# MiniDAC - AD1955 DAC with active I/V



## error401

So here I go again with the ever-more-difficult-and-more-expensive projects. This time I wanted to try out a DAC that doesn't get much exposure around here since you really need a microcontroller to set it up properly - the AD1955. On paper, it's very competitive with PCM1794 and I'd like to give an active I/V a try - everything I have is either voltage out or passive I/V. In my continuing quest for ever-smaller boards, I've managed a layout in only 1.5"x2.5", which wasn't easy. That will make the build fun though, an average of 26 components per square inch in that space on both sides of the board.

 Before I go ahead and send the design off for fabrication, I'm asking you folks to proofread my schematic and make any comments. It's pretty much lifted straight from the AD1955 datasheet with some simple changes. Anyone interested?

 Here's the schematic. Please check out the PDF since it's too large to be readable at forum-size.


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## sleepy dan

I'm interested. Will you be making more than one board? I'd also want to use a THS4131 for I/V as recommended by Twisted Pair.


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## Clutz

What are you going to use to get the I2S? Are you just going to leave that open or are you going to put some sort of SPDIF or USB input? If you leave it open, I might be interested in getting a board too.


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## Filburt

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## error401

Quote:


  Originally Posted by *Filburt* /img/forum/go_quote.gif 
_If you plan to use anything other than the 797, you'll probably need to play around with the exact component values to optimise things if you wish to stick with that schematic.

 As far as layout goes, are you sure a double-sided board is a good idea?_

 

That's actually something I was wondering about. A smattering of posts around the web seems to indicate that people are using that datasheet schematic with different opamps like OPA627 or OP275 without ill effects, but I'm having a hard time finding much detail and the datasheet is rather lacking as well. My layout uses dual opamps; there's no way it would fit with singles though, so I can't use AD797. I was planning on using LME49720 or LM4562 since I've liked them both a lot, they aren't too expensive and are fairly easy to use. Can you give any advice on tuning the component values?

 And no, I'm not sure double-sided will be sufficient. The ground planes are a bit of a mess - but I was able to get everything routed and the main analog and feedback paths have no vias and good separation. I need to take a closer look at the major current loops and see if there's anything really bad, if so I may need to go to 4 layer. I might try it anyway with a 2-layer board since it's pretty cheap to try it out and if it fails I can spend the money on a 4-layer redo. 4-layer prototypes aren't cheap. When the layout is finalized, I'll post more details on what I end up with. I might relax the size requirement a bit to clean up the ground.

  Quote:


  Originally Posted by *Clutz* 
_What are you going to use to get the I2S? Are you just going to leave that open or are you going to put some sort of SPDIF or USB input? If you leave it open, I might be interested in getting a board too._

 

I'm laying out a USB board right now, which I'm planning to fab at the same time, and I've got my Opus SPDIF module (though I'll probably do my own just for kicks, and to use the input mux). It seems like I'm just replicating the Opus line, but it's more for fun and to try some obscure stuff than for the pursuit of the right sound. I've also got PGA2320 volume control and a bipolar power supply board in the mail to go along with these.

  Quote:


  Originally Posted by *sleepy dan* 
_I'm interested. Will you be making more than one board? I'd also want to use a THS4131 for I/V as recommended by Twisted Pair._

 

I'll probably make a couple, but not more than that to start. I'm short on funds at the moment and since I've got extra time, I'm starting a lot of projects. If you like the final layout, we can probably arrange something before I go to fab if you really want in, but keep in mind you'll need an AVR programmer as well to flash the microcontroller. Unfortunately THS4131 won't work with this design, which uses vanilla opamps. Most dual opamps should work though.


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## majkel

Go crazy, built SRPP output! Lampizator CD player upgrade tubed output stage
 It's a CD player modification actually but you can follow the ideas - passive I/V conversion on a resistor and a tube buffer after it.


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## FS2

I've been looking at the same part too, for a certain iPod project (heh). I've got the AD1955 in hand, but not started the board yet.

