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Prune 16th February 2004 07:07 AM

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?

Prune 19th February 2004 07:20 PM

Still hoping for a reply :bawling:

Elso Kwak 19th February 2004 09:31 PM

Analog Filters

Originally posted by Prune
Still hoping for a reply :bawling:

Hi Prune,
On the Analog Devices website or that of Texas Instruments is a program to calculate balanced filters.
Search for FilterPro(R):cool:

Prune 19th February 2004 10:29 PM

I'm using jwb's desing:
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).

Lyra 20th February 2004 01:01 AM

Maybe this :att'n: site could be of help.......


hjelm 20th February 2004 11:39 AM

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?

Prune 20th February 2004 08:44 PM

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.

hjelm 21st February 2004 09:44 PM

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?

Prune 21st February 2004 11:19 PM

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.

Prune 22nd February 2004 03:57 AM

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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