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OPA1642 Dual Opamp Not Working in Balance Differential Mode

Dan R
Prodigy 70 points
Other Parts Discussed in Thread: OPA1642, OPA333

8764.Schematic For Applications Support.pdf

Any help is appreciated on this one.  The OPA1642A is specified with 10^13 Ohms common mode and differential mode input impedance, and  less than 20pA of input bias current.  Why in the world would the non inverting inputs be drawing more than 1.5uA?  The resulting differential DC signal at the inputs causes the outputs to rail, eating up all of my dynamic range.

This circuit works perfectly with many other op amps, but they have higher noise floors than the OPA1642.

If the attachment didn't work, here's the link to the schematic: http://dl.dropbox.com/u/20553231/Schematic%20For%20Applications%20Support.pdf

Thanks,

Dan

  • 0
    TI__Mastermind 23598 points

    Dan,

    The OPA1642 input common mode range is limited to (V+) - 3.5V, and (V-) -  (- 0.1).  This means that you can apply signals from -0.1V to 1.5V for your application (5V single supply).  When you apply signals outside of this range the amplifier will not function normally.  So the 2.5V reverence is outside of the common mode range.  You can use a "rail-to-rail" amplifier to avoid this limitation.  There are many amplifiers that fall into this category.  One popular example is the OPA333.

    Let me know if this helps.  If you need further assistance selecting a device please provide more detail regarding the application.

    Best regards,

    Art

  • 0 in reply to Art Kay
    Prodigy 70 points

    Art,

    That certainly is the problem!  If I increase the positive rail above 7 volts everything snaps to normal.

    I'm surprised I missed that in the datasheet.  I guess I'm just spoiled and am used to rail to rail common mode input amps.

    Thanks for your help! 

    Kind Regards,

    Dan