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Level shift and gain for Arduino ADC

Former Member

I need to amplify and shift the output from a potentiometer where only a small portion of the mechanical travel is used.  The potentiometer data is scaled to 0-1023 for output to a computer.  The Arduino is powered off the computer's USB port.

 

The potentiometer outputs roughly 3.8v to 4.4v. This small range doesn't give me sufficient resolution.

 

If I can shift this range down towards 0v, for example to 0.1v to 0.7v, I can use a voltage divider to adjust Aref on the Arduino down from 5v to 0.7v and actually use the full range of the ADC.  Alternatively, if I can add some gain to the signal, I wouldn't have to fiddle with Aref.

 

I tried using a 741 and 358AN to build the circuit below:

http://i.stack.imgur.com/CCn0j.png

 

But while the level shifting works (down to 0.6v, won't go lower) I get a gain of about 1/10 or less depending on  how I adjust the potentiometer, so that I hardly get any signal out at all.  It's been too many years since I worked with op amps (I mostly write embedded code).  Is there some other circuit I can use, or what's wrong with this one?  Is it my selection of op amps?

 

Thanks

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  • 0 in reply to billabott

    Ooops, my bad.  image      You're right, billabott.

     

    I misinterpreted this line "If I can shift this range down towards 0v, for example to 0.1v to 0.7v..."

     

    BTW, Peter V look for a rail-to-rail OpAmp that works with low voltage, (like the AD820). The 741 might be the least suitable for this.

     

    I'm still curious about the output from the potentiometer.  Why such a low voltage margin, are the connections ok?

    Is the pot ok?

    Is it a linear potentiometer or a logarithmic one?

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  • 0 in reply to Former Member

    Hi folks,

     

    the pot output is so low because it is a linear 300 something degree pot in an apparatus that only uses 50 degrees of travel.  The pot dimensions are very odd and this is the best that I could do.

     

    The whole thing is powered off USB so the supply will be 0-5v.  I'll look for a rail to rail op amp-- wasn't sure if this was needed.

     

    I found that I can wire up the pot and modify it a little such that I can mechanically rotate it so the range is near 0v to about 1v.  Then I tune Aref via a voltage divider to near 1v. However, I think the op amp solution is still going to be better, thanks, I'll try the above circuits when I get the op amps.