PCM179x Vref/Iref and Iout relationship

roender · 7th December 2006, 07:14 AM

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

cviller · 7th December 2006, 01:54 PM

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.

roender · 7th December 2006, 02:02 PM

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.

cviller · 7th December 2006, 02:53 PM

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...

roender · 7th December 2006, 05:51 PM

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The I/V stage is proprietary and can not be published

roender · 7th December 2006, 05:53 PM

I/V stage
http://www.audiokit.ro/img/schematics/iv_conv.gif

cviller · 8th December 2006, 08:45 AM

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. )

roender · 8th December 2006, 11:09 AM

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

cviller · 8th December 2006, 12:58 PM

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.

roender · 8th December 2006, 01:08 PM

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 ?

7th December 2006, 07:14 AM	#1
roender diyAudio Member Join Date: Sep 2006	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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7th December 2006, 01:54 PM	#2
cviller diyAudio Member Join Date: Dec 2005 Location: Copenhagen Blog Entries: 2	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.

7th December 2006, 02:02 PM	#3
roender diyAudio Member Join Date: Sep 2006	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.

7th December 2006, 02:53 PM	#4
cviller diyAudio Member Join Date: Dec 2005 Location: Copenhagen Blog Entries: 2	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...

7th December 2006, 05:53 PM	#6
roender diyAudio Member Join Date: Sep 2006	I/V stage http://www.audiokit.ro/img/schematics/iv_conv.gif

8th December 2006, 08:45 AM	#7
cviller diyAudio Member Join Date: Dec 2005 Location: Copenhagen Blog Entries: 2	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. )

8th December 2006, 11:09 AM	#8
roender diyAudio Member Join Date: Sep 2006	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

8th December 2006, 12:58 PM	#9
cviller diyAudio Member Join Date: Dec 2005 Location: Copenhagen Blog Entries: 2	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.

8th December 2006, 01:08 PM	#10
roender diyAudio Member Join Date: Sep 2006	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 ?