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Old 16th February 2004, 07:07 AM   #1
Prune is offline Prune  Canada
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Default Analog filter

The CS43122 only needs a simple analog filter at the differential outputs. In the datasheet, it's two 300R in series with each output followed by 2200pf between + and -. When I put this in LTSPice, it shows a pretty big dB dip and phase shift at the high end.
Would it be allright to change this by say using a lower capacitance, or otherwise modifying the values? Or is there a reason not to do this?
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Old 19th February 2004, 07:20 PM   #2
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Old 19th February 2004, 09:31 PM   #3
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Cool Analog Filters

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Originally posted by Prune
Still hoping for a reply

Hi Prune,
On the Analog Devices website or that of Texas Instruments is a program to calculate balanced filters.
www.analog.com
http://www.analog.com/Analog_Root/st...er/filter.html
www.ti.com
Search for FilterPro(R)
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Old 19th February 2004, 10:29 PM   #4
Prune is offline Prune  Canada
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I'm using jwb's desing:
http://atari.saturn5.com/~jwb/43122.png
There are no opamps. The CS43122 has voltage output. jwb used the same passive filter as in the CS43122 datasheet (2 resistors and a capacitor), followed by a voltage follower.
When running this output stage in LTSpice, lowering the capacitor value slows falloff in the high frequencies. I was just wondering if there would be negative side effects in doing that (I want to even out the frequency response).
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Old 20th February 2004, 01:01 AM   #5
Lyra is offline Lyra  Norway
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Maybe this site could be of help.......

Lyra
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Old 20th February 2004, 11:39 AM   #6
hjelm is offline hjelm  Sweden
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I do not think it is that bad. It falls less than a dB and 9degrees phase shift, or i might have done something wrong.

I am also kind of interested to see what people think of these low pass filters. If i look at the CS43122 spec it seems that if you are running it in mode0(whatever that is) the attenuation is at least 100dB in the stopband so i would guess that this circuit would not cause any trouble in any amplifier that follows it even without filter.

Is it important sounwise to have absolutely flat frequency response 20-20k?
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Old 20th February 2004, 08:44 PM   #7
Prune is offline Prune  Canada
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Yes, it falls just under a dB; however, I'm getting close to 24 degrees phase shift. Considering that the DC blocking output capacitors shift the low end phase to the other direction, that's pretty large.

I tried without a filter with my tube headphone amp, and didn't notice a problem. I'll be making the Aleph-X, so I wonder if that might be a problem.

As I don't have any smaller SMD caps for this, I wonder if it would be OK to just put two of these 2200pF in series.
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Old 21st February 2004, 09:44 PM   #8
hjelm is offline hjelm  Sweden
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Just a gut feeling i have but i would not put two caps in series. You decrease the resonant frequency of the cap by four, might still be ok though.

Could you decrease the value of the resistors instead or would that load the dac too much?
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Old 21st February 2004, 11:19 PM   #9
Prune is offline Prune  Canada
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Well I did it anyways, and it worked fine.
Capacitors in series halves the capacitance. But I don't know about resonant frequency. Can you point me to some info? I had trouble finding what you were talking about on Google.
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Old 22nd February 2004, 03:57 AM   #10
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BTW will the CS43122 be able to drive a potentiometer? I need a volume control and I want to put it before the voltage follower, else I'm limiting what kinds of loads I can drive. And what value pot would be optimal?
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