info of 74hc4051 maximum volatge needed

[michael nikolaou]

Hi

I want to read many analog inputs from 0....24 V . I want o use one
multiplexer.
I think the 74hc4051 is the most common. My question is in case of
overvoltage
over the supply rails at the inputs can one get away without protection
diodes with the use
of appropriate input resistors ???. I mean the chip will go into latch-up
the minute an
overvoltage is detected or it needs appropriate current to create a latchup
or other
malfunction ?.
Any help will be appreciated .



Michael

 

[Vladimir Vassilevsky]

Hi

I want to read many analog inputs from 0....24 V . I want o use one
multiplexer.
I think the 74hc4051 is the most common. My question is in case of
overvoltage
over the supply rails at the inputs can one get away without protection
diodes with the use
of appropriate input resistors ???.

NO! NO! NO!

The 4051 can't be used beyond +/- supply rails. If the voltage can
occasionally go beyond the rail (even with the protection diodes and
current limiting), there will be a really weird problem with the signal
leak from one input to another. This happens with the generic HC muxes;
the ADGs and MAXs don't behave like that.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

 

[KMoffett]

Have you looked at the datasheet?

Absolute Maximum Ratings:
Switch I/O Voltage (VIO) VEE -0.5 to VCC +0.5V
Clamp Diode Current (IIK, IOK) ±20 mA

This will not work as an analog switch for 24VDC analog signals.

Ken

 

[michael nikolaou]

You would do better to buy the Motorola/ON-Semi versions. I did an
exquisite job of minimizing the "bow" in the on-resistance versus
signal input voltage, also minimizing distortion in the process ;-)

...Jim Thompson

Thanks for the reply Guys .

Can i ask if i use schottky diodes tied to the supply rails so voltage
is withing +-.2v within the supply rails will it be ok ?????.

 

[Bill Sloman]

That will result in either clipping or a loud bang, followed by a puff of
smoke.

It can be more subtle than that - when I once relied on diode clipping, with
a 4051 everything looked as if it was working fine until I started checking
linearity and precision, when the current that leaks out of or into the
substrate
gave me small but unnecessary errors when the signal was very slightly
outside
the rails.

Easy enough to fix when you realise what is going on, but in my case3 it
took
a day or so of careful work before I noticed that I'd got aproblem.

John Larkin called it correctly.

 

[Spehro Pefhany]

Hi

I want to read many analog inputs from 0....24 V . I want o use one
multiplexer.
I think the 74hc4051 is the most common. My question is in case of
overvoltage
over the supply rails at the inputs can one get away without protection
diodes with the use
of appropriate input resistors ???. I mean the chip will go into latch-up
the minute an
overvoltage is detected or it needs appropriate current to create a latchup
or other
malfunction ?.
Any help will be appreciated .



Michael

You might be able to 'get away with it' (as in the chip would not
immediately fail), but I would not advise it. Aside from latch-up
(which is specified by the abs max ratings), it can affect the
accuracy of the channel you are measuring -- you would be exeeding the
recommended operating conditions (Vee <= Vin <= Vcc) and such things
happen.

If you put a voltage divider on each input st the output voltage won't
exceed Vcc/Vee (eg. 10:1 with +/-5V supplies), then you can mux the
divided-down voltages.



Best regards,
Spehro Pefhany