[sdiy] Analog Computational Unit

[James Patchell]

At 11:09 PM 1/31/2004 -0600, Ryan Williams wrote:

> > Yes, those are fun analog computational blocks.  I have used them for
> > computing ARCTAN and Square Root Sum of Squares.  However, there is one
> > caveat .... the input voltages can only be positive (because they are log
> > circuits).  It is possible to get around these limitation.  Also,
>bandwidth
> > is a function of signal level.  At the lower input voltages (lower input
> > current really), the bandwidth is significantly lower.  These are just
> > things you need to be aware of.  It might make an interesting way of
> > processing control voltages.
>
>right now I'm justing thinking about how this could be done, and the main
>problem I'm having is the part about only positive voltages being allowed. I
>guess I'll get that "non-linear handbook" that Paul mentioned.

Yes, the Analog Device Nonlinear Handbook is, as far as I am concerned, a 
Bible.  At first glance it may seem there isn't much in there for a 
synthesist,  but you soon learn that it has a lot of very neat circuits for 
doing things.  I have gotten a lot of my ideas from that book.


> > You can pretty easily build one of these by just using an LM3046
> > transistor array for the log transistors and your favorite opamp.  You
> > should be able to make one that will perform adequately for about $2....
>
>I"m *trying* to analyze the circuit shown in the functional diagram of AD538
>and I think I understand pretty much what it's doing. to lazy to do all the
>math tonight (I ran out of space on the single sheet of paper I allocated
>for this task) but I think I'll just build a version from a 3046 and some
>opamps (like you said) and see how that works first. that is after I figure
>out the part about inputing negative voltages.
>
>I'm assuming you want the 3046 because it's important that the temperature
>and matching of the transistors is close? It looks to me like if they are
>perfect then the temperature effects will be canceled out?

         Yes, that is correct.  Probably for a little more precision the 
MAT04 could be used, but I am willing to bet the LM3046 will more than do 
the job...


>--Ryan (apparently starting another project when I have about a million
>other things I want to build)

         -Jim
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