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OPA2348: OPA2348 un-expect output

Intellectual 1470 points

Replies: 7

Views: 1873

Part Number: OPA2348

Dears,

My customer using OPA2348 to monitor the NTC sensor which is integrated on IGBT module. the input voltage of IN+ is range from1.8V~3.7V. the VCC of OPA2348 is +5V.

Normally, the IN- (connect to Output via a IN4148) will follow the inputs, but sometimes the IN-(connect to Output via a IN4148) is 0.7V. It may happen when testing for days, sometimes is 10 days. 

is there any risk of using the below schematic? thanks!

  • The full schematic as below:

  • In reply to Gavin Jiang:

    Gavin,

    I believe the problem you see is caused by the back-to-back input protection diodes between the input terminals of OPA2171 - see below.

    In order to eliminate the issue, please use instead OPA2196 or OPA2191, which do not include those input diodes - see below.

    Marek Lis, MGTS
    Sr Application Engineer
    Precision Analog - TI Tucson

  • In reply to Marek Lis:

    Gavin,

    Be sure to measure all the device pin voltages to get a complete picture.

    Regards,
    Ronald Michallick
    Linear Applications

    TI assumes no liability for applications assistance or customer product design. Customer is fully responsible for all design decisions and engineering with regard to its products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning Customer's design. If Customer desires engineering services, the Customer should rely on its retained employees and consultants and/or procure engineering services from a licensed professional engineer (LPE).

     

  • In reply to Marek Lis:

    Hi Marek,
    thanks.
    Is OPA2348 also has the back-to-back input protection diodes? The issue came from OPA2348.

    Another strange thing is that: When OPA2349 enter the "un-expect output status", once we use multi-meter to test the voltage between IN- and GND, OPA2348 will return to normal output within 1 second.
  • In reply to Gavin Jiang:

    Gavin,

    OPA2348 does not have back-to-back input protection diodes but the diodes from OPA2171 still may effect its output-see below.

      

    With all the capacitance around the inputs, the circuit gets into a lock-up state, which gets reset by discharging the cap with the multi-meter.

    Marek Lis, MGTS
    Sr Application Engineer
    Precision Analog - TI Tucson

  • In reply to Marek Lis:

    Hi Marek,
    There is an 10k resister connect the cathode of IN4148 to the IN+ of OPA2171. I am afraid it's very hard to lock-up the voltage of 0.7V at the cathode of IN4148 (also IN- of OPA2348) while the OPA2348 output short circuit current up to ±10mA and the voltage of OPA2348 IN+ is 2.5V around.
    Could you please tell more about it? thanks a lot.
  • In reply to Gavin Jiang:

    Gavin,

    The circuit is called a half-wave recifier because when the output is loaded in a way to require output current sinking, it will only show positive half of the input waveforem - see below.  

    I have simulated the circuit for various values of C1 and the results are shown above. What it means is that depending on R1*C1 time constant, the output will have dramatically different shape, which results in a large differential voltage between the inputs of OPA348 – see above.  The output will eventually snap back to follow the input but for a large values of C1like 100nF, it may limit the frequency below 1kHz (10k||100nF => 1ms).

    OPA348 Gavin.TSC

    Marek Lis, MGTS
    Sr Application Engineer
    Precision Analog - TI Tucson

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