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THS4281: Different result in Tina Sim and Implementation!

Amirhosein Akbari
Prodigy 40 points
Part Number: THS4281

Tool/software:

Hello

I am using THS4281 to determine if a part of my circuit is open-circuited or short-circuited. My main circuit is mainly high voltage AC, so I am using a voltage devider configuration along with a comprator structure, In Tina's simulation, for the short-circuited case, I expect to see 0V at the output of the opamp, and when there is an open-circuited situation, I expect to see alternating voltages at the output of the comparator. Vin is a Sin wave with 600V and 1khz frequency.

In the implementation, and for the short circuit condition, the output of the op-amp is VCC instead of being zero! I know that there are comparators available, and I already tried them in the simulation, and I still get the same response. I also know that op-amps and comparators in open-loop configurations are essentially unstable (the output is either 0 or VCC), and I was thinking maybe this causes the output of the op-amp in reality to go high instead of low. I attached the simulation results and was wondering if anyone could help me understand this. 

Thank you very much

----------------------------------

Open circuit:

Short Circuit:

  • 0
    TI__Genius 16585 points

    Hi,

    We usually do not recommend using amplifiers as comparators due to limits in differential voltage and internal input stage clamps that can cause a catastrophic failure if overdriven. Op-amps will also be slower than a comparator, which is why we recommend using a dedicated comparator. For this device the abs max input differential voltage is +/-2V, which although would keep the device alive is not recommended as this could still degrade the reliability and life span of the device. Regarding your simulations, is there a difference in the circuit when running a short circuit test? I am not sure what the difference was in your testing. Regarding the slamming, in a lab setting the output would slam depending on the differential input voltage polarity, which could technically slam to either supply. I am not sure if it is just the model capturing a positive input offset which is causing the device to slam only to VCC.

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Prodigy 40 points

    Hello Ignacio

    Thank you very much for your answer

    Well, since it is Sin wave, polarity is always changing, and I think having it connected to the - or + terminal side of the op-amp does not matter. I already did the simulation for different polarities, and still, I get the same result!

    Based on my design, the voltage at 600V will not exceed the 2V range of the op-amp. Additionally, since the input voltage changes at 50/60Hz, the fast or slow response of the op-amp is not particularly important.

    By any chance, do you have any other solution for this purpose? In this circuit, I am checking if the fuse is blown or not, which is why I am measuring the voltage across it and comparing it to see if the fuse is ok or not.

  • 0 in reply to Amirhosein Akbari
    TI__Genius 16585 points

    Hi,

    We do not have solutions with this type of application/setup as we do not highlight our devices in an open loop. I believe the comparator team would be able to find a device that can easily fit in this application without straining the device like you would potentially be doing with the THS4281.One option could be to implement a difference amplifier circuit if you would like to measure a voltage difference. However if a comparator type application is what makes the most sense in your circuit I can transfer your thread to the comparator team as they will have an ideal solution for your application.

    Best Regards,

    Ignacio