Kirk Humphries

multiple A/D

Discussion created by Kirk Humphries Employee on Jan 27, 2006

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Posted: Mon Nov 14, 2005  10:45 am


Hello all,


I need to sample about 3 analog outputs, while I have (as much as I know) only 1 A/D input. What should I do,


Thank you,



Posted: Mon Nov 14, 2005  1:21 pm


You could use 3 transistors & 3 general purpose I/O pins to multiplex the analog input. Make sure you take into account the voltage drop across the transistor.





Posted: Mon Nov 14, 2005  7:34 pm


>or a single chip that can turn on/off switches to channel one of many into the one

> input pin.


You could use the 74HC4051 to multiplex your 3 analogs to a single input pin as long as you have 2 extra port pin to toggle A,B pin of the 4051. Make sure to tie C low of said chip.



Posted: Mon Nov 14, 2005  11:01 pm


Thank you very much for the help you give me,


I would like to make sure I understood the principle correct:


I should connect 3 transistor in parallel,while all their source legs are connected together, and each drain is connected to a different analog output I would like to sample. The gate of each transistor is from a different general purpose I/O pin.


(for nmos types- I hope I wrote it correct).


This configuration should simulate a mux, and should work for multiple analog inputs, and only 1 ATD input?


Thank you again.



Posted: Tue Nov 15, 2005  7:56 am


Most HC12 and HCS12 devices have one A/D converter, but many A/D input pins (8 or more), that can be multiplexed internally in the microcontroller to the A/D. This allows sampling up to 8 different analog signals using the internal A/D (first sampling input 1, then sampling input 2, etc. etc.). To use this method, you will need to read the details of the A/D peripheral on the specific HC12 derivative you are using.


The other suggestions you previously got, relate to the case that for some reason you have used all 7 other HC12 A/D input pins for other purposes, and have only 1 pin left to be used as an actual input to the A/D. In such a case, people suggested you will use an external analog multiplexor.


Hope this helps,