 I believe AD uses the same schematic for their eval board, which is said to sound fantastic. And I've seen the same circuit in a analog design article somewhere. Unfortunately I didn't bookmark it, but I think it was a Walt Jung article and I was searching for active I/V converters and low pass filters.

 Those 100 Ohm resistors in the op-amp feedback loops are recommended for the AD797 from it's datasheet. I don't know if you'd need them for an alternative op-amp.

 Overall, the circuit is also a fourth-order low pass filter, and the only thing I can remember from the article is that C29,C35,C39 and C40 need to be tweaked to obtain the desired response. But since AD ended up with those values, they should be OK. 
	

	
	
		
		

		
			





 I take it you're using 0.1 uF decoupling per op-amp per rail? The reference circuit also adds a 4.7 uF tant in parallel with each one. I've been looking for alternatives, but they all have much lower ESR than tantalums, which may be a problem.

 How did you derive your values for R5 and R6?


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## error401

Quote:


  Originally Posted by *FS2* /img/forum/go_quote.gif 
_I've been looking at the same part too, for a certain iPod project (heh). I've got the AD1955 in hand, but not started the board yet.

 I believe AD uses the same schematic for their eval board, which is said to sound fantastic. And I've seen the same circuit in a analog design article somewhere. Unfortunately I didn't bookmark it, but I think it was a Walt Jung article and I was searching for active I/V converters and low pass filters.

 Those 100 Ohm resistors in the op-amp feedback loops are recommended for the AD797 from it's datasheet. I don't know if you'd need them for an alternative op-amp.

 Overall, the circuit is also a fourth-order low pass filter, and the only thing I can remember from the article is that C29,C35,C39 and C40 need to be tweaked to obtain the desired response. But since AD ended up with those values, they should be OK. 
	

	
	
		
		

		
		
	


	




 I take it you're using 0.1 uF decoupling per op-amp per rail? The reference circuit also adds a 4.7 uF tant in parallel with each one. I've been looking for alternatives, but they all have much lower ESR than tantalums, which may be a problem.

 How did you derive your values for R5 and R6?_

 

That was my thinking as well: it's in a publication from a major manufacturer, the values should be well-chosen. I need to look into it a bit more though. I wouldn't be surprised if Walt Jung designed this circuit himself, since he has worked for AD for many years. I'll see if I can source the article you mention, as it would be good reading.

 I am using 0.1uF per op-amp per rail, plus a bank of 4x100uF polymer electrolytics shared between them. I believe this should be sufficient, but since I can probably fit them in, I might add those tants as well. There is also 0.1uF and 47uF after an LP2985 for each AD1955 supply.


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## FS2

Well, "I believe" may have been an understatement... I came across a pdf that appears to contain the schematic and BoM for the AD eval board. 
	

	
	
		
		

		
		
	


	




 The bad part is the tantalums they listed were Panasonic EF series parts that have been discontinued, I can't find data on their ESR. As an alternative I was looking at Panasonic FK series in B size (1.35 Ohm impedence and more than 4.7 uF) or their SP-CAP and possibly a resistor in series with it. (Both for an all surface mount board design) That said, I don't know for sure if the BoM is real, or if it was actually used.

 LP2985 is something I'll have to look at. I was considering REG101s for the 5V supplies and 2.8V non-inverting references. I picked up some of the 2.8V ones, but he 5V were out of stock.

 Oh, and just to avoid any misunderstanding, I saw the article on some website to do with analog design, not Analog Devices' website.


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## error401

Quote:


  Originally Posted by *FS2* /img/forum/go_quote.gif 
_Well, "I believe" may have been an understatement... I came across a pdf that appears to contain the schematic and BoM for the AD eval board. 
	

	
	
		
		

		
		
	


	




 The bad part is the tantalums they listed were Panasonic EF series parts that have been discontinued, I can't find data on their ESR. As an alternative I was looking at Panasonic FK series in B size (1.35 Ohm impedence and more than 4.7 uF) or their SP-CAP and possibly a resistor in series with it. (Both for an all surface mount board design) That said, I don't know for sure if the BoM is real, or if it was actually used._

 

Me too: http://home.tiscali.cz:8080/saudio/AD1955EB.pdf and it seems legit. I have no reason to believe it's not accurate, at least to the point of being usable.

