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Stack-a-DAC filter module taking shape

Posted 28th April 2014 at 01:27 PM by abraxalito
Updated 29th April 2014 at 12:36 AM by abraxalito

I'm prototyping this baby which will probably be the first Ozone board to go to PCB layout. It implements the 10th order Chebyshev filter shown earlier and is intended to be mounted atop a 'hexacap' passive shunt. The DAC stack in the centre is made up of six TDA1387s. Input will be I2S and output unbalanced audio 40mV RMS.

Update - over on a thread about AD815 Eldam asked about the inductors - how many I need to buy to get the values in the schematic - so I'll talk about that a little here.

I have bought a big reel of 1k pieces of these 680uH inductors. The price was around $100 but I think I probably should have paid a little more as although the reel is marked up as TDK, they might well be fakes. I suspect that because their appearance isn't as professional as some 1mH parts I bought earlier from another seller. Added to that is the spread of values - having measured at least 100, only one has shown up as 680uH (within 10uH). The rest have varied from 800uH down to 500uH. Most are centred around two values: 630uH and 560uH. This kind of distribution and variation does not shout 'Japanese manufacturing' at me However apart from this weird distribution of values, the other parameter of interest (Rac - the loss factor) is within specification. So there is no problem in using them, but it means a little flexibility in the filter design.

In conjunction with the inductors, I'm using TDK 0603 220nF X7R capacitors. On the meter, these tend to measure around 200nF which surprised me as they do look like genuine TDK parts. However after soldering them in, the capacitance measures much closer to the advertised value.

So the answer to your question Eldam is a bit long and complex, but it turns out that the filter isn't particularly sensitive to the inductor values - that's a big reason why I chose this topology. With my weird distribution of inductors I've selected 560uH and 620uH values to fit in the 600uH and 680uH places and adjusted the source and termination resistors to compensate. They're 47ohms now, instead of 50ohms. I suggest having a play in LTSpice to see how much variation there is. If you buy 100 chokes I feel you'll be able to get 10 which make a filter which isn't too lumpy in frequency response. The rest you'll probably be able to use with tweaks to resistors and/or caps.
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