Hi all,
I am working on a DAC design with WM8742, an old device but I like it.
Will it be possible to use the Analog Reference pins as a method of volume control? My idea is to feed a well filtered DC voltage controlled by a potentiometer to the analog input reference pins (AVDDR, AVDDL) and thus avoid 1) software control of the DAC and 2) not using the full 24 bit resolution as you do in a volume control in the digital input side.
I asked Cirrus but they didn't know.
BN
I am working on a DAC design with WM8742, an old device but I like it.
Will it be possible to use the Analog Reference pins as a method of volume control? My idea is to feed a well filtered DC voltage controlled by a potentiometer to the analog input reference pins (AVDDR, AVDDL) and thus avoid 1) software control of the DAC and 2) not using the full 24 bit resolution as you do in a volume control in the digital input side.
I asked Cirrus but they didn't know.
BN
The datasheet doesn't appear to give any information but mentions 'resistor strings' from AVDDR/AVDDL in the power on reset section, it might work low volumes may have very degraded DAC performance though. Voltage reference pins often expect a wide bandwidth low impedance driving them so you should use a buffer amplifier/active filter on your pot.
So amplifiers (as an example of a circuit inside a DAC) even if they are 'rail to rail' have very degraded performance close to the supply rails. Usualy this wouldn't matter as only the top and bottom 'bins' of the DAC would be affected. However if your operating close to the supply then most of the range with be affected. There are other things I can think of as well like comparators will have reduced differences at their inputs which slows down switching and increases errors due to fixed hysteresis. Just examples of analog building blocks that can have problems, I don't know the internal architecture of the DAC. I would however be pretty confident in SNR and THD degradation with reducing Vref.
It's not an option to just use a digipot on the output of the DAC? (digitally controlled potential divider)
It's not an option to just use a digipot on the output of the DAC? (digitally controlled potential divider)
Ok, so it is a bad idea.
But as I could not find any references on it I had to ask.
Thanks.
Is the software volume control in the WM8742 working the same way as a numeric digital gain control on the input signal? Or is it more clever?
Maybe I should not worry too much about it, but in principle you only get full resolution of the DAC if you are using most or all of the 24 bits.
Just as an example, -48dB (a VERY low volume indeed) is using only 16 bits out of the 24? I guess this is what a simple software volume control does e.g. in a music player.
This is how my present volume control work in my DSP, and if the WM8742 software volume control is better than this, I should try this path instead.
BN
But as I could not find any references on it I had to ask.
Thanks.
Is the software volume control in the WM8742 working the same way as a numeric digital gain control on the input signal? Or is it more clever?
Maybe I should not worry too much about it, but in principle you only get full resolution of the DAC if you are using most or all of the 24 bits.
Just as an example, -48dB (a VERY low volume indeed) is using only 16 bits out of the 24? I guess this is what a simple software volume control does e.g. in a music player.
This is how my present volume control work in my DSP, and if the WM8742 software volume control is better than this, I should try this path instead.
BN
I think Steve Nugent at Empirical Audio has been using it in his DACs. You might try contacting him at AudioCircle.
Empirical Audio
Empirical Audio