VCA chips

[Don Tillman]

   From: "jhaible" <jhaible at metronet.de>
   Date: Tue, 25 Aug 1998 20:24:04 +0200

   I noticed the same thing some time ago, and asked AD about the 2024's.
   Back then (about 1/2 year ago), they said they were still in production,
   but they recommend the 2164 instead. So it's a good idea to ask them again
   about the 2024's current state.

I phoned Analog Devices.  They said, as I suspected, that the SSM2024
is still in production but the SSM2164 is recommended for new VCA
designs.  And they left the SSM2024 off their web site because they
are not recommending it for new VCA designs.

   Doesn't the 2164 has exponential control inputs?  I don't remember that
   exactly,but in that case it would be hard to use in the Scanner.

True, and that would defeat one of the goals of my design.  Though it
might be an interesting variation...

   I haven't done this myself, but I highly recommend that someone tries the
   CA3280 (no typo) for the scanner. It can handle much higher Iabc currents,
   so one could insert the LEDs directly in the Iabc control path. 

Hmmmm....  The data sheet says it'll take an Iabc of 10mA without
exploding, but no curve is plotted for more than 1mA in the data
sheet.  It makes me a little suspicious.  It's certainly an area to
explore.

The CA3280 also has the advantage of a wide output voltage swing, so
you're not forced to follow it with a virtual ground current input
stage (like an inverting opamp circuit).  And the CA3280 has both
positive and negative inputs, so you don't need an additional
inverting stage on top of that if care about polarity.

  -- Don