Signal Presence Detectors?

[Don Tillman]

   Date: Mon, 04 Dec 2000 13:50:28 -0800
   From: Buck Buchanan <voltagecontrolled at home.com>

   I immediatly wondered if a just plopping down another regulator
   (78xx, 78Lxx, etc) and filter cap to drive the display section
   might be easier.  There's a couple of volts dropout to contend with
   but that doesn't seem like a big problem for a display.  Or is it
   that these regulators w/cap still won't provide enough isolation
   for your needs?

A regulator chip won't really help -- it would regulate the voltage on
the LED but do nothing about the various current spikes as the LED
turns on and off.

I probably wasn't very clear about my request...  The context is a set
of audio processing modules optimized to be audiophile clean.  A
signal presence detector is a fundamentally nonlinear operation, so I
want to make sure none of the artifacts from rectifying the signal,
from comparing the signal to a -20dB level or from driving the LEDs
are audible.  Especially considering the audio signal level will be at
a quiet level when the LED trips.

Current steering is an elegant solution because the current drawn
from the power supply never changes, it just gets directed different
ways.  The first diff amp performs the rectification by being biased
off slightly ("not off, to the left!") and the second diff amp steers
current toward the LED.  

  -- Don

-- 
Don Tillman
Palo Alto, California, USA
don at till.com
http://www.till.com