• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 47 subscribers
  • Views 1530 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

XTR300 Current Source

Tanner Songkakul
Prodigy 20 points
Other Parts Discussed in Thread: XTR300

Hello,

I am trying to use the XTR300 as a constant current source in order to measure impedance across tissue using a 1V p-p sine wave as the input.  The circuit diagram can be found below. However, when measuring the voltage across the tissue, unplugging the leads (creating an open circuit from DRV to ground) results in the presence of a constant AC voltage. Shouldn't this value be zero, because no current should be flowing? What could be possible causes for this?

Thanks,

Tanner

  • 0
    TI__Guru 63965 points

    Hello Tanner,

    We tested your circuit in the lab today and did not observe the same results you have.  Would you please attach a scope shot with the problem waveform?

    Our results are below and show the expected operation that when the load is disconnected the output approaches the positive supply rail.

    1.)  Normal operation:

    2.)  Without 1kOhm, with 1nF

    3.)  Without either

  • 0 in reply to Collin Wells
    Prodigy 20 points

    Alright, I'll double check my wiring.  Could you explain why output approaches the positive supply rail for an open circuit?

  • 0 in reply to Tanner Songkakul
    TI__Guru 63965 points

    Hi Tanner,

    This is the expected application for a current source.  The output voltage of a current source with a ground-referenced load is Vout = Iout *Rload.  If the load resistor is removed, Rload in the equation is basically infinite.  Therefore, Vout will go as high as it can which is limited to the positive rail.  

    A current-sink will do just the opposite and will pull to the negative rail for an open circuit.