[sdiy] Outputs grounded! Re: SSI2164 datasheet updated

David G Dixon dixon at mail.ubc.ca
Sun Aug 12 19:40:58 CEST 2018


> Oh well, must be right then.  After all ten thousand is a big 
> number indeed.

a) The 2164 datasheet gives no guidance at all as to what to do with unused
VCAs.
b) The control input is already grounded (across an internal impedance -- 5k
originally, 10k in the new ones) so this is not an issue.
c) The 2164 VCA amplifies current, so zero current in equals zero current
out, except for the input bias (up to 10 nA) and output offset (up to 50 nA)
currents.

This would suggest that one probably should supply paths to ground for these
small bias and offset currents.  I suppose I will start doing that in my
designs now.  However, looking at the inner 2164 circuit in the datasheet,
it would appear as if all of these currents have alternative paths to the
negative rail through the various transistors.  Presumably, when the inputs
and outputs are unconnected, then either these small currents simply don't
flow, or they get out of the device by some other means.

The Dixie II+ involves three unconnected 2164 VCAs.  The Rubicon2 also
involves three unconnected 2164 VCAs.  Both of these VCOs track beautifully
and otherwise work exactly as designed.  Hence, while I'm willing to concede
that we may have done it "wrong", I'm also somewhat dubious that it makes
any difference whatsoever.  I know that I am a relative neophyte to
electronic design, and have no formal background in electronics (my day job
is as a metallurgical engineering professor) other than 10 years of
experience learning by doing.  However, my designs represent millions of
dollars of revenue for Intellijel, and many hundreds of happy customers, so
I figure that counts for something.



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