Re: In D/A conversion, is sample-and-hold necessary?

Jerry Avins <jya@xxxxxxxx> wrote:
(someone wrote)

<>> I have a confusion in the process of DAC.

< I think it is worth expanding on why the output voltage is so low with
< this scheme. The narrow pulses contain less energy than wider ones
< would.

Thanks for the explanation. Even though I knew that, I didn't
think about it that way when I read it. Mostly, I didn't think
they were that narrow!

< In fact, the energy at low frequencies is directly proportional
< to the pulse area, height times width. When the pulses are narrow,
< energy is low, but the response is uniform at all frequencies. As the
< pulses widen, energy at all frequencies increases, but high=frequency
< energy increases less. (The width of the pulse "blurs" the highs.)
< Calculating the magnitude of the effect yields the classical sinc
< rolloff. For me at least, understanding the phenomenon from a physical
< viewpoint enlightens the math.

The "high frequencies increase less", yes, but not so much less
until the width gets to be a significant fraction of the higher
frequencies, which have to be less than the sample rate.

For 5V down to millivolts, the pulse width would be about 1/1000th
of the sampling rate. For sinc, approximately 1-x**2/3, and 16 bits
I wouldn't think you would need to go that narrow.

For x**2/3=1/65536, x=1/148.

-- glen
.

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