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LMP7721: LMP7721 debug

James Lee94
Prodigy 190 points
Part Number: LMP7721


Hi all:
  I have found another phenomenon by Oscilloscope on vout  (LMP7721's PIN4) as following ,there is no input on PH+.

Would you please tell me why? how to resolved it ? And the frequency is 50HZ.Probe.pdf

  • 0
    Guru 140200 points
    Hi James,

    you cannot leave the +input of LMP7721 totally unconnected. No OPAmp can work like this. At least, mount a resistance from the +input to "RGND" to provide a current path for the input bias current.

    Kai
  • 0
    TI__Guru 68291 points

    James,
    You mentioned in the write-up LMP7721 but your schematic only shows LMP7221 - so what is it?
    I am not sure what issue you describe. If you float the input, the input will slowly move over time towards one of its rails and thus the output will follow. This may lead to saturation of the output stage leading to back-and forth tripping of the input stage just like in the case of a comparator.
    In order for the input stage to work properly, there must be a current path for the input current flow to system ground or the input will collapse on its rail. Thus, the way to resolve the issue is not to float the input.

  • 0 in reply to Marek Lis
    Prodigy 190 points

    Hi :

     I test this because when i connect with PH glass electrode on PH+ end.

    The vout pin(LMP7721's PIN4) as following  attachment pitcture. And the frequency is 50HZ.

    the device is LMP7721. 
     

  • 0 in reply to James Lee94
    Guru 140200 points
    Hi James,

    this is simple 50Hz mains hum which your LMP7721 circuit "receives" from the surroundings because your ph-sensor and the LMP7721 circuit is so extremely high ohmic. You need to use proper shielding. Read section 8.1 and 8.2 of datasheet of LMP7721.

    Kai