strong@tc.fluke.COM (Norm Strong) (02/14/89)
}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? What you want is a "multiplying DAC" Put the reference input on the output of the amp, and the output of the dac on the inverting input. Now the gain will be controlled entirely by the digital address. Analog Devices makes several CMOS DAC's of the type you want. -- Norm (strong@tc.fluke.com) -- Norm (strong@tc.fluke.com)
dr@skivs.UUCP (David Robins) (02/19/89)
A simple op-amp gain control is to use an opto-coupled FET, the GE H11F1 , F2, or F3. It is fairly linear with respect to current thru the LED. The FET output is floating, acting as a variable resistor, with a voltage rating of about 27 V. I have used it both in the input resistance and the feedback resistance in op-amp circuits, It is much simpler than using multiplying DAC's or multiplexers. -- David Robins, M.D. (ophthalmologist / electronics engineer) The Smith-Kettlewell Institute of Visual Science, *** net: uunet!skivs!dr 2232 Webster St, San Francisco CA 94115 *** 415/561-1705 (voice) The opinions expressed herein do not reflect the opinion of the Institute!