The ten least significant bits are generated by emitter scaling in the TDA1541. They are referenced between -5V and -15V.
I believe the idea was pioneered in the Marantz C7 and Htp elaborates on it somewhere in this thread (?) I use stacked TL431:in my dac for this reason.
Hi,
Another way of saying this is to state the actual current return point for Iout is not Agnd but +5V, so strictly speaking +5V serves as analogue reference point.
However +5V is also the digotal supply line and needs a low impedance connection there (marce, is the digotal plane under the TDA1541 becoming clearer?(
This is the beginning of understanding, but not complete.
Ideally +5V and -15V and AGND and DGND are held at PRECISELY the same AC potential from a few Hz to a few MHz. The -5V supply is a little different, but can be treated the same (same AC as AGND & DGND).
This is (as said) not really the case and if we cannot make the potentials the same we need to consider which currents matter most.
Layout is just one part of optimisation, non-standard approaches to decoupling supplies and regulation may also be needed. For example, my design uses shunt regulators with around 0.03 Ohm Impedance in the audio range and all decoupling is matched against this impedance, to generate as flat an impedance from DC to tens of MHz...
A lot of this optimisation needs a LOT of resources, I have yet to see a Kit or DIY DAC that comes close except ecdesigns.
Incidentally, I I find it funny that ecdesigns arrived with a lot of cirtuotus detours and effort at something that (clocks and I/V excepted) is not far from AMR's design ca. 2005...
Our clocks are a bit more commercial, we basically get a mil spec supplier to make them for us in a can, I doubt they perform much worse... But you need to buy 100's to get the spec and acceptable pricing.
I/V, well I stick to my own from the late 90's...
Ciao T
Damn... still confused what I should do. I am in the process of designing the final dac now, can someone help:
- +5V is analog and digital supply and should be decoupled to AGND
- -5V is digital supply and should be decoupled to DGND
- -15V is digital supply and should be decoupled to -5V or DGND
Surely you decouple to Dground otherwise you are injecting noise into the other supply rail.
TDA 1541 decoupling
Hi,
please read post number 3820,
ecdesigns : "Building the ultimate NOS DAC"
Grounding schemata is shown, it is like a star ground.
All is done directly under the TDA chip - easy and logic.
Hope, this may be helpful.
In the very last post's you can read about new circuit from next model Mk8.
Regards Harry
Hi,
please read post number 3820,
ecdesigns : "Building the ultimate NOS DAC"
Grounding schemata is shown, it is like a star ground.
All is done directly under the TDA chip - easy and logic.
Hope, this may be helpful.
In the very last post's you can read about new circuit from next model Mk8.
Regards Harry
This is my understanding:
-15V is digital supply and should be decoupled to -5V (and less preferable going to DGND) because some bits rely on this decoupling. Decoupling via earth is then indirect and leads to extra loop noise.
And a good way around to get this, is piggyback: stack the supplies.
(I have read the posts from H ten Pierick suggesting this long time ago, but have not done it myself, lazy lazy).
albert
Thanks , I had neglected this source for some time. I must catch up!
Anyway, instead of really implementing the stacked supplies, a small non-inductive capacitor can be inserted between the -15V and -5V legs. That looks very logical to me. Pin 26 and pin 15. In fact, the sheet implies this (conventions of drawing, that is), where these pins are locally decoupled to the same spot on the ground plane with [small, on my boards ceramic] 100 nF's.
Anyway, instead of really implementing the stacked supplies, a small non-inductive capacitor can be inserted between the -15V and -5V legs. That looks very logical to me. Pin 26 and pin 15. In fact, the sheet implies this (conventions of drawing, that is), where these pins are locally decoupled to the same spot on the ground plane with [small, on my boards ceramic] 100 nF's.
I'm sorry but I cannot see the logic in shorting one supply line to another at hf which is what you are doing. Decoupling is surely meant to short hf noise to ground and not to feed it from one supply to another....
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.