Re: Digital sine wave generation
- From: The Phantom <phantom@xxxxxxx>
- Date: Mon, 03 Apr 2006 19:08:22 -0700
On 30 Mar 2006 10:54:24 -0800, "Tim Shoppa" <shoppa@xxxxxxxxxxxxxxxxx>
wrote:
While my obsession is still digital sine wave generation, I'd like to
note that the resistor values in AOE 2nd Ed, 9.38, for the twisted-ring
counter weighting resistors are not the best values to choose in the
E96 series. They're not bad but not the best.
If someone wants to check, I think Lancaster's _CMOS Cookbook_ has the
same resistor values as AOE. Can't find my copy of Lancaster's book at
this moment.
What is in AOE is:
Q1: 57.6K
Q2: 30.9K
Q3: 23.7K
Q4: 22.1K
Q5: 23.7K
Q6: 30.9K
Q7: 57.6K
My search has shown that you get 80% less sum-of-squares error if you
instead use these standard E96 values:
What voltage levels did you determine give the best approximation to the
sine wave?
Q1: 30.1K
Q2: 16.2K
Q3: 12.4K
Q4: 11.5K
Q5: 12.4K
Q6: 16.2K
Q7: 30.1K
The matching with these values makes it worthwhile to use 0.1% or even
better resistors (which are actually affordable and available these
days!)
It appears to me that a generic 74HC595 output has an output impedance
of 40 or 50 ohms at room temperature and currents under 1mA and 5V Vcc,
and at these impedances it is actually very slightly better to use the
11.5K-12.4K-16.2K-30.1K range rather than the ten-times-higher
115K-124K-162K-301K.
Tim.
.
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