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Can LMC6482 (3.3v single supply) be made to swing its output closer to ground by connecting an output resistor to negative supply?

Mark Bridges
Intellectual 270 points
Other Parts Discussed in Thread: OPA2334, LMC6482

I am considering the use of a LMC6482 single supply (3.3v) to drive an ADC input, and need its input and output to swing from very close to ground to Vcc.  In the past I have used a OPA2334 in a similar application.  The OPA2334 (single supply) is able to have its output swing very close to ground if a resistor is connected between its output and a negative voltage supply.  Please see the OPA2334 data sheet, page 6, figure 2.  I can't use the OPA2334 in this application as its input will not swing within 1.5v of the positive rail.


Can the same output bias technique be used with the LMC6482?  Also if anyone else can recommend a 3.3v single supply dual op amp that will drive rail to rail (within a few mV of ground), that would be a great help.  Speed is not really important in this application.

Thanks for all your help.

  • 0
    TI__Guru 53636 points

    Hello Mark,

    That is the problem with the pull-down trick - you can loose a lot of positive swing.

    Let's see what the LMC6482 sourcing graphs say...

    12.5K//40k = 9.52k  (Parallel combo load of feedback and pulldown resistors)

     3.3V / 9.52kk = 347uA

    According to the LMC6482 3V Sourcing graph (fig 5), at about 400uA, the swing is 40mV to the positive rail (3.26V total)

    The pull-down trick should work on most any CMOS output device, and should also work on the LMC6482.

    Regards,

  • 0 in reply to Paul Grohe
    Intellectual 270 points
    Hi Paul, thanks so much for the help and the thorough explanation. I'll give the LMC6482 a try.

    Mark