OK I've finished by PCB layout for the DAC im gonna build. This is largely based on MWP's DAC but substituting a differnt DAC chip.
So the basic layout is PCM2902 USB interface => DIR1703 SPDIF receiver => PCM1794/8 DAC chip.
Well as im a complete newbie to digital I basically just copied the schematic for the 2902 and the 1703 and then implemented the DAC chip as per TI's data sheets.
Anyway my main concern is grounding.
I have created three seperate ground planes around each section, a USB ground for the 2902 chip, a digital ground for the 1703 and digi side of the 1794/8, and an analogue ground for the analogue side of the 1794/8 and I/V stage.
So on the PCB I have three sets of copper islands around each different section all isolated from each other. My question is how do I link all these together? Do I run a separate feed to the star ground or do I link them together someway on the PCB?
Cheers Matt
So the basic layout is PCM2902 USB interface => DIR1703 SPDIF receiver => PCM1794/8 DAC chip.
Well as im a complete newbie to digital I basically just copied the schematic for the 2902 and the 1703 and then implemented the DAC chip as per TI's data sheets.
Anyway my main concern is grounding.
I have created three seperate ground planes around each section, a USB ground for the 2902 chip, a digital ground for the 1703 and digi side of the 1794/8, and an analogue ground for the analogue side of the 1794/8 and I/V stage.
So on the PCB I have three sets of copper islands around each different section all isolated from each other. My question is how do I link all these together? Do I run a separate feed to the star ground or do I link them together someway on the PCB?
Cheers Matt
Hmm...well, we're assuming I guess that the USB ground is total drek. (I think this is safe) If you have a TTL S/PDIF output from the USB receiver then I would be tempted to transformer couple to the main digital ground. I have never heard of people using transformers over such a short distance (inches) so you may want to wait for the pros to weigh in on this.
If your I/V stage output is balanced out then the analog ground should connect to the digital ground through a nice fat bridge under the DAC IC, preferably along the division between analog and digital circuitry in the die.
If your I/V stage output is balanced out then the analog ground should connect to the digital ground through a nice fat bridge under the DAC IC, preferably along the division between analog and digital circuitry in the die.
"tiroth" gives good advice. Join the grounds ender the DAC with a fat piece of copper - 10mm is good.
Joining the ground for the 2902 can be done with a wide strip under the digital signals that go between the sections.
Think about ground return currents. Every digital signal has a return current and it's best to make this go the shortest distance.
The best fun I ever had with a design was a 3.4GHz radio, 14V and 120V switched-mode PSUs, audio CODECs and lots of DSP all one one 4-layer board. IT HAD ONE SOLID GROUND PLANE ALL OVER and performed excellently.
Joining the ground for the 2902 can be done with a wide strip under the digital signals that go between the sections.
Think about ground return currents. Every digital signal has a return current and it's best to make this go the shortest distance.
The best fun I ever had with a design was a 3.4GHz radio, 14V and 120V switched-mode PSUs, audio CODECs and lots of DSP all one one 4-layer board. IT HAD ONE SOLID GROUND PLANE ALL OVER and performed excellently.
skippy said:
Hooray !
You see guys, it IS possible (as long as you control the currents)
well done
Guido
http://www.tentlabs.com/Info/Articles/Supply_decoupling.pdf
Well the PCB is already designed and its a bit impossible to put copper islands under or ontop of the DAC area because im using SSOP to DIP converters and ive got traces on both copper top and bottom nagating any islands around those area. There are large amounts of copper around everything anyway so I'll just try it and see how it turns out.
I was simply asking if there are any do's and dont's with digital stuff like making the analogue ground digital ground or something or keeping then completely separate appart from the star ground.
I was simply asking if there are any do's and dont's with digital stuff like making the analogue ground digital ground or something or keeping then completely separate appart from the star ground.
OK i have taken a pic of the copper top and bottom so you can see exactly how I have done this. If its complete trash then say so, I have made an active xover board like this and it works superbly. But if you think it will be OK then thats good too. This is already the second attempt at the baord design (the first one was for SSOP, until I realised that might be a bit ambitious so I made is DIP) so I dont really want to go about designing another one.
