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Original question:

INA500: VREF and V- Tied to Ground (positive only configuration)

INA500: INA500C input common mode voltage when supply is 0V

Savas Sahin
Prodigy 240 points

Part Number: INA500
Other Parts Discussed in Thread: TINA-TI

Tool/software:

Hi,

We are planing to use INA500C to read battery cell voltages. It says input common mode voltage is 5*SupplyVoltage which is 5V for our application thus a 0V to 25V input common mode voltage expected. 

I want to learn when device is off, will it be a problem for INA500C to face 25V input common mode voltage where battery cells are not disconnected from the board? 

All the bests,

Savas

  • 0
    TI__Mastermind 33723 points

    Hey Savas,

    Will the overvoltage only be present on IN+ or will there be voltage on IN+ and IN-?

    Best,
    Gerasimos

  • 0 in reply to Gerasimos Madalvanos
    Prodigy 240 points

    Hi Gerasimos,

    25V on IN+, 21V on V- , VSUPPLY = 5V, VREF = GND

    All the bests,

    Savas

  • +1 in reply to Savas Sahin
    TI__Mastermind 33723 points

    Hey Savas,

    If there is 25V on IN+ there is no issue. This is due to the snapback ESD diodes on the input of the device. These diodes are present before the the input resistors, and will not start conducting before I believe 32V. I need to confirm this value. This means that there will not be a conducting path for the 25V on IN+.

    However, if this 21V is on IN-, there will be a conduction path to V+. This voltage will go through IN-, to OUT through the feedback path and conduct through the output to V+

    There are two considerations here. one is the input current. The input current limit is 10mA, and due to the high input impedance of the INA500, this should not be an issue. The next consideration is the impedance of the supply rail. If the supply rail is high Z, this 21V can pull V+ above the absolute maximum value and damage the INA500.

    If the supply can sink the current from the input conduction path and pull the node to 0V, there is no issue. If the supply is high-z, you may need to add a Zener diode with a threshold above typical V+ but below the abs max V+ value of 6V

    Best,
    Gerasimos

  • 0 in reply to Gerasimos Madalvanos
    Prodigy 240 points

    Hi Gerasimos,

    Can I solve this using a 100k pull down resistor on the supply rail, which keeps the rail around 1-2V if i am not wrong? I can tolerate that much current consumption. Will that affect output performance? 

    Another idea is using a resistor divider by  2 on the input side and using the INA500B? 

    Thank you,

    All the bests,

    Savas

  • 0 in reply to Savas Sahin
    TI__Mastermind 33723 points

    Hey Savas,

    You could pull down the supply with a 100kOhm resistor. The drawback here is that you will constantly be sinking 50uA of current (almost 4x your quiescent current for your INA500 when the device is powered.

    You may see an erroneous output on the amplifier if the input current and voltage is sufficient to turn on the INA500.

    Best,
    Gerasimos

  • 0 in reply to Gerasimos Madalvanos
    Prodigy 240 points

    Hi Gerasimos,

    Thank you for your reply.

    Is there a way to simulate this behaviour in TINA-TI or Pspice? Does INA500C TINA model or spice model show how much leakage exist during off state?

    All the bests,

    Savas

  • +1 in reply to Savas Sahin
    TI__Mastermind 18140 points

    Hi Savas, 

    Gerasimos is out on travel this week, so I am covering this thread. Our spice models do not show behavior when the part is in the off state, as it is meant to show typical behavior in operation. The information Gerasimos has been referencing above is from the datasheet. 

    Regards,
    Ashley