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ISO7742: CMTI questions

Kailyn Chen
Genius 9560 points
Part Number: ISO7742

Dear，

　　Our customer has some questions about CMTI.  

1. According to the datasheet of the CMTI, how to know CMTI test is pass or fail? 

I

Below are the test curve, are they pass or fail? I think  the first output has inverted due to digital signal, it could be fail, but the second output is almost high, only has one small drop and then changed high, is it pass or fail？

2.  The CMTI test need 15pF load. How to balance the small and big Cload? If using the big Cload, the output could be smooth, but if using active probe which could be met the 15pF, but the probe is easy to crosstalk. 

3. How to decide the CMTI interference source rise time, the voltage is so defined, such as voltage 100V, time 1ns, Or voltage 2000V time 20ns, this slope is all 100kV/us, but the test results are different, voltage time this is how to consider? In addition, this slope is not the ideal ramp or ramp, how does the oscilloscope take this PASS-capable slope, is it the maximum slope of the segment or the average? 

Best regards

kailyn

  • 0
    TI__Genius 17825 points

    Hello,

    Thank you for your question.

    I have answered your questions below:

    1. CMTI pass/fail
      1. The CMTI specs you see in the datasheet is the pass level for the device. levels above this could cause the device to fail.
      2.  for the first waveform your understanding is correct. If the output has changed from LOW to HIGH due to CMTI, then this is a fail.
      3. for the second waveform since the output voltage does not drop to low logic threshold and only lasts for less than 10ns, it is not read as a false low signal. Therefore, this is a pass
    2. The CMTI test need 15pF load. How to balance the small and big Cload? If using the big Cload, the output could be smooth, but if using active probe which could be met the 15pF, but the probe is easy to crosstalk.
      1. The 15pF load that is added in the CMTI test circuit in datasheet is an optional test load. Customer may not need to add this load to their test. It is OK to use any probe for testing. 
    3. How to decide the CMTI interference source rise time, the voltage is so defined, such as voltage 100V, time 1ns, Or voltage 2000V time 20ns, this slope is all 100kV/us, but the test results are different, voltage time this is how to consider? In addition, this slope is not the ideal ramp or ramp, how does the oscilloscope take this PASS-capable slope, is it the maximum slope of the segment or the average? 
      1. We use VCM = 1200V for CMTI testing, as stated in the datasheet. Customer can use any voltage up to 1200V.
        1. The maximum slope of the CMTI generator output should be less than the datasheet specification for the device to pass

    Regards,
    Aaditya

  • 0 in reply to Aaditya V.
    TI__Genius 9560 points

    Hi Aaditya，

    Thank you very much for your reply.

    The customer also has some questions to confirm again.

    1. 

    Aaditya V. said:
    for the second waveform since the output voltage does not drop to low logic threshold and only lasts for less than 10ns, it is not read as a false low signal. Therefore, this is a pass

    The logic low pulse only lasts less than 10ns and read as pass signal, how long does the low signal last at least to read as fail signal？ 

    2. 

    Aaditya V. said:
    The 15pF load that is added in the CMTI test circuit in datasheet is an optional test load. Customer may not need to add this load to their test. It is OK to use any probe for testing. 

    He would like to confirm how much load of our test platform? 

    3.  The slew rate of CMTI interference source is not stable,  such as voltage 100V, time 1ns, Or voltage 2000V time 20ns, this slope is all 100kV/us, but the test results are different, we decided it by average slope or the fastest slope? 

    Best regards

    Kailyn

  • 0 in reply to Kailyn Chen
    TI__Mastermind 24985 points

    Hello Kailyn,

    Please note that today is a holiday for our teams in the US, but we will be able to get back to you tomorrow with an update.

    Michael

  • 0 in reply to Kailyn Chen
    TI__Genius 17825 points

    Hello,

    Thank you for your patience

    1. The logic low pulse only lasts less than 10ns and read as pass signal, how long does the low signal last at least to read as fail signal？ 
      1. The 10ns pulse you see is an output from the ISO7742 device. Whether this is a pass or fail is determined by the device that is reading the signal. For example, if the bandwidth of the input channel of the reading device is 10MHz, then it will not be able to recognize a 10ns pulse and therefore it would be a pass. Similarly, if the bandwidth is 200MHz, then it will be able to recognize the 10ns pulse and therefore it would be a fail.
    2. He would like to confirm how much load of our test platform? 
      1. Did you mean to say how much load we have in TI test setup? if yes, then it would be 15pF load.
      2. Did you mean to ask how much load to apply on customer test setup? if yes, then it would be up to the customer on how much load they have in their system.
    3. The slew rate of CMTI interference source is not stable,  such as voltage 100V, time 1ns, Or voltage 2000V time 20ns, this slope is all 100kV/us, but the test results are different, we decided it by average slope or the fastest slope? 
      1. Any transients durations less than 3ns are typically not recognizable and all voltage - time combinations where the time is greater than 3ns can cause the device to fail.
      2. please note that 100kV/us is a typical pass level. Due to manufacturing differences between units, some device can fail at 100kV/us while some others can fail at 120kV/us, while some can also fail at 150kV/us. The actual fail level of the device will be greater than 85kV/us but they can be anywhere above 85kV/us. 
      3. To summarize, not all devices will fail at 100kV/us. 

    Regards,
    Aaditya Vittal