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Old 20th March 2010, 06:23 PM   #1
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Default Looking for small SPST SMD "switch" (cmos?)

I'm looking for a small switch to use to change the gain of an amplifier. The way the amp is drawn, a single SPST will work, the smaller footprint the better. I would just switch in a second resistor that would then be in parallel with the first resistor. I have a 5v supply to work with as an aux supply, but also have access to +-12V, but would prefer not to use this. I'd like to have the option of using a mechanical switch to trigger four of these, or using a microcontroller. I've never used a CMOS switch, but I'm guessing this is exactly what I want.. but feel free to correct me
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Old 22nd March 2010, 02:05 AM   #2
soekris is offline soekris  Denmark
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You haven't stated all your parameters, but for audio I like, available at DigiKey:

TC4S66F in sot23-5
DG448 in sot23-6, lower rds-on

Or try their search engines....

Soren
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Old 22nd March 2010, 05:55 PM   #3
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Soren:

I'd like to be able to use a 5v control voltage. Also, I need to handle rail to rail analog voltages, maximum is probably +-15V. Also, as many throws as possible, but when I look at for example some of the sp4t, they have low voltage limitations. Rds-on isn't a huge deal, as it will be in series with a large resistor anyways to set the gain of the amplifier.

Since it's being used in a feedback loop, perhaps even the +-15V can be relaxed, but I'm just not sure. If so, then here is a sp4t that looks nice:

http://focus.ti.com/lit/ds/symlink/ts3a5017.pdf

... now if I could find a 5v version and to avoid a regulator for this chip..
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Old 22nd March 2010, 06:19 PM   #4
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What about this?

ADG1409 | 4 ? Ron, 4 - /8 - Channel 15 V/+12 V/5 V iCMOS Multiplexer | Multiplexers (Muxes) | Switches/Multiplexers | Analog Devices

Haven't used it but was looking at it for a project.
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Old 22nd March 2010, 06:22 PM   #5
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Quote:
Originally Posted by luvdunhill View Post
Soren:

I'd like to be able to use a 5v control voltage. Also, I need to handle rail to rail analog voltages, maximum is probably +-15V. Also, as many throws as possible, but when I look at for example some of the sp4t, they have low voltage limitations. Rds-on isn't a huge deal, as it will be in series with a large resistor anyways to set the gain of the amplifier.
The Rds-on is actually fairly important, because the switch resistance is usually dependent on the input voltage. A switch with low RDS-on will have lower distortion, because the voltage-dependent resistance introduces distortion.
Quote:
... now if I could find a 5v version and to avoid a regulator for this chip..
The ADG736 and others in that series have very low resistance and work at 5v.

Another solution is to use a digital potentiometer. Maxim and Xicor have many to choose from. If you can generate the I2C control signals, the MAX5477 is a good choice. Of course, you could also use a digital volume control such as the PGA4311.
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Old 22nd March 2010, 09:11 PM   #6
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all good suggestions. Thanks everyone. I'll pour over some more choices and report back later!
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