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INA241A-Q1: Use case of high value of series resistor at input path

DEVENDHIRAN KUMARASAMY
Prodigy 120 points
Part Number: INA241A-Q1
Other Parts Discussed in Thread: INA190, INA186, INA237, INA239, INA228, INA229, INA226

Tool/software:

We need to monitor the current through a Shunt resistance of 40mOhm.

For the diagnostic purpose, we are planning to keep 1K ohm resistor in series between the IN+ & IN- pins and the shunt resistor.

So now the questions are,

1. Is it allowed to have 1K ohm resistance in front of IN+ & IN- pins of INA241A-Q1 device?

2. What is the functional impact of having high input resistance in front of IN+ & IN- pins of INA241A-Q1 device?

3. Is there any formula or simulation is available to estimate the impact of INA241A-Q1 device due to this configuration?

  • 0
    TI__Mastermind 29870 points

    Hello Devendhiran,

    DEVENDHIRAN KUMARASAMY said:
    1. Is it allowed to have 1K ohm resistance in front of IN+ & IN- pins of INA241A-Q1 device?

    It is not recommended as the device has an IB on the input to power up the input stage.  This allows the inputs to go beyond the range.  Having the resistors will cause some error.  If you must have a resistor there we recommend under 10 Ω.

    DEVENDHIRAN KUMARASAMY said:
    2. What is the functional impact of having high input resistance in front of IN+ & IN- pins of INA241A-Q1 device?

    This will give you errors due to the IB.   Both offset and gain errors.

    DEVENDHIRAN KUMARASAMY said:
    3. Is there any formula or simulation is available to estimate the impact of INA241A-Q1 device due to this configuration?

    I don't have the exact formula but the model in TINA and Pspice will model this.  

    Regards,

    Javier

  • 0 in reply to Javier Contreras4
    Prodigy 120 points

    Hi Javier,

    Thanks for your response.

    Do we have any differential or Current sense amplifiers available in TI which allowed to use 1K resistor in the input path?

    Regards,

    Devendhiran K

  • 0 in reply to DEVENDHIRAN KUMARASAMY
    TI__Mastermind 29870 points

    Hello Devendhiran,

    The INA190 has low input bias currents and a very high accuracy.  The only down fall is the speed and lower VCM range and lower bandwidth.  If cost is a concern there is a less accurate device the INA186.

    If you need more voltage range some of our digital power monitors can also do this.  Like the INA228, INA229 as examples.  There are less accurate versions as well like the INA239, INA237.

    If you list more of your requirements I can see if we have some thing that will meet your needs.

    Regards,

    Javier

  • 0 in reply to Javier Contreras4
    Prodigy 120 points

    Hi Javier,

    Thanks for your feedback.

    We are looking for a current sense amplifier or any differential amplifier with the following requirements. They are,

    1. Single 5V supply operation
    2. 50V/V gain configuration or adjustable
    3. Bi-directional current measurement shall be possible. (Unidirectional also would be ok if we get the part in low price)
    4. Minimum continuous common mode input voltage need to be ≤ -20V
    5. Maximum continuous common mode input voltage need to be ≥ +40V
    6. Common mode Input impedance must be ≥100K Ohm
    7. Differential mode Input impedance shall be ≥100K Ohm (but it is not mandatory)
    8. Input bias current must be in nA range to make use of 1k resistor in the input path

    Kindly suggest the any part if the specification matches with the above-mentioned requirements.

    Regards,

    Devendhiran

  • 0 in reply to DEVENDHIRAN KUMARASAMY
    TI__Mastermind 29870 points

    Hello Devandhiran,

    These constraints basically eliminate a simple current sense amplifier.  In order to work on a single supply and have input voltage range above and below the rails with the input current in the low nA basically eliminates most of our amplifiers.  This is due to inputs stage being powered up by the input stage and drawing current.  The easiest option is using a digital and moving the supplies to meet your needs.  You would need a negative supply for this to work.

    https://www.ti.com/lit/ug/tidu361a/tidu361a.pdf this is an example of what I am thinking but then you need a supply lower than you most negative voltage and instead of the INA226 one of our newer power monitors with low IB.  https://www.ti.com/amplifier-circuit/current-sense/digital-power-monitors/products.html#726max=80%3B85&sort=726max;desc&

    Also your 100KΩ common mode impedance needs to be higher if you want nA IB.  40V/100k= 400µA .  Current sense amplifiers normally do not specify input impedance because our input bias current does not follow the trend of a resistor.

    Regards,

    Javier.