cd4049UB ESD protection question

[dschwartz]

Hello all!
I'm designing a switching scheme.

It's similar to amplifier's remote switching, and is intended to be used as such with a stereo cable (it should be able to accept typical "tip/ring to ground" switching devices like programmable switchers.

I want to use 4049UB's to buffer the switch input, but i want to be sure it will not blow up from static or other unsuitable voltages.

According to the datasheet, the 4049UB has input protection diodes, and a ESD rating of 1000-1500V..also i plan to use a 100K ohm or higher resistor at the input to limit any exceeding current.. should be enough for typical use? or should i use extra protection like zeners, diode limiters, etc?

thanks for your help!

edit: i added the schem:

its for switching between 2 pots to control 1 parameter on a fv1....  Top inverters switch between pot A and pot B...bottom inverter turns both OFF

[dschwartz]

forgot to ask..or should i just use npn transistors as inverters?

[Rob Strand]

100k is usually enough on CMOS B/UB series devices.

I'm not sure your transistor switches are going to do what you want.

As drawn the emitter current is going to cause an offset which raises the zero voltage.  You could connect the emitter to the pots, which is lower impedance, to help that a bit.

However there's a possibly significant problems.   If you expect a full 0V to 3.3V range at the switched side (ie at the FV-1) then the transistors wont't do that.  The BE drop will come into play.  Also for a transistor switch to work you need some level of base current and you can't get base current to flow without some voltage drops.    Swapping of NPN to PNP won't help because the voltage drop just be transferred from the +3.3V side to the 0V side.

#### You could use a parallel NPN and PNP to hide the voltage drop at each rail but that going to complicate the circuit.    This idea is essentially what is done in CMOS switches.  They use an N-channel and P-channel device in parallel so the switches work near the rails.

[dschwartz]

Bummer...
But I'm not sure.. according to LTSpice, it works perfectly.
I get a range between 8mV and 3.27V at the output.. which is good enough, at least for my purpose.

The transistrors are driven by the inverters at 9V, and at the transistors base there is 4.4v (thanks to the current limiter/slow down resistors) when activated.

I ran into emmiter voltages higher than 3.3v with the pots being 100k, but making them 10k solved it, and the voltage at the output follows the pot value almost perfectly ( with mV error)

Also tried using the emmiters into the pots, i got weird voltage spikes when switching and some kind of crosstalk between voltages..

I know ltspice is not the last word...but i would think their npn models should be accurate at this point.

I'll have to breadboard it...ugh...

[Rob Strand]

But I'm not sure.. according to LTSpice, it works perfectly.
I get a range between 8mV and 3.27V at the output.. which is good enough, at least for my purpose.

The transistrors are driven by the inverters at 9V, and at the transistors base there is 4.4v (thanks to the current limiter/slow down resistors) when activated.

Damn it, I missed the 9V connections.   Don't listen to me 

9V, that helps a lot.   You will see still small offsets but you shouldn't see the more severe problems I was talking about near the rails.   Your numbers from LTspice don't look bad now I see the 9V.

Also tried using the emmiters into the pots, i got weird voltage spikes when switching and some kind of crosstalk between voltages..

You can remove spikes by adding caps.

FYI, I have seen weird voltages on some spice simulators when flipping the emitters and collectors.  They are not real.  They are false solutions in the maths.  Don't know if LT spice has this problem.   The spikes could be real.

[dschwartz]

Thanks Rob!
With 3.3v at the base it didn't work at all
I design "by ear" with "some" ee knowledge, so sometimes i get caught with basic stuff, like transistors.

I like the "on Off" idea i had, though... It will take the output down to 10 or 20mV. Schottky diodes bring them to 0.0V..but reverse current messes everything else up..

[Rob Strand]

I like the "on Off" idea i had, though... It will take the output down to 10 or 20mV. Schottky diodes bring them to 0.0V..but reverse current messes everything else up..

In spice it will look better than reality because the diodes are matched.  Even silicon diodes will work.  The transistor has the advantage that it acts as a switch with a small offset (which to some degree can be though of as the BC diode and BE diode aren't matched).   Matching of diode voltages requires the diode currents to be the same.