DAC output help

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juniorhifikit
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Re: DAC output help

Post by juniorhifikit »

JR. wrote:Glad it worked out. I suspect there was still a simpler fix, but for low volume projects it's not worth losing sleep over.


What is your general sense of the approach, after getting some time on it, without magic smoke leaking out?

JR
Generally, I think it's the perfect approach, especially considering that the solution uses parts I already have in the BOM. A diode would have been simpler, but somehow not as refined. I emailed my situation to the manufacturer of the DAC along with a schematic, and while they were not so inclined to comment on my design in general, they did say that the bias situation could cause a current sinking problem for the output amps on the DAC chip.

Another theoretical benefit to this solution, which I have yet to test, is that any fluctuation in the supply voltage (LM337 = 18v, feeding a 7805 =5v for DAC's & bias) would be somewhat reflected in the DAC output, and ultimately in the VCA output. Now with the bias tracking the 5v supply, it could "cancel-out" any of that minor voltage fluctuation, keeping the VCA output accurate. Neat!
Last edited by juniorhifikit on Wed Mar 07, 2012 3:39 am, edited 1 time in total.
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JR.
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Re: DAC output help

Post by JR. »

Sorry I wasn't clear. I wasn't as curious about the DAC circuitry, as the audio path it is controlling. Is that working to your satisfaction? Better, worse?

I suspect the swinging rail fix is probably over-kill, but we do what is expedient, to put things like this behind us. If you were building thousands of these, they would deserve more attention. I've done more than my share of sharp pencil design, where the odd opamp here and there is considered significant.

JR
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juniorhifikit
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Re: DAC output help

Post by juniorhifikit »

Ah - the audio! Well, the channel input to direct output sounds great - actually it sounds like nothing at all, which is as much as you can ask for in a VCA circuit! I'm very pleased, and will be running some more RMAA tests after the holidays. Now, once I track down a grounding problem that's causing the stereo bus amp to oscillate (Waynes 1570 design) I'll be off to the races!
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JR.
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Re: DAC output help

Post by JR. »

Thanks,, clean is good...

JR
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juniorhifikit
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Re: DAC output help

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Just wanted to report back on this project of mine...

Wayne's 1570 stereo bus amp is performing wonderfully. The oscillation problem I was having was due to a ground loop from some not-so-neat flying leads to correct for a pcb error in the prototype.

As for the DAC problem, it's come back to haunt me. The tracking CV bias supply has stopped the DAC's output amps from frying (when having to sink too much current before they're properly powered up), but I'm still having DAC chips die on me. I've been able to monitor the I2C serial connection between the microcontroler and the DAC, so I can see that the DAC is no longer responding (dead). Today I'm going to monitor the communication with a known-good DAC. I have three hunches:

1) supply voltage spike - but after 2 regulators in series (main +-18V regulator, then down to +5V with another regulator in series), with filter caps on each, I doubt this is it, but it's still a possibility. I'll probably need a better scope to know.

2) -12V bias frying the DAC's output amps - but we already addressed this with the bias supply tracking the DAC's +5V supply. The DAC's are no longer showing the same symptoms of fried output amps (0 volts or full rail output - nothing in between) so I don't think this is the problem either.

3) The microcontroler is powered by one 5V regulator (fed from the +7.5V digital supply) and tied to digital ground; while the DAC is powered from a separate 5V regulator (fed from the +18V analog supply) and tied to analog ground. The two chips are tied together by the I2C bus, which I can't remember if it's biased with the +5V digital supply or the +5V analog supply. Perhaps there's some difference in potential that's causing current to flow where it shouldn't... On the schematic, the I2C bus is biased from the +5V analog, but in the hand built prototype, I can't remember. It's currently being beta tested at a studio at the moment, so I can't check for a few days.

4) Pin 4 of the DAC, which is the LDAC pin, was left floating (as per the instructions in the data sheet), but in a design note they stated it should be grounded. When this pin goes from high to low, it's possible to change the DAC's bus ID, and also power-down individual outputs. That's not something I want to do, so I've since grounded the LDAC pin, replaced all the dead DAC's, and put it back into service. Crossing my fingers and waiting is not a very solid problem solving technique!

