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Old 6th June 2008, 12:57 PM   #1
gmarsh is offline gmarsh  Canada
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Default DSD / 1-bit input DAC?

I'm designing a board around an Atmel AP7000 processor, which has a "audio bitstream DAC" built into it. Basically, it outputs a pair of 1-bit signals at 128Fs (5.6448M @ 44.1) along with a clock. Sort of like DSD but twice the data rate.

I hope to use this for a diagnostic headphone jack. The quality doesn't have to be that great, but it can't be ridiculously bad (buzzing/supply noise/etc) that can make someone think that something is wrong with the application.

On the cheap side, I could take these lines directly, feed them through a 74xx CMOS gate running off its own linear regulator and finally an op-amp doing low pass/headphone driving duties, but I'm concerned that the output of a 74xx won't really be suitable.

On the spendy side, I could use a PCM179x part from TI in DSD mode, but I'd need to add a PLL to boost the bit clock by 4x for the PCM's system clock. So board area, and cost, are starting to get out of the question here.

Is there a specific device that I could be using for this? ie., does someone sell a "1 bit in, 1 bit out" DAC that I should be aware of?

Thanks.
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Old 6th June 2008, 05:16 PM   #2
Giaime is offline Giaime  Italy
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Why you can't just low-pass it passively and then feed it to an opamp as headphone driver?
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Old 6th June 2008, 05:20 PM   #3
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ALL TOTALLY WRONG !!!!!!!
In times of high-quality class-d you are obliged to feed the signal into a switching power stage and then filter it passively - of course !!!!!

Regards

Charles
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Old 7th June 2008, 12:00 AM   #4
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I'd have to check the datasheet again, but the AD1955 might work without a crazy clocking set-up.

You might consider a voltage out part like the cs4398, just for simplicity of not dealing with an I/V for something like this.

-David
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