Switchable opamp gain

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Panasonic makes PhotoMOS relays, and IIRC, some of them use symmetric FETs for AC signal switching. Using symmetric FETs and their inherently floating optical 'gate' control, they can be extremely linear. Here's a simple link: PhotoMOS Relays - Panasonic | Mouser

These will work, but they're priced around the same as a mechanical relay. However, they will behave much more predictably, will never wear out, or be subject to interference due to an adjacent device (relay coils of neighboring devices can alter each other's turn on and turn off times depending on the coil state.)

If you only need a few switches, PhotoMOS can work well, in some ways better than relays. Not sure if they can be made to do a slow turn on or turn off, but they are definitely more predictable than any relay.
 
Regarding the H11Fxx optocoupler, Fairchild opto line was absorbed by On Semi, and the part numbers changed somewhat.


On Semi now carries the devices as H11F1M with PN variations for SMT and wide pinout. Go to Mouser, pull the latest data sheet and select what you need. One piece price just under $3.00US.
 
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