion is basicly the same, what kind of frequency should I design
my capacitor from??

Thank you!

Re: Low pass filter
In comp.arch.embedded,

What are you posting with? Please quote the relevant text and keep the
attributions. I don't feel like tracing back the thread, so doing this
from memory.

Then you should first answer this:

How is your buffer built, Where are the resistors and the cap?
What is your highest frequency of interest?
What is the maximum allowable attenuation at this frequency?
What is the samplerate
What is the required attenuation at the Nyquist frequency?

Have you downloaded filterlab? It gives you schematics for filters and
shows you how to build a second-order filter with a single op-amp. It
produces nice graphs and numeric data with information on your filter
performance.

--
Stef    (remove caps, dashes and .invalid from e-mail address to reply by mail)

According to the latest official figures, 43% of all statistics are
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Re: Low pass filter
On Friday, in article

I will leave my comments about the application and original poster for
the sake of everybody's spleen.

I find that to be a stripped down version or similar to Filterpro from
Texas Instruments, that does high and low pass filters.

<http://focus.ti.com/analog/docs/sampleutilities.tsp?path=templatedata/cm/utilities/data/filterpro&templateId=3&familyId57%&navigationId97%42>

If you have trouble with that link go to

<http://www.ti.com/>

Enter 'filterpro' in the keyword search

the top item in the results list is the page you want.

--
Paul Carpenter          | snipped-for-privacy@pcserviceselectronics.co.uk
<http://www.pcserviceselectronics.co.uk/ PC Services
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Re: Low pass filter
In comp.arch.embedded,

I don't know filterpro, but found filterlab sufficient for this purpose.
Just downloaded version 2 from microchip and found out it now does low-,
high- and bandpass filters (older version was only lowpass). So maybe
they have reduced the stripdown. ;-)

--
Stef    (remove caps, dashes and .invalid from e-mail address to reply by mail)

Common sense is instinct, and enough of it is genius.
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Re: Low pass filter
On Friday, in article

Still apparently according to the docs only does low pass for Bessel filters.

Both have advantages and disadvantages from quick view of options in
documentation (I may not have them all)

Filter  Filter
Lab     Pro
Poles           8       10
Differential    No      Yes
Pass            Low     Low
High    High
Band
Filter types       Bessel*      (*only low pass on Filterlab)
Chebychev
Butterworth
--      Linear Phase (0.05 and 0.5)
--      Gaussian 12db
--      Custom (Q and FnQ)

Configurations     Sallen Key
MFB (Roche)

Selection       tabs     All on one screen
& wizards

Plots           Freq    Freq
Phase
Delay

Spice output    Yes     NO

Component range 1%      Exact   (FilterPro has different ranges for R & C)
Exact   R       C
E12     E6
E24     E12
E48     E24
E96
E192

Show Sensitivity NO     Yes             (sensitivity of each R and C)

Cutoff Freq min 0.1Hz   0.001Hz
max 1MHz    100MHz

They both have their uses, I find other abilities like override R, C and
gains for EACH stage useful in Filterpro. I find the adjustments easier
and quicker to see in FilterPro.

I would suggest looking at BOTH for filter designs as they both give
useful information.

--
Paul Carpenter          | snipped-for-privacy@pcserviceselectronics.co.uk
<http://www.pcserviceselectronics.co.uk/ PC Services
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Re: Low pass filter
On Fri, 04 Feb 2005 23:53:33 +0000 (GMT), paul\$@pcserv.demon.co.uk (Paul

Yikes! Not very agressive.

Give Filter Wiz a try. The "LE" version is pretty inexpensive and
handles a reasonable variety of types and topologies.

http://www.schematica.com /

--
Rich Webb   Norfolk, VA

Re: Low pass filter
On Saturday, in article

That is one limitation of one of the TWO packages shown.

Yes it is 'inexpensive', but considering the other tools I was comparing
are actually FREE tools that actually cover most requirements for most
people from hobbyists to prefessionals. I find FilterPro quicker and easier
to operate than most filter packages as the calculations/graphs are done
one EVERY parameter change on one screen not dialog boxe, which makes
the tweaking stages easier.

Which means you can quickly get a basic design that you may want to do
more extensive simulations on.

It is worth considering.

If I need to purchase tools, my simulation tool of choice from experience
would be SImetrix from http://www.catena.uk.com/ instead of a mix of
programmes to work up to full simulation, still cheaper than *most* full
SPICE modelling tools.

Similarly a good op-amp and filter primer is "Op Amps For Everyone"
(TI document number SLOD006B) 464 page PDF, that covers many aspects.

--
Paul Carpenter          | snipped-for-privacy@pcserviceselectronics.co.uk
<http://www.pcserviceselectronics.co.uk/ PC Services
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Re: Low pass filter
In comp.arch.embedded,

[...]

[...]

Nice comparison, just one remark: Filterlab does show Phase and delay in
the graphs.

That Filterpro doesn't sound too bad, I'll think I'll give it a go next
time. ;-)

--
Stef    (remove caps, dashes and .invalid from e-mail address to reply by mail)

I am covered with pure vegetable oil and I am writing a best seller!

Re: Low pass filter
On Sunday, in article

Just rechecked it and I had missed that one, hopefully I caught the main
things.

Saved me a lot of time on some 3rd and 5th order filter and playing around
with value and gain changes.

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Paul Carpenter          | snipped-for-privacy@pcserviceselectronics.co.uk
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Re: Low pass filter

Ive just seen an article stating that National have a filter design software
(free) on their website. I haven't looked at it. http://webench.national.com

Re: Low pass filter
On 06 Feb, in article

Yes you have to register and use it online, even with broadband relies on
their servers to do the calculations. The other two are downloads, which makes
it quicker and easier to tweak and experiment and document.

<rant mode=on>
What it also shows is 'My Webench', I become nauseous with the
simplistic 'My xxx' used on anything...
</rant mode=off option=maybe>

--
Paul Carpenter          | snipped-for-privacy@pcserviceselectronics.co.uk
<http://www.pcserviceselectronics.co.uk/ PC Services
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Re: Low pass filter

Thanks Paul youve saved me some time, I might have guessed it would be no good.