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APU mixer function efficiency
http://forums.nesdev.com/viewtopic.php?f=3&t=14927
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Author:  lidnariq [ Wed Oct 12, 2016 4:54 pm ]
Post subject:  Re: APU mixer function efficiency

rainwarrior wrote:
(Is the actual mix an analog combination of 3 DACs or a digital sum output to 1 DAC?)
Analog, for both output pins.

Effectively, the current out of a pin should be

ID = K(VG-VS-Vth)²(1+λ(VD-VS-Vdssat))
and the voltage of the pin should be
VS = 100Ω·ID.

K is the sum of the digital values for each of the channels, each of which have their own linear scaling factors. (Because each of the MOSFETs making up the DACs in the 2A03 have different widths and lengths)

VG and VD are both 5V. Vth, λ, and Vdssat are very approximately 1V, 0, and irrelevant.

Solving for VS—which is to say VOUT—yields (1+800K-sqrt(1600K+1))÷200K ... but obviously actually sitting down and more exactly determining the other parameters would produce a better result.

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