 DigiKey part numbers are listed and they seem to still have the EF series datasheet on their site if you want it: http://www.panasonic.com/industrial/...ABF0000CE5.pdf . ESR doesn't seem to be specified however, but I wouldn't be that concerned about it. They are decoupling capacitors and should be 'the lower the better'. Thus, I think my use of UCC PSA series (25mR ESR) should be fine.

  Quote:


 LP2985 is something I'll have to look at. I was considering REG101s for the 5V supplies and 2.8V non-inverting references. I picked up some of the 2.8V ones, but he 5V were out of stock. 
 

REG101 seem to be getting hard to source so I've stopped using them. LP2985 specs almost as well and is much easier to source. It's also about half as much per device. If you're planning on using an active device for the reference voltage (I'm using the resistive divider with decoupling), you might look at a voltage reference instead of a regulator. You should be able to find one with much lower noise and better temperature stability than either REG101 or LP2985. Personally I think the divider is sufficient since the bias voltage only drives opamp non-inverting inputs, it should be a very static, very small load. An active device may actually be worse since it's probably more temperature dependent.

 And you previous asked about R5 and R6. These are calculated based on a formula in the datasheet: R = (V+ - VBIAS) / (IBIAS + (VBIAS / Riv)). In my case, this works out to R = (15 - 2.80) / (3.24mA + (2.80 / 2000)), R=~2.63K. As far as I can tell, Riv is the feedback resistor and Ibias is given in the datasheet as -3.24mA.

 It's a nice circuit. Very well designed.


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## error401

The USB board is pretty much done. Check out the PDF. Just a basic PCM2707 implementation, nothing special at all, but the board size and layout matches the plan for the DAC so far...


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## FS2

Right, found the article that has a _very_ brief mention of the circuit:

Using Op Amps with Data Converters - Part 5

 last one on the page, and the values are different.

 I don't think DigiKey has any stock left of the EF capacitors in the schematic, hence my searching for an alternative. And unforutnately, this is one case where lower isn't necessarily better...

 If you look at the impedence frequency plot of a cap, it's v-shaped. The left hand side decreases due to capacitance, but the rising arm on the right is due to ESL. The minimum point is determined (among other things) by the value of the cap, a larger valued cap will have a minimum at a lower frequency.

 Now when you put a large valued cap in parallel with a small one, eg the classic 4.7 uF and 0.1 uF, the ESL of the larger cap and the capacitance of the smaller one form a resonant tank circuit, leading to a unexpectedly high impedence peak between the two valleys.

 The way to get around this is to add some series resistance into the circuit, this has the unwanted affect of raising (rounding off) the valley of the cap you put the resistor in series with, but the plus is with the right value, you can wipe out the unwanted peak. (Or in RF terms, you add a Q-spoiling resistor.) And tantalum caps just happen to have about the right amount of resistance to do that...


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## error401

I actually built this board up almost 3 months ago, and I haven't posted here because I wasn't able to get it to work! Both 5V power supplies were being dragged down to ~1V and were in current-limit. I couldn't find any obvious bridges and reflowing didn't help.

 The major error I found today after a few hours of probing was that I had ordered 2.21 and 2.80 OHM resistors instead of 2.21K and 2.80K for the VBIAS divider! 5 ohms to ground on AVDD! I didn't have any proper values on hand, but I was able to make some substitutes that get VBIAS at about 2.9V, which is pretty close. After making that substitution, I still had the same basic problem, and finally decided to replace the AD1955 itself, figuring it must have died at some point. It was a tough decision since they're almost $15 each and replacing a 28-SSOP isn't exactly trivial.

