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Old 15th October 2015, 02:52 PM   #1
zdr is offline zdr  Belgium
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Default Pcm2707/1794 usb dac

I am designing my first usb dac, based on pupdac. I need to make it as small as possible and keep the quality sound. I want to be able to swap opamps, so the power supplied will be +-5v instead of 2.5. For power inverter, i will use lm2776. I plan to use only x7r/nop ceramics across the board. Does all this make sense so far? =)
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Old 15th October 2015, 03:32 PM   #2
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Sure, why not.

I put DIR9001, SCR4192, and PCM1794A all on a 25x55mm single sided board.

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Old 2nd November 2015, 06:16 PM   #3
zdr is offline zdr  Belgium
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Board is finished, will be dual sided. I added tlv702 regulators on all pins, plus 72301 on negative rail. I also used external clock xosm57 from Vishay.

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Old 3rd November 2015, 06:58 AM   #4
marce is offline marce  United Kingdom
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Using X7R for decoupling duties is common practice, in fact every board I lay out uses x7rs for decoupling duties, with the occasional COG/NP0 used in the odd critical place (oscillators).
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Old 22nd November 2015, 10:45 AM   #5
zdr is offline zdr  Belgium
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I am stuck with LM2776 that does not seem to work. It should invert usb 5v into negative, but instead it outputs -2V only. I used schematics from datasheet: one 2u2 x7r at the input, one 2u2 x7r at the output, and one 1u x7r as flying capacitor. I tried three different samples and they all misbehave. There is nothing connected to the output or anywhere else. What could I be doing wrong here?
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Old 22nd November 2015, 01:59 PM   #6
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Quote:
Originally Posted by theAnonymous1 View Post
Sure, why not.

I put DIR9001, SCR4192, and PCM1794A all on a 25x55mm single sided board.

Click the image to open in full size.
Nice work, it appears that a steady hand was used for all of that SMD soldering. What type of analog side solution do you connect it to?
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Old 22nd November 2015, 02:25 PM   #7
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Quote:
Originally Posted by zdr View Post
I am stuck with LM2776 that does not seem to work. It should invert usb 5v into negative, but instead it outputs -2V only. I used schematics from datasheet: one 2u2 x7r at the input, one 2u2 x7r at the output, and one 1u x7r as flying capacitor. I tried three different samples and they all misbehave. There is nothing connected to the output or anywhere else. What could I be doing wrong here?
That does seem odd. It is unlikely that you have three defective devices, and the application circuit could hardly be more simple. Some suggestions:

1) Make certain that you have the EN input (pin 4) tied logic high, or to Vin.
2) Feel with your finger whether device is obviously warm to the touch, in which case it may be experiencing thermal shutdown for some reason.
3) Should the above two suggestions pass, try placing a 1k resistor from the output to ground so as to provide a defined load. The datasheet speaks about how the operational mode of the device changes if the output is lightly loaded. It may simply not be able to regulate properly without some loading.
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Old 22nd November 2015, 08:31 PM   #8
zdr is offline zdr  Belgium
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Tried all this already, I am banging my head:

1) When I remove Cout, device is not supposed to work, because there is no cap to hold negative charge - yet, Vout goes as low as -4.4V!
2) EN is tied to +5V, no doubt about it
3) Tried with different resistors on the output (1k and 100R), but absolute value of Vout drops even more.
4) Tried different output caps - lowest voltage is without Cout, and highest is with 10uF: -1.8V
5) I am using three different samples on three different PCBs - there is no way soldering or defective components are on all of them.
6) Even tried soldering LM2776 wrong way, lucky to have USB power protection
7) Device is cold on touch.
8) Gave PCB a good scrub in white spirit to eliminate excess flux.

Device does work on 2MHz, so placement of components might be critical. I have flying cap as close to LM2776 as it can get. Cin and Cout could be closer, they are on the opposite sides of chip connected through GND plane. They are all mounted on top layer as recommended. Caps are 1206 size.
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Last edited by zdr; 22nd November 2015 at 08:37 PM.
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Old 22nd November 2015, 09:10 PM   #9
zdr is offline zdr  Belgium
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Blimey, this was it! Cin, Cout and GND were too far away for 2MHz operation! I shorted them with a wire as short as possible and voila - 4.9V!
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Old 17th February 2016, 01:58 PM   #10
zdr is offline zdr  Belgium
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DAC is finished and even works quite well

Measurement done with internal Audigy ZS sound card.
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