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INA225: Current sense amplifier

Meenalochini D
Prodigy 150 points
Part Number: INA225

Hi Team,

In INA225 current sense amplifier datasheet, It was mentioned that input voltage at terminal may exceed beyond 40V if the current is limited to 5mA.

Under this condition, Is it possible to operate this device continuous/transient operation - ie Maximum common mode voltage of >40 V.

 As per datasheet, Input impedance is given as 25Kohm and Maximum voltage is 40V. So by default the current is limited  to <2mA.

Our design is having  bus voltage=common mode voltage of  40V+/-3%.

Please confirm how to use this device for >40V continuous operation.

Regards,

D.Meenalochini

  • 0
    TI__Mastermind 32395 points

    Hello Mr. Meenalochini,

    Thank you for the question. The 25kΩ is after the internal ESD diodes at the input pins, so this will not provide any current limiting, nor will any resistance in the input bias stage. All current-limiting will have to come from external resistors placed at the input pins.

    The device's performance is specified up to 36V and was not tested for voltages in between 36V and 40V.

    Overall it might be impossible to have the device function normally on a 40-V bus without accuracy issues and possibly needing calibration. At some point past 40V, the internal ESD diode will begin to enter a reverse breakdown and conduct current from bus, through reverse biased ESD diode, and to GND. Assuming ESD starts turning on at 41.2V, this means you will need at least 8.24kΩ = 41.2V/5mA at both input pins to keep device just from overheating. But the ESDs might not turn on until a higher voltage, which means device is operating at 40V common-mode and thus will not show datasheet accuracy.

    Having these high input resistor will attenuate overall circuit gain and add significant offset error, which can be theoretically calculated out. However, the error can become larger and unpredictable since ESD diodes may or may not begin slowly turning on and adding to the input bias currents of device, which are what can generate such large offset error. These ESD cells are not characterized in this way.

    To learn more about how input resistor create error for our current-sense amplifiers and how to calculate and calibrate these errors, you can watch this video/document here:

    https://training.ti.com/ti-precision-labs-current-sense-amplifiers-input-filter-error 

    However, since you are operating the part outside specifications and on the border of the Absolute Rating, the errors will be unpredictable and even though they might be calibrated, the error (ESD turn on currents) may fluctuate over temperature and or bus voltage fluctuation.

    The best option is to just use a part that is rated to operate at 40V.

    Sincerely,

    Peter