 Long story short: *it works!*. And to my ear, it sounds pretty fantastic. Definitely more crisp, with more bass and cleaner mids than the AlienDAC I had hooked up just prior to trialing this one. My worries about the active I/V were unfounded, as it sounds great and has minimal DC offset (~3mV). Definitely this is going to be my new 'benchmark' DAC, burned terminal blocks, lifted pads and all. I think I like the sound better than my WM8740-based Opus, but that could easily be placebo 
	

	
	
		
		

		
		
	


	




. It's definitely on the same level though. I haven't done any microcontroller code though, so it's just running with default values right now. I wonder if it's even worth bothering, it sounds that good!

 Now my entire signal chain except for the amplifier is self-designed and self-built. I've got a MiniPow for power, MiniUSB -> MiniDAC -> MiniVol for signal, and the CKKIII as my amp right now. Sublime.

 I don't think I'll be producing more of these, the build is pretty difficult, it's far too dense to be practical. Here's the specifics for anyone interested though:

MiniDAC schematic and layout (pdf) (yes, the bottom slikscreen is a disaster)
MiniUSB schematic and layout (pdf)

 If anyone wants more specifics (e.g. BOM, gerbers, whatever) feel free to ask and ye shall receive.

 *goes to listen to some Pink Floyd*


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## Mazuki

We should do a group PCB buy for this, not many AD1955 DACs out there.


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## holland

I was thinking the same thing.

 Can you do an RMAA for comparative purposes?


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## BrianDonegan

Quote:


 LME49720 or LM4562 
 

Same opamp inside.


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## tcpoint

If you decide on a group buy, I'm in.


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## error401

Quote:


  Originally Posted by *holland* /img/forum/go_quote.gif 
_I was thinking the same thing.

 Can you do an RMAA for comparative purposes?_

 

Unfortunately I don't have a reasonable capture device at all! It's on the list and I'm hoping to get an Audiophile 2496 or something similar soon, but at the moment all I have is a broken PCM2900-based thing with serious crosstalk and frequency response issues. I will post results when I can get them, I might be able to run the test at the girlfriend's place or at the upcoming Vancouver meet. Or I could just buy the hardware that I need anyway.


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## error401

Finally I've obtained a nice capture card so I can publish some RMAA results. Capture device is an e-mu 1212m (original revision), unmodified (though I do have plans to upgrade the input and output stages). Unfortunately I don't have a pair of 1/4" TS->1/4" TS cables right now to do a loopback measurement. I might try to get some 1/4" to RCA adapters soon. Test setup has the DAC powered by a MiniPow at +/-15V and connected to a PCM2707-based receiver mentioned earlier in the thread. It's connected to the e-mu 1212m by about 6ft of StarQuad with the shield unconnected. Results at 48KHz here: RightMark Audio Analyzer test : MiniDAC 48KHz

 I believe the poor THD+N result is due to a long ~6ft unshielded interconnect running behind my computer near DVI, AC power and other signals. I'll try to come up with a better test environment later.

 The frequency response is a bit more puzzling, as far as I understood my circuit, it should have flat response out to around 100KHz - but I could be misunderstand it as it was ripped off from Analog. Anyone have any ideas about this?

 Noise is low aside from the strange appearance of the relatively strong signals at 1KHz, 2KHz and 3KHz. I think this could also be improved with a better PSU to push 60Hz further down towards -120dB. I'm guessing the other fundamentals are coming from the IC's proximity to other signals.

 Crosstalk is a weird one too. My guess is that this is primarily caused by coupling in the interconnect since I'm using one length of StarQuad for both signals. I might build some shielded cables that are set up with separate wires for each signal to test further. The log increase seems telling - but I'm not sure of what - is this just the result of higher frequencies coupling more readily?

 I might obtain results at higher sample rates later using an S/PDIF receiver instead. I'd then have to actually write the firmware to configure the AD1955 .


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## borisov57

Hi error401,

 I found Rightmark great tool for evaluating dac-s and amps. I also had some issues when I have started using it. I have custom ADC for measuring and it has fairly low input impedance ( cca 500 Ohms ). So resistance of contacts becomes an issue if ground is shared between channels. But this type of crosstalk is resistive and has same amplitude for all frequencies. Yours is not, it rises with f.
 So my guess is, that it originates from shared power supply, but that is just a guess.

