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INA241A: Input bias current when measuring very low currents

Igor Brkic
Prodigy 40 points
Part Number: INA241A
Other Parts Discussed in Thread: INA190

I'm designing a precise high side current measurement circuitry which needs to switch measurement shunts through a few measurement ranges (that part is mostly done). I noticed that INA241A, while having all other parameters seemingly perfect for my use case (bandwidth, error, gain options, common-mode voltage...) has input bias current of 25-45uA. Does that mean I wouldn't be able to use it to measure shunt current in uA range. My common mode voltage is between 0 and 18V and power supply is 5V (although I have other higher voltage rails, such as 20V, as an option for powering it). If that's the case is there a way to improve it (maybe add some high input impedance buffers to Vin+ and Vin-) or would I need to use a different amplifier?

  • +1
    TI__Mastermind 29710 points

    Hello Igor,

    Most of our current sense amplifiers that have a high VCM range (larger than VS) will have a larger input bias current than most amplifiers.  We do have other ones that have the current range you are looking for VCM range but may be lacking the bandwidth you are requiring.  This is the INA190.  This device has a different architecture that limits the bandwidth to 47kHz on the lowest gain option.  

    We do have instrumentation amplifiers that also can be used that would also need but these devices will have limitations on the input range.  We have a Analog Calculator to see the limitations on the input VCM depending on the device.  You will also see the bandwidth limitations as you increase the gain.  A screen shot below.

    Another options buffering the input of the Rshunt before going to the INA241 but that seems to add much more IC.  If you do this you need to look at bandwidth and offset of the amplifiers.  You would also need to power it up with 20V to not limit the swing. 

    I will ask around to see if I am missing other options but the exact gain you need and bandwidth may help in the selection.  Also is this going to an ADC that requires the outpu to be limited to 5V?

    Regards,

    Javier

  • 0 in reply to Javier Contreras4
    Prodigy 40 points

    Hi Javier,

    thanks for your reply, that cleared up some things for me. In the meantime I also studied some more about the difference between various current sensing amp circuits (which maybe I should've done before asking the question here :)). Due to the relatively high shunt resistor for lowest current range I'll probably have to go with the instrumentation amplifier but I'll have to do some calculations first, now that I know a bit more about the topic.

    Best,

    Igor