How to make a sharp anti-alias LP-filter?

[Elektro]

I need to make an anti-alias LP-filter. The filters cut of frequency
should be 110 kHz and it should ideally attenuate 98 dB at 250 kHz.

The A/D-converter is a 12-bit SAR converter with a sampling frequency
of 500 kHz.

How is it possible to make such a sharp LP-filter? Should I use a
passive L/C filter?

 

[Tim Wescott]

I need to make an anti-alias LP-filter. The filters cut of frequency
should be 110 kHz and it should ideally attenuate 98 dB at 250 kHz.

The A/D-converter is a 12-bit SAR converter with a sampling frequency
of 500 kHz.

How is it possible to make such a sharp LP-filter? Should I use a
passive L/C filter?

If I read your post right your highest frequency of interest is 110kHz,
and you are trying to get 16 bits worth of rejection at Nyquist. But if
your highest signal frequency is 110kHz then then you shouldn't really
have to worry about signals between that frequency and the first alias
frequency, or 390kHz. Is there something going on in your environment
outside of sampling that makes 250kHz special?

A passive LC filter won't gain you any attenuation over an active filter
with a like number of poles, and at those frequencies it probably won't
be smaller. Even if you can put your 98dB point at 390kHz you're still
talking about 10 poles for a Butterworth filter, and probably not too
many fewer for a Chebychev.

Is there any way that you can push up the sampling rate? You should be
able to get 12-bit ADCs that will do in excess of 1MS/sec; if you
sampled faster then you could have a filter/decimate step in your
digital hardware. The overall cost with the faster sampling rate may be
lower than fiddling with such a sharp filter.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

 

[Tim Wescott]

I need to make an anti-alias LP-filter. The filters cut of frequency
should be 110 kHz and it should ideally attenuate 98 dB at 250 kHz.

The A/D-converter is a 12-bit SAR converter with a sampling frequency
of 500 kHz.

How is it possible to make such a sharp LP-filter? Should I use a
passive L/C filter?

Forgot to mention -- there is a 5-pole continuous (_not_ switched
capacitor) active filter out there. I just can't remember who make it.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

 

[Elektro]

Sorry, I meant a 16-bit SAR, not a 12-bit, sorry.

And you are right about 390 kHz, I didn't think of that. So I need
-98 dB at 390 kHz ideally.

I must check if it's possible to select a different ADC with higher
sampling frequency.

 

[Tim Wescott]

Sorry, I meant a 16-bit SAR, not a 12-bit, sorry.

And you are right about 390 kHz, I didn't think of that. So I need
-98 dB at 390 kHz ideally.

I must check if it's possible to select a different ADC with higher
sampling frequency.

The 16-bit part makes it harder to find a higher sampling rate; there
are parts out there but they're expensive.

While you're pondering attenuation vs. frequency you may also want to
ask if you need 98dB of alias rejection at the high end of your
frequency range -- you may find that you need high accuracy at DC but
that various other forces conspire to screw up your accuracy at high
frequencies anyway, allowing you to relax the spec.

And check the LTC1563 -- it's a monolithic 4th-order filter who's
frequency you can set with one resistor; with another resistor you can
turn it from a Butterworth into a Chebychev. With two or three of those
you should be able to build your filter pretty quickly.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

 

[Tam/WB2TT]

Sorry, I meant a 16-bit SAR, not a 12-bit, sorry.

And you are right about 390 kHz, I didn't think of that. So I need
-98 dB at 390 kHz ideally.

I must check if it's possible to select a different ADC with higher
sampling frequency.

You might want to look at a combination of active and passive filters.

Tam

 

[Charles Schuler]

Sounds like an active Chebyshev to me ... given the limited information.

 

[Tom Bruhns]

A sixth-order elliptical will do it, with 1dB or less passband ripple.
We used to do things like that with op amps, but beware that you need
to use a pretty fast op amp to get good performance. Presumably, you
want distortion products down 100dB from full scale, too, or you're
wasting effort on the AAF part. There are some freeware and trialware
filter design programs out there which could help you explore both
active and passive (LC) filters. The free filter program from aade.com
is very decent for LC filter designs.

But for a 110kHz bandwidth and 16 bits, could you just use a
delta-sigma converter and make your antialias life much easier? I
certainly understand that there are times when a delta-sigma isn't
appropriate, but maybe it's at least worth asking.

Cheers,
Tom

 

[Ken Smith]

Forgot to mention -- there is a 5-pole continuous (_not_ switched
capacitor) active filter out there. I just can't remember who make it.

I think you are thinking of Linear. The make a filter that can be shifted
between 500KHz and 1MHz cut off.

 

[Iwo Mergler]

I need to make an anti-alias LP-filter. The filters cut of frequency
should be 110 kHz and it should ideally attenuate 98 dB at 250 kHz.

The A/D-converter is a 12-bit SAR converter with a sampling frequency
of 500 kHz.

How is it possible to make such a sharp LP-filter? Should I use a
passive L/C filter?

That's quite steep. You may want to consider sampling faster,
with less bits, and do some of the filtering and downsampling
digitally.

Kind regards,

Iwo

 

[Elektro]

I found a free filter design program at Linear called FilterCAD. It
suggested a 1596-7 filter IC. It is a "Linear Phase, DC
Accurate,Tunable, 10th Order Lowpass Filter" You set the cut of
frequency with a resistor.

 

[Joseph2k]

That's quite steep. You may want to consider sampling faster,
with less bits, and do some of the filtering and downsampling
digitally.

Kind regards,

Iwo

That is in the class called "brick wall filters" almost always implemented
digitally.
The high attenuation is in and of itself is difficult to realize
practically. In the real world you would want to do about 24 dB of this
filter in the analog world and do the rest digitally.