 If 0.5 dB roll-off at 20 kHz bothers you, you can always move your filters up in frequency.

 With 16-bit measuring, you are limited to about 98dB SNR/DNR measurements, so you can try RMAA at 24bits. Changes, that you made to your circuits, will be more apparent.

 If you want, I can send you my measurements of AD1955 dac for comparison purposes.


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## error401

Interesting idea about the crosstalk, I think e-mu 1212m has 47K input impedance (though it's not really specified anywhere that I can see). I'm pretty sure the almost-perfect log response points to something obvious, but I'm not knowledgable to know what. You could be right about the power supply, looking at the LME49720 datasheet I do see that PSRR degrades along a similar curve, down to about -75dB at 10K. I'd think the decoupling would be sufficient to keep this down though. I really don't know enough to say for sure, and I'm not sure what power supply topology would help. What power supply setup are you using for your I/V stage?

 I will leave the filters for now, it doesn't bother me so much as I can't explain it. I may have simulated the filters incorrectly, but I believe their response extends out to 100KHz before rolloff.

 Is it possible to set up RMAA to measure at 96/24 while playing back at 16/48? My USB module can only handle up to 16/48 and I haven't made any measurements with S/PDIF input at higher rates yet. That's next on the list when I have time 
	

	
	
		
		

		
			





 I would be interested to see more on your DAC. Do you have a website or such for it?


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## borisov57

Hi,
 I am using Jung regulators for I/V and for analog portion of dac.
 Yes, with rightmark 5.5 you can play 16bit/48kHz and record with 24bit/48kHz, but I guess it is O.K. if you set all to 24bit/48khz and usb drivers will take care off extra bits.
 I do not have a website, so please tell me, what are you interested in. I have some sort of "lego" modular dac. I have usb and spdif input moduls, different dac modules ( AD, TI, AKM, Crytal) and different IV converters.


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## error401

I'm mostly interested in your power supply and I/V. I assume you're using separate Jung regulators for Avdd and for the I/V. My power supply is LM317/337 @+/-15V for the I/V, and separate LP2985 off the V+ supply for Avdd and Dvdd.

 I/V is taken from the AD1955 evaluation board schematic.

 I like the idea of a modular DAC, I'm considering it as a future project if I turn out to be unsatisfied with AD1955. I've also run into a couple of 'issues' in that I didn't think about getting sample rate information from outside (ie. S/PDIF receiver) to configure AD1955 properly since it doesn't have an auto mode. Oops. I will see if I can jumper LRCLK into the microcontroller to detect. So I should rebuild it anyway to integrate into my future preamp (aka HiFi receiver) project, though I may just use WM8741 for this since I have a couple (thanks Jambo) and it's much easier to use.


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## borisov57

I am using three separate windings, greatz-s and jung regulators for AVDD and for +/- supply of I/V converters. I am getting 119.7 SNR measured with 24bits/48kHz.
 Yes, sample rate detection and configuring registers is problematic. You can get pass this issue if you use ASRC before DAC chip. Anyway, AD1955 will work with default settings with 44.1/48/96 and sometimes even with 192 kHz.
 PCM1794A can also be your choice, it has current output like AD1955 and works with anything you feed it with, as long you choose I2S/LJ/RJ format right.
 WM8741 is only DAC, that I did not try, but I would like to, if I get one.


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## FS2

error401, looks like I missed the earlier posts where you finished your amp. Sounds like a nice job, despite the measurements. I'm curious, what parts did you use for op-amp decoupling in the end? Still just the 0.1 uF? 

 And is there a sweet spot for the sample rate settings fof the AD1955?

 Me, I'm still stuck trying to layout my power supply.


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## error401

Quote:


  Originally Posted by *FS2* /img/forum/go_quote.gif 
_error401, looks like I missed the earlier posts where you finished your amp. Sounds like a nice job, despite the measurements. I'm curious, what parts did you use for op-amp decoupling in the end? Still just the 0.1 uF? 

 And is there a sweet spot for the sample rate settings fof the AD1955?

