Nah, talk more shit

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alexis 2023-01-02 16:47:48 -07:00
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# TIMDAC Theory of Operation
TIMDAC is _not_ a PWM DAC, where a waveform is generated whose average value
equals the desired output, and then filtered all the way down to DC. While this
is definitely a feasible way to implement a precision DAC, the level of
filtering required to achieve usable 15-bit or 16-bit performance severely
limits the update rate, especially if using a scanning sequence to generate
more outputs than available PWM channels. Also, to truly achieve equivalent
specs to TIMDAC, other circuits implemented here (such as the constant-impedance
reference chopper) become necessary. While you could use a PWM DAC and get most
of the way there, TIMDAC takes it that little smidge farther into commercial-
equivalent DAC performance.
equals the desired output, and then filtered all the way down to DC. This method
is often proposed, but is not really feasible at high precision — to achieve
1 LSB of filtered ripple, at 15 bits with a 36 MHz counting frequency, a
first-order filter will settle in about a _minute_. (A basic second-order filter
will settle in about a second, which is much much better, but still very
limiting.) Furthermore, some of the same measures TIMDAC uses to mitigate other
problems at high precision, for example the constant-impedance reference
chopper, are still necessary to get these specs.
Instead of PWM, TIMDAC uses an integrating sample-and-hold topology. A single
pulse is integrated, producing a ramp from 0 up to the product of the pulse