Are there any analog switches that are closed when unpowered (Vcc=0)? This is to clamp an audio output going to a powered speaker with auto-power switching, so the speaker won't turn itself on from tiny noise inputs. I know there are other ways to do this, but.
-- John
Do you need this to be completely passive (using only signal power) or can it be powered by a voltage rail in the unit when it is powered on?
Perhaps a delpetion mode device like a Jfet or DMOS (which analog switches are mostly made of) but you will need a couple of volts (negative) to turn it off (open) when you want the signal to go out.
Any reason it needs to be an analog switch, mechanical switches for signal level can be had cheap and similar size compared to the board footprint of a SOIC-16 analog switches usually come in.
My suggestions would be to make sure the wiring is tightly coupled and ground loops are eliminated to minimize the noise entering the system to start with. Then make the input as low of an impedance as practical so the driving side can handle it but when unpowered there is some resistive loading to keep things from floating around. Audio inputs are usually in the 3K range. You could also put some capacitance across it to make a RC in the high frequency range where most of the RFI is picked up on wires. Think common mode and differential mode.
I've had cases where there was 60 humm in the system and placing a 1 ohm resistor in series with the signal ground was enough to eliminate it and force those currents to return on the proper conductor.
Depletion mode mosfets can do this. Also there are some normally closed solid state relays (which themselves use depletion mode mosfets). Or, of course, an actual mechanical relay which would seem to be the simplest solution here!
-- John Devereux
On a sunny day (Sat, 09 Nov 2013 14:43:30 +0000) it happened John Devereux wrote in :
Yes, or for example a JFET. When power is present the gate is negative, and the JFET is off. When power is off the FET is a few hundred ohm, and shorts the input sigal to the now zero volts supply. The circuit assumes a high impedance load. I just guessed some values, other values for R and C may be better. Reed relais is perhaps better...
Forgot resistor R2 to + to keep source high whne power present .... Maybe more things... :-) Maybe source needs to be 1/2 Vb, well to lazy to start spice.
What's the matter with the usual NO analog switch in series? There are versions that input protected while powered down.
There are normally-closed solid-state relays. Google for them.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
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