Attachments
Mmm,
Does not look too well to me. Seems the current loops will be quite big, since the returnpath is not directly under a trace but has to go all the way around. There should be ground under a trace to provide the returnpath. In that way you know where that current will go and you need to make shure there are no other currents near. Now all will go through same ground.
Two stories here (not much experience, so you might ignore this post
):
One guy made a testpcb one day using dig circuit on veroboard.
Did not use groundplane but one big track for ground and connected all to it. Then he had 0.5 volt difference on the groundplane, say over 10 centimeters (from memory, long ago).
This difference looked ugly (wasn't DC, but noise, spikes, crap).
So he was soldering big massive wires on the groundplane, to reduce resistance, result: no change.
Story two: made a pcb myself for a dsp at school. ADC on the edge, than fast memory and then DSP. ISA bus on the underside.
No groundplane, except around the ADC. Two layer only and lots of traces needed. So ground was also big trace around the edge of the pcb. So we got the pcb, stuffed it and plugged it in. Turned the knob, downloaded testprogram and ...... it worked (led started blinking). Hurray!!! So we started using it. All fine, except now and then the DSP did not see the ADC interrupt anymore and needed a reset. Happend randomly. So we got a new DSP and plugged that in (lots of $$$ for the school). Same result, bit less but still it stopped working sometimes.
In the end a solution was a wire close to the ADC-DSP traces (say ~i2s connection) connected to ADC and DSP ground.
Than it worked and we never had to reset again (though the groundloop was
). Resolution was not that good, we did not get the 16bit of the adc. Too much noise, but than again it was not sheelded (yet) and inside a PC. Think we got 14bit.
So i would get a groundplane. If you have too many traces to connect, use wires over the ground layer instead of cutting through it.
Hope this helps,
Regards,
Guido B (so not T )
Does not look too well to me. Seems the current loops will be quite big, since the returnpath is not directly under a trace but has to go all the way around. There should be ground under a trace to provide the returnpath. In that way you know where that current will go and you need to make shure there are no other currents near. Now all will go through same ground.
Two stories here (not much experience, so you might ignore this post
):One guy made a testpcb one day using dig circuit on veroboard.
Did not use groundplane but one big track for ground and connected all to it. Then he had 0.5 volt difference on the groundplane, say over 10 centimeters (from memory, long ago).
This difference looked ugly (wasn't DC, but noise, spikes, crap).
So he was soldering big massive wires on the groundplane, to reduce resistance, result: no change.
Story two: made a pcb myself for a dsp at school. ADC on the edge, than fast memory and then DSP. ISA bus on the underside.
No groundplane, except around the ADC. Two layer only and lots of traces needed. So ground was also big trace around the edge of the pcb. So we got the pcb, stuffed it and plugged it in. Turned the knob, downloaded testprogram and ...... it worked (led started blinking). Hurray!!! So we started using it. All fine, except now and then the DSP did not see the ADC interrupt anymore and needed a reset. Happend randomly. So we got a new DSP and plugged that in (lots of $$$ for the school). Same result, bit less but still it stopped working sometimes.
In the end a solution was a wire close to the ADC-DSP traces (say ~i2s connection) connected to ADC and DSP ground.
Than it worked and we never had to reset again (though the groundloop was
). Resolution was not that good, we did not get the 16bit of the adc. Too much noise, but than again it was not sheelded (yet) and inside a PC. Think we got 14bit.So i would get a groundplane. If you have too many traces to connect, use wires over the ground layer instead of cutting through it.
Hope this helps,
Regards,
Guido B (so not T
)
guido said:
OK im not really understanding this term return path etc what exactly is this all about. I was of the understanding that the ground was a refernce of zero volts for things to work from and was there for dumping of current.
What do you mean by the return path is not directly under a trace? but has to go all the way around, ground under a trace to provice the return path.
All the way around what?? to get from where to where? Im sorry if this seems a really obvious question but im completely lost.
5th element said:
You dont dump current into ground and then it is gone. It will continue to flow towards the originator of the current closing the loop.
If you have a groundplane, you know where that current flows: under the trace from sender to receiver. Without it, it will flow in the groundtrace you have made around the edges of the pcb.
As will the currents of all the other signals. Result: interference.
