Problem with WM8501 datasheet

[jkeny]

I'm doing some work on a Squeezebox receiver Squeezebox Receiver Modifications & this is the on-board DAC chip.

I already have this working with an external DAC (see link) connected via I2S but I also wanted to see how well the on-board DAC could perform as it can drive headphones easily.

One of the most critical parts of the DAC is it's VMID pin which functions as a bias (2.5V) & Vref for the output stage. Like all vrefs, noise has to be kept to a minimum as it will directly effect the output. Now good coupling is an approach to minimising noise on this pin - the datasheet recommends 10uf cap bypassed by 0.1uf in close to the pin. It suggests that alternatively, an external 2.5V can be supplied to this VMID pin.

An external reference could be used to drive into the VMID pin if desired, with a value typically of about midrail ideal for optimum performance

This is where I see a problem! The 2.5V ref is internally generated on VMID pin by an internal resistor voltage divider across AVDD & GND according to the block diagram of the chip (attached). So when the DAC is powered on with it's 5V AVDD supply 2.5V immediately appears on the VMID pin.

How can connecting an external 2.5V supply to VMID do anything? Am I missing something obvious?


Edit: BTW, this DAC does sound very good when output through a transformer to block 2.5V bias (I use an external 2,5V on the other primary leg to cancel this bias - works wonderfully)

[ilimzn]

You have to keep in mid that the diagram does not represent the actual inner workings of the DAC, just the principle. In your case, this sort of DAC has internal switched capacitor filters and charge transfer stages as well as output amps which tend to use VMID internally. Using external VMID means that this voltage reference becomes a 'hard' reference rather than just a RC filter, so performance vs frequency may be optimized.

[jkeny]

Originally Posted by ilimzn

You have to keep in mid that the diagram does not represent the actual inner workings of the DAC, just the principle. but In your case, this sort of DAC has internal switched capacitor filters and charge transfer stages as well as output amps which tend to use VMID internally. Using external VMID means that this voltage reference becomes a 'hard' reference rather than just a RC filter, so performance vs frequency may be optimized.

If I read you correctly, the VMID being supplied externally should act as a better voltage reference & optimise the performance? This is what I was hoping for. There is a statement in the datasheet that says the chip is powered down by setting the ENABLE pin low & it looks like this is achieved by pulling down the voltage on VMID pin but it's unclear. The Wolfson support engineer hasn't been much help in the last 3 days of emails - he hasn't clarified this confusion.

I know I could just go ahead and do it & see but I didn't want to kill the DAC chip so I was check with Wolfson first - as it happens I killed the Squeezebox receiver anyway - ironic isn't it?