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
An HC4041 isn't usable as a mux for signals past the supply rails.
With HC, the problem isn't so much latchup, as long as you reasonably limit the current into the esd diodes. The real problem is that, if you drive inputs past the rails by even the esd junction drop, you'll turn on the switch fets common-gate mode and current from "off" channels will blast through to the mux output.
Resistive dividers can be used to scale the voltages down.
Some fancier mux's don't do this.
John
The Fairchild datasheet says the analog input range is +/- 6 Volts, so it won't pass your 24 volt analog signals.
The datasheet also indicates that the operating analog input and output voltages must be between Vee and Vcc - if you are using +/- 5 volt power supplies, your analog input voltage must be between those limits if you expect the output voltage to be the same as the selected input.
-- Peter Bennett, VE7CEI peterbb4 (at) interchange.ubc.ca GPS and NMEA info: http://vancouver-webpages.com/peter Vancouver Power Squadron: http://vancouver.powersquadron.ca
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
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
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| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine Sometimes I even put it in the food
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
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 ?????.
That will result in either clipping or a loud bang, followed by a puff of smoke.
Check out the multiplexers here and you'll be a lot more happy:
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
You did the 74HC40xx series for ON Semi? Hmm, the world is indeed small, I've used those. So we are connected :-)
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
Yup. I redid many of the 74HC... parts so that, when processed by the Chartered Semiconductor foundry, the old standard 74HC specifications would still be met.
Besides the analog switches I did many of the logic parts as well.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine Sometimes I even put it in the food
latchup
And you couldn't have convinced them to introduce the 74HC equivalent of a CD4007? That would be so cool.
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
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.
-- Bill Sloman, Nijmegen
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
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