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SN65HVD3082E: problem of the positive/negative-going differential input threshold voltage of its receiver

Yining Liu
Prodigy 50 points
Part Number: SN65HVD3082E

Hello team,

When I was testing the threshold parameter of the reciever refering to the datasheet, two methods were used and their results are different.

            

The first method is adding a squarewave directly to the two input pins of the reciever, and by changing its high level and low level, we can get the threshold voltage when the receiver is not able to maintain a determined output, as shown above;

The second is adding a common mode voltage between -7~12V at the input side, and we use a signal generator to add a squarewave as the differential mode signal, a transformer is used for isolation, as shown below

                       

However, this method is only able to change the peak-peak voltage of the signal, and when it is lower than 700mV, the output is not able to maintain a stable voltage, so the threshold voltage is much higher than the given range on the datasheet.

So which of the testing method is correct? and why these two method lead to different results?

Thanks a lot.

  • 0
    TI__Prodigy 50 points

    these are waveform and the schematic of the second method

  • 0 in reply to Yining Liu
    TI__Mastermind 25705 points

    Yining,

    Vit is a DC spec, which means you only need to input DC voltage to check the receiver response. For example, if you want to check Vit with 12V common mode voltage, you can set B at 12V and sweep A from 11.5V to 12.5V to see where the receiver toggles. If you do the test with square signal input, please make the frequency very low like 1Hz. Please let me know if you have more questions about the test.

    Regards,

    Hao

  • 0 in reply to Hao L
    TI__Prodigy 50 points

    Hi Hao,

    Thank you for your reply.

    I understand this method to test the threshold voltage now, but in the second test method, the voltage given by the generator is also a differential one, why could it be much larger than the specified value of 200mV? And in the practical applications when transfering data, the frequency of the reciever input waveform might be much higher than 1Hz, would the phenomenon mentioned above result in the loss of the transmitted data?

  • 0 in reply to Yining Liu
    TI__Mastermind 25705 points

    Yinling,

    It's a good question. Historically Vit is defined as a DC parameter in RS-485 standard. As the data rate goes higher, due to the bandwidth limit of the receiver input, the effective Vit will become higher. Therefore it's recommended to leave some margin for the receiver to work correctly in real applications. Please let me know if it makes sense to you.

    Regards,

    Hao