mark@mips.COM (Mark G. Johnson) (11/07/89)
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}