ART100@PSUVM.BITNET (Andy Tefft) (02/12/89)
Probably a simple question, but: I want to be able to control the gain of an opamp by an external control. Normally I would just put a potentiometer in the feedback loop of the amp but I want to be slick about this. I want the gain to be controlled by the number stored in a register (probably converted to an analog voltage or current in a D/A converter). How can I do this? Is there some sort of inexpensive, small "voltage-controlled resistor" or something that will do this for me? Please mail responses if possible. I don't get too many chances to read this group. Andy Tefft ART100@PSUVM.BITNET, a1t@ecl.psu.edu
wolpert@caip.rutgers.edu (Seth Wolpert) (02/15/89)
In article <71213ART100@PSUVM>, ART100@PSUVM.BITNET (Andy Tefft) writes: > Probably a simple question, but: > > I want to be able to control the gain of an opamp by an external control. > that will do this for me? > > Andy Tefft > There is a CMOS device called an analog multiplexer that includes; 4051 triple 2-1 analog multiplexer, 4052 double 4-1 analog multiplexer, and 4053 single 8-1 analog multiplexer. All three are controlled from a CMOS-compatible binary source, and offer a very low parasitic resistance, especially when powered with a 15-volt supply. They are inexpensive and very available. I have used them in several projects with good results. They can be banked if more than eight settings are required. If you need any details, please let me know. I'd be happy to elaborate. Seth Wolpert ******************************************************************** SW** I LOVE mankind-it's PEOPLE I can't stand! Linus Van Pelt **SW ********************************************************************
jim@trsvax.UUCP (02/16/89)
I've seen someone use a quad analog CMOS switch (4066?) to switch a couple of resistors hooked-up like an R-2R DAC in the feedback loop. This will take fewer resistors than the 4053 and such for anything over 3 bits or so. It also works on capacitors in analog filters and oscillators if you are willing to search for nore-or-less exact values. If you need an explaination of an R-2R DAC, send mail, I'll reply. James T. Wyatt UUCP:decvax!microsoft!trsvax!rwsys!jim KA5VJL
ART100@PSUVM.BITNET (Andy Tefft) (02/21/89)
Thanks to everyone who answered my question about controlling op amp gain digitally. I didn't expect to get so many answers! Here's a short summary of some of the suggestions I received (sorry I can't attribute these to specific people, but they know who they are anyway): Some people suggested an analog multiplexer, with a different resistor on each line. Good idea, possible for intermediate testing, but in the final version we will want more resolution. Many suggested a discrete FET. We want to stay away from discrete transistors. I also don't think we could get the range we want out of this. One idea which I can't remember the details of was a sampling-type circuit which varied the frequency of a square wave which was multiplied by the output. Varying the frequency would change the average value of the signal. Unfortunately I'm kind of worried this would kill an audio signal. Other suggestions included an optocoupler or phototransistor/led combination. This was one of my first ideas but I really didn't want to do it. Another common suggestion that I considered was using a Xicor EEPOT. I was unable to locate any of these on short notice, and also found out that it wouldn't quite fit our needs. We'd like to be able to read off and display an indication of how much gain we have. Short of a non-volatile counter or something of that nature, we couldn't think of a great way to do this. Besides, I never got any indication that the EEPOT could be reset to some preset value. I think we're settling on the most common suggestion, a multiplying DAC. I didn't even know such beasts existed... Anyway there are apparently several different ways this can be incorporated into our design, and we'll try a bunch. We can play around with varying the values fed into this to give us different ranges, and it's still easy to display or reset the value of the gain, unlike the EEPOT. Well, I tried to thank everyone personally but I know some didn't make it through, so if you sent me a suggestion, thanks. I'll post later to let everyone know how this works out. Andy