Any thoughts would be greatly appreciated, as usual!

Sorry for the huge image - smaller was unreadable. Might have to open it in another window
Image
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JR.
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Re: DAC output help

Post by JR. »

juniorhifikit wrote:Just wanted to report back on this project of mine...

Wayne's 1570 stereo bus amp is performing wonderfully. The oscillation problem I was having was due to a ground loop from some not-so-neat flying leads to correct for a pcb error in the prototype.

As for the DAC problem, it's come back to haunt me. The tracking CV bias supply has stopped the DAC's output amps from frying (when having to sink too much current before they're properly powered up), but I'm still having DAC chips die on me. I've been able to monitor the I2C serial connection between the microcontroler and the DAC, so I can see that the DAC is no longer responding (dead). Today I'm going to monitor the communication with a known-good DAC. I have three hunches:

1) supply voltage spike - but after 2 regulators in series (main +-18V regulator, then down to +5V with another regulator in series), with filter caps on each, I doubt this is it, but it's still a possibility. I'll probably need a better scope to know.

2) -12V bias frying the DAC's output amps - but we already addressed this with the bias supply tracking the DAC's +5V supply. The DAC's are no longer showing the same symptoms of fried output amps (0 volts or full rail output - nothing in between) so I don't think this is the problem either.

3) The microcontroler is powered by one 5V regulator (fed from the +7.5V digital supply) and tied to digital ground; while the DAC is powered from a separate 5V regulator (fed from the +18V analog supply) and tied to analog ground. The two chips are tied together by the I2C bus, which I can't remember if it's biased with the +5V digital supply or the +5V analog supply. Perhaps there's some difference in potential that's causing current to flow where it shouldn't... On the schematic, the I2C bus is biased from the +5V analog, but in the hand built prototype, I can't remember. It's currently being beta tested at a studio at the moment, so I can't check for a few days.

4) Pin 4 of the DAC, which is the LDAC pin, was left floating (as per the instructions in the data sheet), but in a design note they stated it should be grounded. When this pin goes from high to low, it's possible to change the DAC's bus ID, and also power-down individual outputs. That's not something I want to do, so I've since grounded the LDAC pin, replaced all the dead DAC's, and put it back into service. Crossing my fingers and waiting is not a very solid problem solving technique!

Any thoughts would be greatly appreciated, as usual!

Sorry for the huge image - smaller was unreadable. Might have to open it in another window
That huge image didn't show the whole thing anyhow... maybe you need to make it bigger. :lol:

I still think the tracking bias rail is over kill.

We've probably gone around this tree before but you should be able to diode clamp the resistor string going to the control voltage node so a) it can't go very negative, and b) with resistor in series that current can't be enough to hurt anything.

I seems from a casual inspection that perhaps breaking the R from the DAC into two series R's and a diode clamp there to ground should work and still allow - control voltage more than -1 diode drop... probably several ways to skin this cat...

If you need to you could put a diode in series with the r from the DAC so clamp would hold DEC side to no lower than ground, but that shouldn't be necessary.

The divider from the dac could also be redesigned, say with a first divide from the output of the dac to ground. then from that node another resistor feeding the divider resistor to -V this should put less stress on the DAC.

The two diodes approach could insure that DAC never sees any negative voltage.

?? Is there any transient or otherwise difference between analog ground and logic ground voltage (VSS)?

Those parts look pretty robust... should be harder to kill than that.




JR

PS: I am not immune from blowing stuff up... I recently fried a 44pin PIC and two LED latch drivers, on a proto when I lost a posiitve bias string on A/D inputs and apparently hit the PIC input with too much negative juju for too long... FWIW I changed the design to cap couple all A/D inputs so even a serious DC fail in the future will only be transient.

