Counter based adc w/ voltage other than 3.3?

TJHJ

So I saw somewhere once talking about using a different voltage for the counter based ADC Circuit. I would like to avoid using an opamp to scale it down to a 3.3 to 0V range from the issues of opamps themselves.(OPamps hate me).
(Sorry I had to put it into a code box to make it look right)

                                  3.3V < Can this be connected to 12v for example to give a 0 - 12V range?
                                   |
                                  1nF    
                     APIN &#9472;&#9472;&#9472;&#9472;&#9472;&#9523;&#9472;&#9472;&#9547;&#9472;&#9472;&#9472;&#9472;&#9472;&#9472;150k&#9472;&#9472;&#9472;&#9472;&#9472; Analog In             
                               |  1nF
                    BPIN &#9472;&#9472;100k&#9496;   |
                                  Vss   

I am thinking all this will do is blow up the prop chip unless I am thinking about the RC network incorrectly.

Thanks
TJ

Kit Morton

To read voltages that are larger then 3.3 volts you can use a voltage divider.

Like this:

     
                                        Analog In
                         3.3V             |                       
                          |              26K
                         1nF              |
             APIN ─────┬──╋──────150k─────┤              
                       | 1nF              |
            BPIN ──100k┘  |              10K
                         Vss              |
                                         VSS

This will divide the 12 volt signal down so it is between 0 and 3.3 volts.
-Kit

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TJHJ

Doah
I dont know why i didnt think of that

Thanks a bunch

Rayman

You can just pick different values to change the input voltage range.· Try this applet:

http://www.pulsedpower.net/Applets/Electronics/SigmaDeltaADC/SigmaDelta.html

Phil Pilgrim (PhiPi)

Rayman,

You need to include a cushion in your calcs, due to the fact that the threshold for inputs is not exactly Vdd/2 and can vary from part to part. The 150K/100K combo is what's recommended for a 0-3.3V range. I made the msitake of trying 100K/100K for a 3.3V range and got clipping at the high end.

TJ,

Keep in mind that if you use a voltage divider, you can load the analog device driving it, possibly skewing your results. You're better off sticking with the two-resistor design and tailoring those values to meet your needs.

-Phil

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Post Edited (Phil Pilgrim (PhiPi)) : 7/29/2008 3:47:51 PM GMT

Rayman

Phil,

The applet is meant to be general purpose, but I'll make a note regarding this with the Propeller. Thanks.