Problem with WM8501 datasheet - diyAudio
Go Back   Home > Forums > Source & Line > Digital Line Level

Digital Line Level DACs, Digital Crossovers, Equalizers, etc.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 24th January 2010, 06:42 PM   #1
jkeny is offline jkeny  Ireland
Banned
 
Join Date: Feb 2007
Location: Dublin
Default Problem with WM8501 datasheet

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.
Quote:
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)
Attached Images
File Type: jpg WM8501 Block Diagram.JPG (41.9 KB, 101 views)

Last edited by jkeny; 24th January 2010 at 07:02 PM.
  Reply With Quote
Old 27th January 2010, 06:26 PM   #2
ilimzn is offline ilimzn  Croatia
diyAudio Member
 
Join Date: Feb 2005
Location: Zagreb
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.
  Reply With Quote
Old 27th January 2010, 10:21 PM   #3
jkeny is offline jkeny  Ireland
Banned
 
Join Date: Feb 2007
Location: Dublin
Quote:
Originally Posted by ilimzn View Post
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?
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Datasheet shon35us Chip Amps 3 9th March 2009 03:10 PM
stk442130 datasheet and problem shon35us Parts 0 21st March 2007 02:04 AM
Datasheet help Stocker Parts 0 10th March 2004 01:36 PM
Datasheet Piersma Digital Source 3 29th January 2003 07:49 PM
Need datasheet Michael_N Digital Source 5 21st January 2002 11:07 PM


New To Site? Need Help?

All times are GMT. The time now is 08:41 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2