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γ24 DAC core module development

gamma3, gamma24 (plug-in for the γ3)

γ24 DAC core module development

Postby amb » April 10th, 2015, 2:33 am

Updated November 24, 2015

Current status

- The project is now complete and released.

Introduction

This subforum and thread is split from the γ3 high-resolution DAC development thread to focus specifically on the γ24 module, which will contain the digital interface receiver (DIR), digital interface transmitter (DIT), asynchronous sample rate converter (ASRC) and digital-analog converter (DAC) portions of the circuit.

Photo of the γ24 module

Here is a photo of an assembled γ24 module:
Image

Schematic diagram

The following is the current schematic diagram of the γ24 module, shown in two pages. You may need to maximize your browser window to see everything. You can also associate this diagram with the γ3 backplane board schematic, found in the first post of the γ3 development thread, in order to make sense of all the connections.

Page 1
Image

Page 2
Image

PDF version of the schematic

3D rendering

A 3D rendering of the PCB:
Image

PCB layout

Here is the top silkscreen and all four copper layers of the PCB (without ground plane fill for clarity). Click the image for full resolution:
Image

The four copper layers, individually, in full resolution:
- Top layer
- Inner layer 1
- Inner layer 2
- Bottom layer

Parts list

Please refer to the γ24 website for the latest parts list.

Note: Since this post had been updated as the project went along, some portions may contain descriptions of future tasks which are now done.
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Re: γ24 DIR/DIT/ASRC/DAC module development

Postby LV26 » April 10th, 2015, 2:54 am

I think using two LDO for WM8741 AVDD (+5V) good idea.
For info:
Look at LP38798.
Ultra-Low Output Noise: 5μV RMS (10 Hz to 100 kHz)
High PSRR: 90 dB at 10 kHz, 60 dB at 100 kHz

But IC package not good for DIY-ers. Need hot air soldering station. But I personally vote for LP38798 ;)

P.S.
Use dedicated LDO for Crystek OSC.
If you worry about cost - you can add shunt resistor to other 3.3V source and not place additional LDO.
Generally - using dedicated LDO`s for oscillators (especially for low-jitter) is a good design practice.
Everything should be made as simple as possible, but not simpler. // Albert Einstein
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Re: γ24 DIR/DIT/ASRC/DAC module development

Postby amb » April 10th, 2015, 3:10 am

DIY solderability is a large factor in device selection. I was looking at TPS7A4701 which has output noise of 4μV RMS (10 Hz to 100 kHz) and PSRR of 82dB at 100Hz and >55dB from 10Hz-10MHz. But I didn't choose it because of the tiny VQFN package.

I chose TPS7A4901DGN with MSOP-8 package that's still small, but can be soldered with a normal iron. It has good noise performance of 12.7μV RMS (20Hz to 20KHz) and PSRR of 72dB at 120Hz. This device has a thermal pad on the bottom that will not be conducive to DIY soldering, so I added an extra pass transistor to make it take the thermal load. This in effect changes the regulator from a strict LDO-type to a hybrid LDO-NPN regulator with increased dropout voltage, but in this application we'll be dropping from 10V down to 5V, so it's not an issue.

I tried to run PSpice simulation of this regulator circuit but unfortunately, the Spice model from TI is encrypted, and my version of OrCAD PSpice does not support that format. So I'll have to build one to test.

I like the idea of separate regulators for the two WM8741s, and I may do that.
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Re: γ24 DIR/DIT/ASRC/DAC module development

Postby l-train » April 10th, 2015, 7:11 am

How about the Analog Devices ADM7150? It has even lower noise (1.6uV) and comes in an SOIC-8 package with exposed pad.
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Re: γ24 DIR/DIT/ASRC/DAC module development

Postby dsolodov » April 10th, 2015, 7:38 am

Will ADM7150 be considered http://www.analog.com/media/en/technical-documentation/data-sheets/ADM7150.pdf? Analog Devices offers it in SOIC-8 package with 1.27mm pitch. ADM7150 has 1.7μV/sqrt(Hz) at 1MHz and 90db PSRR up to 100kHz. ADM7150 is rated for 800mA. The IC seems to fit well to criteria described.

Linear offers LT3042 in MSOP with 0.8μVrms and 79dB PSRR at 1MHz and is rated for 200mA.

I would try to avoid using a bypass transistor along with LDO for powering the audio DAC IC due to extra noise. The choice of available ICs is great. If input is 10V, I'd add another pre-regulator IC at entry to drop to 7.5VDC and kill some noise as well. Why is the input V so high?

I'd suggest to use a daughter board for local regulators, so that one could replace them as desired in the spirit of DIY instead of being locked into a single IC.
Completed AMB projects: M³, σ11, σ25, ε24, γ1, γ2, LCDuino
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Re: γ24 DIR/DIT/ASRC/DAC module development

Postby amb » April 10th, 2015, 11:39 am

ADM7150 looks interesting. I would probably still need to add a pass transistor despite the 200mA rating. It's the thermal dissipation that I'm concerned about. One single pass transistor shouldn't contribute much to the noise. My current plan is to use BC550C, which has very low noise.

The reason why the input is 10V is because it's intended to come from the output of the σ22 (configured for ±10V) for the α24 analog boards. Here is the basic power supply scheme:

1. One +5V regulated, always-on power supply for the LCDuino-1 and ζ1. The ζ1 has a local +3.3V LDO regulator, and gets enabled via the TPS22929D load switch chip on the backplane board when the γ24 powers up.
2. One +5V regulated power supply for the γ24 digital section, locally regulated to +3.3V and +1.8V for the oscillator, SRC4392, WM8741, etc.
3. One ±10V dual-rail regulated supply for the α24 analog boards. It also powers the γ24 analog section, regulated down to +5V locally.

That's three power supplies, most builds will use three separate power transformers, and I don't want to make it any more complex.

There is no need for pre-regulators for noise reduction purposes, because the inputs are all pre-regulated.

No daughterboards for the regulators, sorry. It's only going to make the γ3 more expensive. One thing you need to realize also, is that for every additional board there is, I have to tie up a good sum of $$ for inventory.
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Re: γ24 DIR/DIT/ASRC/DAC module development

Postby MisterX » April 10th, 2015, 2:56 pm

No ideas for discreet regulators?
Might be tough to match the performance of the LDO + pass transistor though....
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Re: γ24 DIR/DIT/ASRC/DAC module development

Postby amb » April 10th, 2015, 3:15 pm

It would have to be something like a mini σ22. Too many parts for the γ24. I'm trying to keep it from getting too big.
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Re: γ24 DIR/DIT/ASRC/DAC module development

Postby amb » April 10th, 2015, 11:31 pm

OK, it looks like I can do without adding a pass transistor if I use two ADM7150ARDZ-5.0, one for each WM8741. It will still run a bit hot, but within safe margins.

Given that this regulator has such superlative noise specs, should I use it for all the other regulators? The only drawback is cost -- they are more than 10x the price of the TPS793xx series, and we'll have six of them on the γ24.
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Re: γ24 DIR/DIT/ASRC/DAC module development

Postby Balcmeg » April 10th, 2015, 11:53 pm

I think the absolute cost is more interesting than the relative. How much more in real money is the ADM7159ARDz-5.0?
/Stefan
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