PCM179x Vref/Iref and Iout relationship - diyAudio
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Old 7th December 2006, 07:14 AM   #1
roender is offline roender  Romania
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Default PCM179x Vref/Iref and Iout relationship

Hello all,

I want to build a DAC based on PCM1792 and I need to reduce by hardware Iout response from DAC, modifying Iref.
Accordingly with data sheet, a 10k resistor is used to obtain a DAC Iout response between -2.3mA to -10.1mA (7.8mAp-p), with center point at -6.2mA.
What happening if I use 20K Iref resistor? The center current (digital 0) point will be translated up at -3.1mA with Iout response between -1.15 to -5.05mA (3.9mAp-p)?
In other words, is possible to manipulate Iref for analog out attenuation?

Thank you
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Old 7th December 2006, 01:54 PM   #2
cviller is offline cviller  Denmark
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I think the guys at TI figured out that 10k was the value to use to get the best specs, so I think you'll be better of by changing the volume in or after the I/V section.
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Old 7th December 2006, 02:02 PM   #3
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I don't want to make volume control over Iref. All I want is to reduce with 6dB Iout. At that level, the I/V converter has the best THD and dynamic range.
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Old 7th December 2006, 02:53 PM   #4
cviller is offline cviller  Denmark
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What I/V stage do you intend on using?

Can't you just change a resistor value? It is easier to get better snr with higher currents...
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Old 7th December 2006, 05:51 PM   #5
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Quote:
Originally posted by cviller
What I/V stage do you intend on using?

Can't you just change a resistor value? It is easier to get better snr with higher currents...
Yes, better SNR with higher current CAPABILITYS from I/V stage but with smaller imput currents.
The I/V stage is proprietary and can not be published
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Old 7th December 2006, 05:53 PM   #6
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I/V stage
http://www.audiokit.ro/img/schematics/iv_conv.gif
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Old 8th December 2006, 08:45 AM   #7
cviller is offline cviller  Denmark
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Do you have a board with the iv section already? You could use pcm1798, which is basically the same chip as they (www.audiokit.ro) use together with the IV you posted. If you want to get the better specs from 1794 you'll need to use their recommended values.
Otherwise you might be able to change dynamic range with the value of R1... (just a guess - you could try with a simulator to see what you have to change to lower the gain. )
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Old 8th December 2006, 11:09 AM   #8
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There is a spice model for PCM1792/4?
I have the Rockna RD2 DAC and I will copy their I/V stage in my DAC based on SRC4392 and PCM1792
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Old 8th December 2006, 12:58 PM   #9
cviller is offline cviller  Denmark
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I was thinking you could use a current generator and feed it into the a simulated I/V stage. Then you might be able to guess how to change some values to match the larger current.
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Old 8th December 2006, 01:08 PM   #10
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I already do that.
The I/V stage is "perfectly" designed and any changes will degrade SNR.
There is any spice model for PCM1792/4 ?
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