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Multiple 12V analog inputs


#1

For my project I need a lot of analog inputs that are driven by 12V. Usually that would involve a voltage divider for each input.

I’ve come up with the idea to use a analog multiplexer to switch the input before stepping it down to 3v levels. But most multiplexer IC’s cannot cope with large analog voltages. I would be needing a multiplexer that handles 3.3v digital pins select drive the 12v analog input.

But maybe this would work? : http://uk.farnell.com/jsp/displayProduct.jsp?sku=9664548&CMP=e-2072-00001000


#2

I use the 4051… up to 15v 8 inputs to 1 output

I think you’ll find that the multiplexing works as low as 3 volts … Perfect you only need one voltage divider…

Cheers Ian


#3

Thats wonderful news. Do you need to put 12V at the chip too, or are the inputs tolerant?

Thanks Ian


#4

Unfortunatley Not! Minimum 5vdc supply with vdd+5vdc max analogue signal ie… on 5v supply + 5vdc = max vin of 10 volts analogue in, But even if you supply 15vdc you cant go above with your analogue… as it’s 15v max throughout

CD4051BCN by fairchild…

Cheers Ian


#5

Eeerm I read that twice but I still can’t follow…

How much should I put on the chip (Vdd/Vcc versus Vss/Vee) to drive +12V inputs with a 3v digital pin? Sorry if this sounds stupid, but I am stil a new to this


#6

It just means that you can only raise the analogue in voltage 5 volts above the power rail, but no more than 15 volts…

Just power the thing at 12 volts and use 12 volt signals and stick a voltage divider on the 4051 output and if you supply 0 - 3.3v from the FEZ to the multiplex signals you’ll be good to go.

Cheers Ian

P.S. you okay with the voltage divider?


#7

[quote]Just power the thing at 12 volts and use 12 volt signals and stick a voltage divider on the 4051 output and if you supply 0 - 3.3v from the FEZ to the multiplex signals you’ll be good to go.
[/quote]

Great,thanks. I just wanted to make sure the Fez supply would be enough to drive to signal selectors… Will any 4051 do this or do I need to select a specific supplier? Over here in the Netherlands there’s not much choice in shops/parts and i don’t want to spend $5 shipping on a $0.25 part.

Also, yes: I am really getting the hang on this voltage divider stuff.


#8

most CMOS devices will do just check the data sheet your looking for 3-15v digital input as they have level shifters built in…

Cheers Ian

P.S. it also specify’s a different rail voltage


#9

Thanks I will let you know my results