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-   -   VREF as volume control (http://www.diyaudio.com/forums/digital-source/30641-vref-volume-control.html)

Prune 21st March 2004 01:11 AM

VREF as volume control
 
DACs like the CS43122 have output that's scaled by VREF. Would there be any performance issues if VREF is varied for volume control, as macaque suggested in another thread? The datasheet only specifies typical voltage and does not discuss performance for other VREF values.

jackinnj 21st March 2004 01:21 AM

it's a technique used in instrumentation -- autoscaling, programmable gain -- you just have to be careful at low levels as the noise on VRef can swamp the DAC or ADC. In audio (or photometrics) you don't necessarily want a linear relationship, however.

Texas Instruments and Linear Tech have application notes on this.

Algar_emi 21st March 2004 02:29 AM

Vref as volume control
 
This technic is used in my Mark Levinson ML38. The AD DAC8222 DAC is used as volume control. The signal to control is applied to Vref. The processor control the DAC to change the volume.
An AD823 op-amp convert the DAC current output to voltage.

It is very close to the perfect volume control.

Bye...

Prune 21st March 2004 03:11 AM

So is it safe to assume that the performance would not be affected by using a VREF different as what is given as typical in the datasheet?

I was trying to contact the manufacturer to ask about this w.r.t. the CS43122 specifically, but when trying to register with My Cirrus, a blank page comes up...

jackinnj 21st March 2004 11:35 AM

Quote:

Originally posted by Prune
So is it safe to assume that the performance would not be affected by using a VREF different as what is given as typical in the datasheet?

I was trying to contact the manufacturer to ask about this w.r.t. the CS43122 specifically, but when trying to register with My Cirrus, a blank page comes up...


again, be mindful of noise, RFI and EMI. at low levels variations induce d by these products will cause the control to become "wonky"

Petter 23rd March 2004 12:28 PM

Quote:

Originally posted by Prune
So is it safe to assume that the performance would not be affected by using a VREF different as what is given as typical in the datasheet?

No this is not safe at all. The level of chip optimization is such that the whole chip design is optimized for a particular Iref/Vref. How much you will loose by going down this path is another matter - something you will have to offset against alternative methods.

Petter

macaque 23rd March 2004 01:44 PM

The absolute maximum ratings for VRef on the data sheet is -0.3 < VRef < VA and maximum VA is 6.0V.

The typical operating condition is 5.5V for VRef. Note that the peak to peak and common mode differential output voltage are quote in terms of VRef (ie 1.33 * VRef and 0.5 * VRef).

If you look at the description for VA it claims it supplies power for the analog and reference circuits, which make you wonder why there would be a separate Vref pin and not an internal precision reference.

In the CS43122 white paper (Download it here) there is a description about what makes the Vref implementation on this chip different than most DACs and the description suggests to me that you should be able to vary Vref within some limits, probably enough to have a usable volume control. You are in no danger of exceeding the maximum limits so it is definitely something to try.

macaque 22nd April 2004 09:27 PM

I thought I would add to this thread out of an interesting portion of the AD1853 data sheet:

"Analog Attenuation
The AD1853 also offers the choice of using IREF (Pin 10) to
attenuate by up to 50 dB in the analog domain. This feature can
be used as an analog volume control. It is also a convenient
place to add a compressor/limiter gain control signal."

Very interesting that the data sheet claims 50 dB attenuation using this pin athough they don't give any indication of performance changes by doing this...


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