So all the way around from receiver of the signal to the sender.
Think loops, not just current from A to B
Greetings,
No there is no ground plane. I have simply made copper islands around the empty free spaces around the circuit then joined all the places that require grounding in the circuit, to a place on the copper island.
If you are refering to a ground plane as one layer of copper is dedicated to ground then that is simply not possible for me to do, I need the two layers to connect it all together.
I intend on joining the top and bottom copper islands together via pins or something.
Is analogue that much different to digital? As I said before my active xover was made like this and it is deathly silent.
If you are refering to a ground plane as one layer of copper is dedicated to ground then that is simply not possible for me to do, I need the two layers to connect it all together.
I intend on joining the top and bottom copper islands together via pins or something.
Is analogue that much different to digital? As I said before my active xover was made like this and it is deathly silent.
Ive got an exam this mornin so dont really have time to look for that website to conferm my brain wave about grounding, then again it may be complete rubbish.
Am I gettin the wrong end of the stick with regards to this return path. Is the interference thing more like this. Instead of all the grounds actually being connected directly with the earth, you have to imagine they are all connected together, so they will tend to want to actually flow into each other, like a signal will want to travel from your CDP to your amp thru the interconnect. And its this transfer of signal from one ground point within a circuit to another ground point whithin the same circuit, that causes interference??
Also tiroth you say that you dont think it will work at at all? This I dont understand. Surely if the PCB is schematically perfect its gonna work? Even if the grounding could be a little better.
Am I gettin the wrong end of the stick with regards to this return path. Is the interference thing more like this. Instead of all the grounds actually being connected directly with the earth, you have to imagine they are all connected together, so they will tend to want to actually flow into each other, like a signal will want to travel from your CDP to your amp thru the interconnect. And its this transfer of signal from one ground point within a circuit to another ground point whithin the same circuit, that causes interference??
Also tiroth you say that you dont think it will work at at all? This I dont understand. Surely if the PCB is schematically perfect its gonna work? Even if the grounding could be a little better.
After all your constructive criticism and an email from Peranders it got me thinking that ive not really had a good word about the way ive done this. Having looked at something PA linked me to it got me playing with my PCB layout program. Needless to say I remade the grounding scheme on the PCB
What do you all think of this one now?? Better?
Green is copper top, red pink bottom.
What do you all think of this one now?? Better?
Green is copper top, red pink bottom.
Attachments
Some traces seem to be on both layers ??? (ic under at the left)
Try to get as much traces on one side (bottom) and then ONE groundlayer on the other side (top)
If traces need to cross, don't interrupt the groundplane, but use a wire (depends a bit of the traces you are routing, for fast signals you don't want to interrupt the groundplane underneith such a trace (if it is not interrupted, you know where the current
flows: on the groundplane underneith the trace).
So route the faster signals first (e.g. clock, data etc).
E.g. a 'reset' signal is not so critical.
It seems some traces are crossing the boundaries, going from one groundplane to the other, the return current has to go a long way.
But it is getting more difficult now to give comments,
since i don't know the design.
Seems the chip in the middle is a dac (?). Make shure
agnd and dgnd are connected at the dac.
You might have seen the dac pcb i made some time ago (2*tda1541). There are some (?) mistakes in there too...
Maybe the experts (guys that do routing for a living) can give some comments ?
gr
Try to get as much traces on one side (bottom) and then ONE groundlayer on the other side (top)
If traces need to cross, don't interrupt the groundplane, but use a wire (depends a bit of the traces you are routing, for fast signals you don't want to interrupt the groundplane underneith such a trace (if it is not interrupted, you know where the current
flows: on the groundplane underneith the trace).
So route the faster signals first (e.g. clock, data etc).
E.g. a 'reset' signal is not so critical.
It seems some traces are crossing the boundaries, going from one groundplane to the other, the return current has to go a long way.
But it is getting more difficult now to give comments,
since i don't know the design.
Seems the chip in the middle is a dac (?). Make shure
agnd and dgnd are connected at the dac.
You might have seen the dac pcb i made some time ago (2*tda1541). There are some (?) mistakes in there too...
Maybe the experts (guys that do routing for a living) can give some comments ?
gr
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