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Improving AK4490 THD(+N) figures
Improving AK4490 THD(+N) figures
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Old 14th January 2019, 09:26 AM   #21
nihtila is offline nihtila  Finland
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Originally Posted by sgrossklass View Post
Your issues would seem to reflect LP3878 noise levels, which are quite high below 1-2 kHz in particular according to the datasheet. In hindsight (which proverbially is 20/20, I know), maybe not the best candidate for +5VA. I'd suggest trying a larger capacitor on the BYPASS pin, which would seem to be filtering the voltage reference. The datasheet insists on something low leakage - if you have some 220-470 nF films around, try one of those.

It is unfortunate that the capacitor in parallel to R1 is already used for some pole/zero trickery, but what you could try is another RC combo in parallel to both - an electrolytic in series with maybe the same value as R2 (or half that). This would make the regulator act like a 2 V (or 1.5 V) one for AC, with correspondingly reduced noise gain. I'd try 1 kOhm + 10-22 F (low ESR not needed for obvious reasons... quite the contrary, you want lowish leakage).

With as much capacitance as you have on the VREF pins now, a bit of series R (as mentioned before) is in fact also worth a shot. RC filtering always works much better if R isn't just regulator output impedance, for obvious reasons.
Indeed, it is not the best regulator. In fact, seems LT/Analog have so much better choices for demanding applications although they are more expensive as well.

Thanks for suggestions, will see if I play with it but if I move to on-board regulators, the model will be something else.
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Old 14th January 2019, 10:23 AM   #22
bicefalo is offline bicefalo  Italy
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Hi ,


this is from a well known product.

I find it intresting....
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Old 14th January 2019, 10:40 AM   #23
nihtila is offline nihtila  Finland
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Hi ,


this is from a well known product.

I find it intresting....
To me it looks like they don't really know what they're doing, just fire all types of caps there thinking each one cures different type of issues. I suspect could remove most of them without much impact.

In general it is not a good idea to put so many different value caps in parallel, especially not so close values. You get nasty resonance spikes in the impedance curve. For digital decoupling you basically want many equal value caps in parallel to bring the parasitic inductance down but without creating these nasty spikes. In analog/low-f you want big caps and maybe one small in parallel.

I may be wrong of course Maybe they have evaluated that. But for example Henry Ott has good material on measurements of parallel caps, and showing that different values often create more issues than benefits.
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Old 14th January 2019, 11:57 AM   #24
bicefalo is offline bicefalo  Italy
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Hi,


this schematic is from pioneer cdj2000nx2, sold in thousends of units...


Should work well.
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Old 14th January 2019, 12:11 PM   #25
nihtila is offline nihtila  Finland
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Hi,


this schematic is from pioneer cdj2000nx2, sold in thousends of units...


Should work well.
Ok, interesting to see that in a high volume product. I would imagine they have done their tests then. I don't doubt it not working but think there could be an easier and cheaper way for the same result - if all those caps are even populated.
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Old 14th January 2019, 09:05 PM   #26
kipman725 is offline kipman725  United Kingdom
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Try treating Vref as a low impedance node where packets of charge are taken from rather than something held up by capacitance. Firstly low pass filter your voltage reference to remove high frequency noise, then feed into an appropriate bandwidth (select based on converters true conversion clock) low noise opamp which feeds the Vref pin on your converter. http://www.ti.com/lit/ml/slyc147/slyc147.pdf (see figure 6).
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Old 14th January 2019, 09:10 PM   #27
sgrossklass is offline sgrossklass  Germany
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Getting wideband low impedance is surprisingly nontrivial. Looks like that's what they were shooting for here. Pretty much a cap every decade sounds about right if you don't want any nasty surprises. (And look at all the unpopulated spots.) I'd imagine this should get the job done well into the dozens of MHz. Not sure why Vref needs this (I mean, the actual DAC is operating in the double-digit MHz, but still), but the datasheet of another AKM DAC puts great emphasis on VREFH-VREFL having to be very clean (up to the point of using 10 ohm resistors in series with both to exploit the bypass caps for RC filtering), including keeping all kinds of clocks away from them. So clearly "Vref hygiene" is Important Stuff.
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Old 14th January 2019, 09:17 PM   #28
Markw4 is offline Markw4  United States
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The figure 6 below looks a lot like what we use for AVCC (the dac reference voltage) for ESS mobile dacs (ES9038Q2M). Except we use LTC6655 as the reference, and the opamp is OPA1612 or AD797. It works very well for that application.

EDIT: To be a little more clear, we use one OPA1612 or two AD797. One opamp for each dac channel maintains best stereo separation.
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Old 14th January 2019, 09:39 PM   #29
KSTR is offline KSTR  Germany
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Originally Posted by nihtila View Post
Ok, interesting to see that in a high volume product. I would imagine they have done their tests then. I don't doubt it not working but think there could be an easier and cheaper way for the same result - if all those caps are even populated.
We can only hope they probed the impedance right at the chip's pins with a network analyser...
In general, I agree with you. Paralleling ceramics (or any low-ESR caps) of different values staggered in more than 3:1 rations most often leads to disaster.
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Old 14th January 2019, 09:52 PM   #30
DPH is online now DPH  United States
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Originally Posted by Markw4 View Post
The figure 6 below looks a lot like what we use for AVCC (the dac reference voltage) for ESS mobile dacs (ES9038Q2M). Except we use LTC6655 as the reference, and the opamp is OPA1612 or AD797. It works very well for that application.

EDIT: To be a little more clear, we use one OPA1612 or two AD797. One opamp for each dac channel maintains best stereo separation.
Interesting they're using that over something like the LTC3042 or its brethren. Then again, mobile application is going to have different needs, and be originally running from a battery, so the bandgap vref is probably the noisiest thing around!
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