 Me, I'm still stuck trying to layout my power supply. 
	

	
	
		
		

		
		
	


	


_

 

I think it still measures very well. It wipes the floor with my AlienDAC, at least.

 Decoupling is 0.1uF directly below the opamp V+ and V- pins to ground, and a shared pair of 100uF OSCON-type caps for each power rail nearby. I should've made it an LC filter, and don't really know why I didn't, but it's too late for that now 
	

	
	
		
		

		
		
	


	




.

 I haven't fooled with the AD1955 registers yet, there's really only one relevant setting that controls the oversampling mode. At least according to the datasheet, only certain sampling rates work with each OS mode, and I've only been using it at 48Khz, so I haven't even written the firmware to change it yet. I think I will write firmware that should be able to read LRCLK and detect the sample rate and set the register appropriately. Also, I'm wondering about what boris mentioned, and maybe the DAC will run in 8x mode at higher rates.

 One thing I didn't really mention elsewhere is that with the active I/V this lets me go for a completely DC-coupled signal path from source to headphones. No caps here!


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## Dougie085

Was there ever any PCB's ordered for this? I'm looking for a DAC based on the AD1955.


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## error401

I had some PCBs made, but only enough for my own use. It's a very challenging build and I wasn't even sure it would work so I didn't make more.

 If you'd like the gerbers to have your own made, I think it'd cost you about $30 for qty 1 from BatchPCB where I had them done. I can't stress enough how difficult stuffing the board is though, and I'm not certain the silkscreens are correct (or obvious due to part density), and I don't have a placement guide for you. You'd be pretty much on your own.


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## Dougie085

Are there any other AD1955 solutions? I searched around a bit but couldn't find a whole lot. I heard a CDP recently that used AD1955's and loved the sound they produced.


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## error401

Not that I know of, hence my motivation. You can purchase the ready-made AD1955 evaluation board directly from ADI for about $500 that implements a similar circuit. I know lots of DIYers have used this DAC, but I guess not many have released their designs.

 If I were going to do this again I think I'd not try to cram it in such a tight space. Maybe a future project - reimplement the AD1955 evaluation board for less money as it's said to sound extremely good 
	

	
	
		
		

		
		
	


	




.


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## Dougie085

Yeah I'm not really looking to put 500 into that + PSU and what not. I'd love to build something 
	

	
	
		
		

		
			





 I'd love to see a version that was a little more spaced out. Is it a single sided PCB? If it were I could probably just etch it my self since I have PCB material already. I'm not super worried about compactness. I just thought I'd look around and see what people have done and amazingly for how nice of a DAC it is there isn't a lot.


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## error401

I think people are scared off by the fact that the DAC can only be configured by a microcontroller. Really though there is very little configuration and for standard CD source you don't even need a microcontroller connected, the default settings are fine. I still haven't got around to writing the code since I don't have a high sample rate or DSD source to use with it.

 I think pretty much any board with this chip is going to be double sided, and you'd have a tough time home etching the 0.65mm pitch pads for the AD1955. A project for this is much better suited to a professional board house.

 The DAC and I/V sections are pretty much a verbatim copy of the AD1955 evaluation board. I had a detailed PDF on it with full schematics but I've lost it - if anyone has a copy please let me know. They used AD797 for I/V and slightly different supply voltages, so the bias scheme is slightly different (formula in the datasheet). It wouldn't be too difficult to lay out a larger version of it with DIP-8 opamps and the like, but I'm not ready to take this on at the moment as a project. Maybe in the coming year.


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## FS2

I agree about the double sided thing - I'm trying to keep the underside as a solid ground plane, and I've had to make some comprimises. I'm still not done with my full layout as I'm putting a PCM2707 on the same board as well and trying to head off any possible noise sources. Ideally I'd love to do it as a 4 layer board, but that would cost.

 I've etched a 0.5 mm pitch before at home. It's doable using a sheet of glass to hold a printed transparancy over a photosensitized board. But in this case, considering the overall complexity of the board, cost of parts and the quality of the chip, a PCB is much preferred.


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