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Old 21st March 2002, 03:02 PM   #1
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Default GIC low pass filter?

A few years ago, Burr-Brown propagated the use of a GIC low pass filter at a DAC's output or ADC's input instead of the more common Sallen-Key and multiple feedback configurations. Advantages were supposedly lower distortion and noise.

In the newer data sheets, even that of the PCM1704, they have converted to mulitple feedback. Why is that? Lower component count? Were there unknown disadvantages to the GIC topology? Anybody tried it?

Eric

P.S. the attached app note is really pdf format and should be openend with Acrobat, but the forum insisted it does not like pdf file extensions...
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Old 21st March 2002, 05:05 PM   #2
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It's easier than that........there is less garbage to filter out with higher upsampling rates.

I only use one pole in my designs. And sometimes a capacitor on the output to get rid of the RF leaking out.

Jocko
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Old 21st March 2002, 05:45 PM   #3
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well, the PCM1704 data sheet now proposes a second order multiple feedback filter, in spite of 8x oversampling

Eric
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Old 21st March 2002, 11:21 PM   #4
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Yes, but I've been using one pole for about 10 years now, with no problems.

Jocko
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Old 21st March 2002, 11:34 PM   #5
jam is offline jam  United States
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Jocko,

You and Harry Haller(Hairy Holler) seem pretty well versed in this digital stuff, so how about sharing some of you D/A designs with the rest of us.
I could be wrong but I have always found that GIC filters hurt the sound more than traditional filters.

Regards,
Jam
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Old 21st March 2002, 11:48 PM   #6
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Whaddya want to know? Some of this stuff is proprietary, as it is designed for companies I am not at liberty to name. Even though most are out of business. (That'll teach them to do business with a goofball like me!)

You have to remember: Burr-Brown and AD also sell.......OP-AMPS!

Jocko
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Old 22nd March 2002, 12:04 AM   #7
jam is offline jam  United States
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Jocko,

Since you asked how about a 96k 24 bit D/A with upsampling using PCM 1704's, balanced outputs(true), display of sampling and bit rate, switched phase inversion, selectable filters, four inputs..... have I left anything out. Don't violate any agreements but if a company is out of buisness, are their patents still valid?
You could leave the power supply and I/V conversion to us.

Regards,
Jam

P.S. I am not asking for too much, am I?
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Old 22nd March 2002, 01:48 AM   #8
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Let's see: 4 inputs? Isn't 2 enough. Hard to get sampling frequency form the Crystal RX, unlees you use the non-96 kHz version. And they aren't a lot of help on getting the de-emphasis flag, either. Switching phase is easy, a pin on the 1704 does it for you. Selectable filters?? Digital or analog. And why?

But since you asked.....I may have something in the works (in the way of a PCB) that I might be able to sell you. Stay tuned.

Nothing I've built is patented. Not so sure about Hairy's stuff. I don't believe in them.

Jocko
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Old 22nd March 2002, 04:40 AM   #9
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Some further thoughts.........

Lucky you didn't ask about the P/S and I/V stages. Those are the key to making this stuff sound good. The digital stuff can come straight from the data sheet. You can get away with quite a lot here. Provided the grounding and bypassing are good. And I can draw it out, but how you implement it is the trick. The main thing is to keep the current loops small to minimise EMI. The stuff is very forgiving. As long as it gets the right voltage without a lot of pertubations, it works ok.

Same for the front end interface. The schematic is not all there is. I built my first D/A in one night. Spent the next 6 months getting the input circuit right.

As for whom I may have done work for........

This is, say we say, a very egocentric industry. No one wants it to leak out that some goofball in Texas, or North Carolina, designs part, or all, of their stuff. Even if they are out of business now, they won't be happy if I let the cat out of the bag. They will remember that next time they need a "hired gun".

Jocko
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Old 22nd March 2002, 06:36 AM   #10
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If my memory serves correct, the problem with GIC filters is one of power supply headroom.

I did a GIC notch filter once and discovered that the quality and depth of the notch was dependent upon an equal and opposite signal to "buck" the main signal. Once an opamp hits a power supply rail the quality of the filter goes awry.

If you look at the basic GIC topology it looks almost like a "T" attenuator - much of the signal passes through cleanly right across the top. The opamps in the "leg" counter the incoming signal to remove it (low pass).

Too much feedback for my tastes. It's like a cat continually chasing it's tail. If you can't do it with Ls and Cs, then stick with the basic Sallen-Key or similar.
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