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SSM2142 Balanced Line Driver breaking input impedance

Hi all,

my problem concerns the breaking of the input stage of the chip SSM2142 which occurs some time during a test cycle.

My testbench is attached for information.

How can see, the SSM2142 is connected to the "secondary" windings of a LVDT excited with a 5Vrms/10kHz sine wave on the "primary" windings by means of the SSM2143 differential line driver connected to an arbitray waveforms generator. An oscilloscope is connected to the differential output of the SSM2142 Line Driver.

After about 2hours of test I see that the signal at the output of the SSM2142 is reduced by 1\2. Measuring with a multimeter the resistance at the input of SSM2142 I found that the value was lowered to about 92.76ohm. In normal condition I measure a resistance of about 50kohm.

Do you have idea why this could happen?

What is it the possible root cause of the input resistance "breaking" of the SSM2142'buffer stage of the cross-coupled topology?

Note: The "Test Board" is "hot inserted" in the test bench (i.e. the power supply and the signal generator are "ON" and "fully operational").

Best Regards

Francesco Carbone

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  • Thanks Francesco for the additional information.

    Thanks for the clarification on how you are measuring the input impedance. You are correct that the specification is dynamic and not with the part powered down. So that does explain the difference.

    Yes, the +-7.071 V input level will translate to the 10.5 Vrms output voltage. Now I need to point out that these specifications are derived from the part operating at +-18V power rails.

    One thing that is missing on the datasheet is a specification for the absolute maximum voltage on the input pins. Normally this is around 0.3V above the positive rail and -0.3V below the negative rail. This is speculation on my part as far as the exact numbers but I can say that you cannot expose the pins to voltages exceeding the voltage rails. This will forward bias the ESD protection diodes/devices and cause excessive currents. You may want to go back to a damaged part and measure the resistance to the power rails. I think you may find a direct short to one of them.

    So have a look at your input conditioning circuit and see if you can add more dividers or other protection devices to limit the current and/or limit the voltage swing. You can place some diodes to the voltage rails to clamp the input pins to the rails. This is especially useful for the condition where the part is powered down and the input signal may still be present. This is where a part can be damaged without realizing the problem.

    Since this is an older part I do not have any information about any failure analysis that may have been done during development.

    Thanks,

    Dave T.

Reply
  • Thanks Francesco for the additional information.

    Thanks for the clarification on how you are measuring the input impedance. You are correct that the specification is dynamic and not with the part powered down. So that does explain the difference.

    Yes, the +-7.071 V input level will translate to the 10.5 Vrms output voltage. Now I need to point out that these specifications are derived from the part operating at +-18V power rails.

    One thing that is missing on the datasheet is a specification for the absolute maximum voltage on the input pins. Normally this is around 0.3V above the positive rail and -0.3V below the negative rail. This is speculation on my part as far as the exact numbers but I can say that you cannot expose the pins to voltages exceeding the voltage rails. This will forward bias the ESD protection diodes/devices and cause excessive currents. You may want to go back to a damaged part and measure the resistance to the power rails. I think you may find a direct short to one of them.

    So have a look at your input conditioning circuit and see if you can add more dividers or other protection devices to limit the current and/or limit the voltage swing. You can place some diodes to the voltage rails to clamp the input pins to the rails. This is especially useful for the condition where the part is powered down and the input signal may still be present. This is where a part can be damaged without realizing the problem.

    Since this is an older part I do not have any information about any failure analysis that may have been done during development.

    Thanks,

    Dave T.

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