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Hi there,
I'm very interested in the DAC modules. It looks great, but I find some things a bit strange. Specially the lack of an analog filter as described in the Wolfson datasheet. I only see a series cap and a 100k resistor to ground. Is there a specific reason for this?
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The WM8740 has an internal alalog filter (-3dB @ 195kHz). The suggested output filter in the datasheet had additional filtering (optional) but is mainly a Balanced (Differential) to Single-ended converter. We want Balanced outputs, so no filter.
Incidentally, our Balanced to Single-ended converter module (almost ready) will have spots for additional frequency filtering (optional) if desired.
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Thanks Brain, I thought it would be something like that. Can the series cap also be ommited? The datasheet shows a DC offset of AVDD/2. There are probably more gallant ways to get rid of it? Doesn't the TXD take care of common mode voltage?
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Hello,
The TXD as in its current form has VOCM tied to GND (Version 2 will have a slightly different scheme) so I am pretty sure you cannot just take the output DC coupled. At least I am not brave enough to try yet. :)
Cheers!
Russ
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Interestingly enough I simulated it and it works!!! There will be a very slight offset to GND (same magnitude) from the outputs, but in the simulation it is only around 30mv. I will try it out when I get home from work and see how it works in the real world. I will use two TXD on the output of the DAC and then run it through two Joshua Tree attenuators, then finally out to TXO-2.
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Yes, it's all in the datasheet of the THS: common mode rejection is 80 tot 95 dB :)
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Oh I knew that quite well, the only question I had was that because I was tying VOCM to GND I was worried it would have too much offset. the reason is that other differential amps work this way. :) So I a minimum wanted to simulate it first. :) I will try it at home now that at least I know it simulates fine. Edited by user Monday, June 25, 2007 9:46:16 AM(UTC)
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I did this ones with an OPA1632, more or less the same as the THS4131 as a filter for a DAC. It also showed just a few mV's on the output when driving with 2.5V of common mode. Couldn't get to sound right though  .
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Ok I just tested Opus DC coupled into TXD/TXO, it works just fine! No issues at all. In fact it sounds excellent! I am leaving mine this way. Just one note. I use the TXD right after the DAC, like this: DAC --> TXD --> Joshua Tree --> TXO-2 Also tried omitting the TXD before the Joshua Tree, and that works great too, but you will get a very very slight pop at the higher volume setting as the relays switch because of the DC bias. Also there is a very very faint "chirp" when DC coupled when you turn the DAC off. Nothing that will harm anything, but that chirp is not there when the caps are in place. To do the modification you simply use a wire jumper in place of C5, C6, C7, and C8. Edited by user Monday, June 25, 2007 2:28:08 PM(UTC)
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Rank: Advanced Member
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Great stuff! Did you any DC measturements after TXD?
Any idea when the I2S multiplexer and USB boards are ready?
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USB boards I hope to order before end of week. I2S multiplexer will probably be a couple months out.
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Oh yes, forgot, DC measurements after TXD are very low < 10mv. The TXD is setup for unity gain.
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So if I want to use DC coupling in balanced mode (remove the oscon caps) then the TXD will be the solution?
Thanks...
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