• State
  • Locked Locked
  • Replies 4 replies
  • Subscribers 47 subscribers
  • Views 1092 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.

THS4131 Oscillation

Chuck Smyth
Expert 6320 points
Other Parts Discussed in Thread: THS4131

I am seeing a strange oscillation on the output of my THS4131.  I am using the op-amp as a I/V converter in single-supply mode with +5V.  My feedback resistor is 500 ohms and my source resistance is about 5 ohms.  My input now is just white noise, but the output looks like the attached picture. 

  • 0
    TI__Genius 13935 points
    Hi Chuck,

    That doesn't look like a typical oscillation. Can you post a schematic? What is your current source?

    Regards,
    Loren
  • 0 in reply to Loren Siebert 1
    TI__Expert 6320 points

    Loren,

    Thanks for the reply.  My schematic and datasheet excerpt are below.  My feedback res is 500ohms, the feedback caps are removed and the AC coupling cap is shorted. OPAMP+ is +5V and OPAMP- is tied to GND. I don't have pull-up resistor as suggested in the application circuit, nor do I have any load.  The device output is current mode so I don't want an Rg. As you can see, we are trying to use the THS4131 as the I2V Sum amp seen in the datasheet circuit. Also, VocmI is 0.9V

    Thanks, Chuck

  • 0 in reply to Chuck Smyth
    TI__Expert 6320 points

    I guess e2e still hasn't figured out copy and paste.  Please see attachment below.  Also, my circuit seems to work fine with +/-5V supply.

    I2V OpAmp Circuit.docx

  • 0 in reply to Chuck Smyth
    TI__Genius 13935 points
    Hi Chuck

    From the datasheet it looks like the Vocml is too low. The outputs can only swing to about 1V from the rail. That means with a Vocml of 0.9V there is no swing left on one of the outputs.

    I would advise against having no load. At least a 1k Ohm resistor would be good.

    I understand a current amp should not have an Rg, but then the voltage gain is infinite and the Vos will get gained up. There needs to be some way to null out the offset voltage.

    Is there a reason you don't want to use +-5V? Nearly all ground referenced applications are much easier with split supplies.

    Regards,
    Loren