Odd to see the failure vector reach right through the PIC A/D inputs and out to a couple LED latches through the serial com lines. but fault current has to go somewhere, and positive supply only chips don't like negative voltages at all....
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juniorhifikit
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Re: DAC output help

Post by juniorhifikit »

Thanks JR
JR. wrote: That huge image didn't show the whole thing anyhow... maybe you need to make it bigger. :lol:
Yeah - sorry. Hard to keep the values readable. I consolidated and shrunk the schematic, but it still might need to opened in a new browser window.

I thought about the diode clamp idea from the last conversation, but the DAC's output amps don't seem to be frying on an individual basis like last time. Now the DAC's seem to be dying en mass, one at a time - as if it's a supply problem or an issue on the I2C bus, similar to your anecdote:
Odd to see the failure vector reach right through the PIC A/D inputs and out to a couple LED latches through the serial com lines. but fault current has to go somewhere, and positive supply only chips don't like negative voltages at all....
Since the serial bus is biased from the same supply as the DAC's, that could be a suspect. Perhaps I should consider clamping the output of the 5V supply as well? I bought a new scope and have some investigating to do - will report back...
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JR.
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Re: DAC output help

Post by JR. »

juniorhifikit wrote:Thanks JR
JR. wrote: That huge image didn't show the whole thing anyhow... maybe you need to make it bigger. :lol:
Yeah - sorry. Hard to keep the values readable. I consolidated and shrunk the schematic, but it still might need to opened in a new browser window.

I thought about the diode clamp idea from the last conversation, but the DAC's output amps don't seem to be frying on an individual basis like last time. Now the DAC's seem to be dying en mass, one at a time - as if it's a supply problem or an issue on the I2C bus, similar to your anecdote:
The DAC data sheet says no more than .3V below ground.

The DACs have a pretty healthy output, (25ma or so) so it seems hard to imagine the outputs being pulled that low, but something is happening. Maybe they power up with the outputs turned off, so get pulled negative then.

Odd to see the failure vector reach right through the PIC A/D inputs and out to a couple LED latches through the serial com lines. but fault current has to go somewhere, and positive supply only chips don't like negative voltages at all....
Since the serial bus is biased from the same supply as the DAC's, that could be a suspect. Perhaps I should consider clamping the output of the 5V supply as well? I bought a new scope and have some investigating to do - will report back...
The 5V supply if regulated should be safe. My anecdote involved the fault reaching through a broken part... AFAIK you only have the DACs releasing smoke, nothing else.

Perhaps use your scope to look at turn on behaviors, or perhaps turn off, if one supply comes up sooner or collapses faster at turn off...

JR
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juniorhifikit
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Re: DAC output help

Post by juniorhifikit »

JR. wrote:The DACs have a pretty healthy output, (25ma or so) so it seems hard to imagine the outputs being pulled that low, but something is happening. Maybe they power up with the outputs turned off, so get pulled negative then.
This gives me food for thought... Previously, the DAC's pin 4 (LDAC) was left floating, which is responsible for programing the DAC's ID, and for choosing the each output's on/off state. I tied that pin to ground and so far have had no more issues. New scope arrives today, and so I will recover the unit from the field, do some tests, and report back.

Thanks again for all the nuggets!
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mediatechnology
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Re: DAC output help

Post by mediatechnology »

JR. wrote:
The DACs have a pretty healthy output, (25ma or so) so it seems hard to imagine the outputs being pulled that low, but something is happening. Maybe they power up with the outputs turned off, so get pulled negative then.
juniorhifikit wrote:
This gives me food for thought... Previously, the DAC's pin 4 (LDAC) was left floating, which is responsible for programing the DAC's ID, and for choosing the each output's on/off state. I tied that pin to ground and so far have had no more issues. New scope arrives today, and so I will recover the unit from the field, do some tests, and report back.
I think I suggested the same thing back in posts 4 and 6.

Me:
With analog switches in series with the op-amp output, the OA is likely not there to help sink the current if, during intialization, they're open.
That makes the current path likely flow through the ESD diode and/or some parasitic to the substrate.
Glad you found the floating pin.
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