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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

  • Hello Francesco,

    Can you give me a little more detail as to the transformer? Does it step up the voltage? Because if it does then that is an issue. 5V rms is +- 7.07V peak and that is the maximum voltage level that should be applied to the input pin. If the transformer steps up then you are exceeding the maximum and damaging the part.

    Does the part return to normal after it is shut off for a while or is it bad at that point?

    The other interesting thing is that the datasheet specifies 10K for the typical input impedance. You say you are measuring 50K. That is quite a difference.


    Dave T

  • Hi Dave,

    The LVDT transducers is typically a “step-down transformers”…in my application excite the primary with a max differential sine wave of 20Vpp@5kHz and the maximum  LVDT sine wave response is maximum 3Vpp@5kHz (as a function of “the core” material and position of the inserted in the LVDT).

    About the maximum input voltage range of SSM2142 balanced line driver, Does the Vin =+-7.071V is the maximum input voltage that provide the clip level output voltage of 10.5Vrms? Is it correct? Does the maximum input voltage equal to the power supply of SSM2142?

    The failure is permanent. The devices no longer works after a power-off\power-on of the power supply.

    About the input impedance of SSM2142, I’ve measured the “static” impedance with the board without power supply applied with a digital multi-meter in ohm configuration. Probably the input impedance of 10k typical provided in the datasheet refer to a “dynamic” impedance providing the power supply to the device.  The “static” impedance measurement in my application is useful to detect static failure in the board as short\open condition.

    Do you have others suggestion on what’s could be the “root” cause of this failure?

    Do you have a failure analysis of SSM2142?

    Thanks for the support.

    Best Regards.

    Francesco C.

  • 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.


    Dave T.

  • Hi Dave,

    thanks for your response.

    I checked the power supply of Line Driver (+-15V) both by measuring static impedance with a multi-meter and by measuring the current during functional mode and they are OK. No overcurrent detectd. Only the input impedance of line driver is not ok.

    From your point of view the only or probably "root cause" of the failure could be an electrical over-stress of the input? Considering my test bench rule it out and I've no idea of how to break the input stage!:-(

    Best Regards



  • Hello Francesco,

    Well, that was just a guess on my part that there may be a short to one of the power rails. To find the "root" cause will require a lot more investigation, information and time. We would need a faulty part to do some failure analysis to see what was over-stressed to give us an idea of where to look at your design. Then we would need your schematics and PCB layout information and more details on the input signals to your board. This is all an expensive and time consuming undertaking. Since this part is quite mature and has been in production for some time I doubt there is a problem with the part. So that is why I suggested to look closely at the input pin protection circuits on your board. This is based on a "most likely" cause based on the info you have supplied so far.

    Feel free to send me a private message or an email and we can discuss this in more detail.


    Dave T

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