ADAT DAC Question

[kmt]

Hello to everyone!

I'm trying to design a ADAT-to-analog-converter, but I ran into several problems. I hope there's someone out there who can help me with this. First a short description of my circuit:

I use a AL1402 optoRec by Alesis(Wavefront) semiconductor, connected to a TORX178B opto receiver (47uH and the 100nF capacitor are connected, too). The AL1402 is connected to a Burr-Brown PCM1606 DAC with following signals:

AL1402 PCM1606
DVCO --> 50 ohm resistor --> SCKI
BCLK --> 74LS04 ---> 50 ohm-> BCLK
WDCLK --> 50 ohm resistor --> LRCLK

The AL1402 works in mode 0 (master) and format 01 (left-justified, data valid on falling edge of BCLK?!?).
The PCM1606 works in format 11 (left-justified, data valid on rising edge of BCLK)

A 74LS04 inverter is connected in the BCLK signal to change the polarity of the BCLK signal. (rising/falling edge).
The De-emphasis of the PCM1606 is switched off. (BTW, what is it for at all?)

My problem is: i hear a distorted signal from the analog output. On the scope there are visible peaks in the analog signal, all pointing from signal to GND. Changing the SCKI-source from SVCO to DVCO improved the quality, but there is still a distortion in the signal. So I have several questions:

1. is a TORX178B-receiver possible for ADAT use? Or will I have to use the TORX173?

2. I don't get a stable WDCLK-signal (scope) on the output of the AL1402. Is that normal, or is here my problem?

3. Does anybody have a reference design with the AL1402 or the PCM1606? The datasheet of the AL1402 doesn't show enough interconnections to the DAC...

4. Is the LS04 fast enough for the BCLK-signal?

Any comments are appreciated!

Greetings
KMT

[kmt]

I think I fixed the problem now. I had to invert the signal coming from the TORX178B (I don't know why, actually, but that fixed the problem). The only problem left is that there is much noise (white noise?!?) on the analog outputs. Is there a way to improve that?

Greetings
KMT

[meta]

Hi...

This sounds a very interesting project - could u post some schems perhaps? What opamps are u using for the outputs? And whatabout clk generation? I'm wokrin on a similar project for an adat adc box...

Best

D

[kmt]

Hi, meta!

Glad You're interested in my ADAT-DAC circuit.
Many problems disappeared til now, and an evaluation circuit is working. As opamps, I use the LM833, but I guess a good
low-noise opamp OP... by Burr-Brown would do the same.

Attached You can see a schematic of the test PCB. Supply voltage for the AL1402 and CS4228 is +5VDC, for the LM833 is +/- 15VDC. The connection to the AT90S8515 is to initialize the CS4228. Unfortunately, the use of a microcontroller seems to be the only way to set the interface format, mute control etc. of the CS4228.

A thing I'm going to change in the next versions is that the analog and digital power supplies are (yet) tied together. To improve SNR etc. I'm going to separate them.

The CLK generation doesn't seem to be the problem when receiving ADAT, because the AL1402 generates all required CLK signals for driving the CS4228. The only problem was the polarity of the BitCLK signal, which I had to change to drive the CS4228.

BTW, I've got a question, too: whats the intended use for the De-emphasis in the DACs? Should I turn it on always or is there a "hidden status bit" in the ADAT data stream which tells me to turn it on or off?!? I noticed that the sound is a little less clear with the De-emphasis turned on. An effect I'd like to avoid.

Greetings from Germany!

KMT

[swak]

Hi ktm,

How is your circuit working?

May I ask where did you buy the Alesis chip in Germany?

I am thinking of multi-channel DAC to use with the computer soundcards ADAT output. Have you seen by chance an implementation of this?

The DAC will be simple Non oversamling using the TDA 1543... will try to go as simple as possible.

regards,
Sebastian

[kmt]

Hi, Sebastian!

I changed the used DAC to the AK4356 by AKM, and it's working pretty fine. It sounds very good, very low noise and the AK4356 is much easier to handle than the CS4228. It has symmetrical outputs, and I connected it to the proposed filter circuit by AKM (only a few resistors, caps and a NE5532A). The sound is really pretty fine, "jenseits von Gut und Böse". The only problem is to get the AK4356. I had to order them in USA...

Greetz from Hamburg
Kai Markus

[Skwit]

@kmt

Why do you have the resistors on the DVCO, WDLK etc. connections? Did you care about seperate ground planes for digital and analog ground? And did you do any measurements about SNR? Is there an improvement from TORX 173 to 178?

I for myself am developing the same application: Adat -> 8 analog using AL 1402 and at the moment Cirrus CS 4382. In the next iteration I will switch to AKM AK 4358.

Furthermore, do you have any source for the AL1402 for private use in germany?

regards,
Christian

[bitman]

Skwit,

I am gathering the documentation to do what you are doing. I also wish to interface a AL1402 to a CS4382.

Do you have any success stories or schematics of what you have done so far as to keep me from going down any dead end roads before I start soldering?

I would much appreciate it.


Ron

[Skwit]

Do you have any success stories or schematics of what you have done so far

Yes, I have success stories, the device works. The CS4358 was replaced by an AKM 4358, the AKM was cheaper and did not get as hot as the Cirrus. If you use an AL1402 you have to invert the BCLK-Line.

I tried several methods of grounding, I ended with two ground planes, the digital parts in the digital domain, the analog parts including the DAC in the analog domain. They can be connected by a jumper. I think I measured 109dB(A) SNR, with which I'm quite happy.

I can't give away schematics or layout, I made this device for a company, I think they won't be happy if I give it away.

Where did you get the AL1402?

Christian

[kmt]

Hi to everyone!

It has been a long time since I built up my surround distribution amplifier, but I try to remember the design rules I applied that time...

The schematics I posted are obsolete. As I said in my posts before, I replaced the DAC with a AK4356, a multichannel DAC with balanced outputs. It works pretty fine. Attached You can see the "new" schematics of the DAC. The only thing connected to the output stage of the DAC is an active LPF as described in the AK4356 data sheet.

As You see in the schematics, I completely separated the analogue and digital power rails in order to improve SNR. I must confess, I never really tested the overall SNR of the amplifier :-) I only did a "rehearsal" at my kolloquium at the end of my diploma work. The amplifier was connected to my DIGI002R soundcard via ADAT and XLR, and at the other end, connected to a very expensive surround sound studio monitor system by "Musikelektronik Geithain". There was no "audible" noise caused by the amplifier :-)

I got my AL1402 in UK, by www.profusionplc.com
As You said, it's sometimes very hard to get such devices in Germany. The wholesale dealers here aren't interested in selling 1-10 pieces of a device. I always purchase special parts in the UK or in the USA. It takes a little bit time, but I made the experience that dealers are much friendlier to DIY users... That's what I call "service"!

Greetz from Hamburg
KMT