[sdiy] (oops) EN #76 question

[Phil Macphail]

I haven't seen the schematic, so I can only comment on what has been
posted here. If the resistors are fed from successive taps of the shift
register then the Gaussian reference suggests that the "DAC" is in fact a
weighted FIR filter. The PRBS will be clocked at high speed, and the noise
would otherwise extend to much higher frequencies than required. The
original edition of The Art of Electronics contained a noise source like
this, and it is a rather clever (and powerful) example of mixed-signal
design at its best,

Phil.

On 27/11/2012 17:24, "Russell McClellan" <russell.mcclellan at gmail.com>
wrote:

>> My question was more about how the 8b circuit worked and if there is a
>>mistake or oversight in the operation. A random source that can only hit
>>8 different levels seems a bit useless no matter how gaussian it is.
>
>I just got out my copy of EN76 - this is a very specific DAC for a
>random noise generator.  It's not a very standard technique.  There is
>a long pseudorandom sequence of bits in a shift register loop, with
>taps to the DAC.  The "DAC" is just a sum of the eight taps of the
>sequence, all weighted exactly the same.  There are indeed only 8
>possible output levels of the DAC (how many of the 8 bits are set to
>"1"?).  If the sequence of bits was initially random, this will not be
>white noise but rather gaussian noise where the midpoint is much more
>common than either extreme.  That's why Bernie calls this the
>"Gaussian" weighting.  There's nothing inherently "Gaussian" about an
>equal weight-summer, that term only makes sense in the context of the
>rest of the circuit.
>
>I think the idea is you're supposed to use a reconstruction filter
>after the DAC to get an analog signal, so the number of physical
>levels the DAC is able to produce is not really incredibly relevant.
>Certainly I would imagine 8 levels is enough.
>
>-Russell
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