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  • TI Thinks Resolved

TLV3691: Input impedence abnormal

Part Number: TLV3691

Hi team,

My customer is using the TLV3691 as a comparator to monitor the voltage from a Potential transformer (PT). The negative input of TLV3691 is connected to the PT output and the positive input of TLV3691 is connected to a reference voltage. When customer is doing the test, they found that the input impedance of the negative input is abnormal. The input impedance is only several ohms instead of several Mega ohms. I already checked with customer that the negative input voltage is within the range from GND to VCC. Do you have any comment on what may be the reason of this input impedance issue? 

Best regards,

Wayne

  • Hi Wayne,

    What was the supply voltage used to VCC? How was the input impedance measured? What reference voltage is used?

    A quick sketch of the involved circuit would be helpful.

    Best.

    -Jian Zhang
    LPAC Apps
  • In reply to Jian Zhang5:

    Hi Jian,

    please see the schematic as below. The PT outputs a current signal and then converted to a voltage signal by a 80 ohm resistor. Customer found that during the test, the voltage on the 80 ohm resister is lower than the normal value. So they measured the input impedance of the negative input and found that the impedance is abnormal. There' are two TLV3691 on the board and both of them have the same issue. For one TLV3691, the negative input is measured as 80 ohm. For the other TLV3691, the negative input is measured as 50ohm. The measument is done by multimeter. Do you have any comment what may cause the input impedance issue?

  • In reply to Wayne Wang20:

    Thanks for the quick reply. I'm curious what happens if they reverse the multimeter test leads? The low impedance could be the result of the forward impedance of the low-side protection diode, in this case it could be verified by swapping the test leads.

    Other than the measured low input impedance, does the circuit function at all?

    Best.

    -Jian
  • In reply to Jian Zhang5:

    Hi Jian,

    Thanks for your help. Customer firstly found the the voltage abnormal on the 80 ohm resistor. The measured value is half of the normal value. Then they measured the input impedance of the TLV3691 to be 80 ohm. The external 80 ohm resistor and the 80 ohm input impedance are in parallel to get a 40 ohm resistor value. That accounts for why the voltage is half of the normal value. So I don't think the low input impedance is caused by the multimeter test leads direction. Do you have other comments? Thank you.

    Best regards,
    Wayne
  • In reply to Wayne Wang20:

    Hi Wayne,

    TLV3791 has a maximum input bias current of 20nA which is equivalent to 100s of MOhms input impedance. If the customer is seeing a solid resistance measurement of input in both directions, most likely the input is a melted junction (short). In this case, the device in question is most likely damaged.  There is a thread elaborating in details over a similar issue:

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