🚀 go-pugleaf

In article <9719@june.cs.washington.edu> dylan@cs.washington.edu (Dylan McNamee) writes:
    >I'm interested in designing a high quality digitally controlled
    >attenuator for audio use.  I read about a method used by Berning Audio 
    >that uses no potentiometers in the signal path...
    >I think I may have a scheme, which is based on a voltage divider,
    >that may do the trick.
    >
    >    [shows schematic of voltage divider with many paralleled legs,
    >     each one in series with a switch; switches determine division
    >     ratio.  He proposes using BJT's for the switches]


You can buy this in a single IC; try

	National Semiconductor   LM1036     (2 digital pots in 1 IC; stereo!)
	National Semiconductor   LM1040        "
	Signetics                TDA1074       "
	Harris / RCA             CA3259        "

All of these are available from Digi-Key by mailorder.  In addition,
Analog Devices carries a line of chips to do this function.


If you want to go discrete, don't use bipolar junction transistors to
do the switching.  They'll introduce an offset ("VCEsat") which is
uncontrolled from device-to-device, and which varies with signal level
and with temperature.  Also, unless you do the base control circuit
carefully, they'll operate differently on the negative-half of a waveform
than on the positive-half.

Much better is to use one of the CMOS analog switches to connect the
resistors to ground.  They have no offset and they are reasonably
symmetric.  You do have to account for their nonzero on-resistance,
but that's easily handled by purchasing a relatively newly designed
part (e.g. *not* CA4016 or 4066).

The Cadillac design is, of course, discrete JFETs.  Or a JFET-switch-
based IC like the ones from Siliconix.
-- 
 -- Mark Johnson	
 	MIPS Computer Systems, 930 E. Arques, Sunnyvale, CA 94086
	(408) 991-0208    mark@mips.com  {or ...!decwrl!mips!mark}