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Finding a current sensing amplifier for my design conditions

aman garg1
Prodigy 55 points
Other Parts Discussed in Thread: INA321, TINA-TI, INA199

Hello,

I am aman and I am working on a sensor circuit design to measure the ethylene concentrations. My sensor circuit presents an output range of 0.04uA to 4uA. But I need to add a current sensing amplifier to measure the current at any particular moment which can also amplify the signal or output a voltage which can be then interpreted by the ADC.
I have been trying to find it but it is complex and difficult. Could you please help me out in finding the most suitable amplifier for my requirements.

 thanks

aman

  • 0
    TI__Genius 14825 points
    Aman,
    At this time, our current shunt monitors have large input bias currents (on the order of 10s of uA) which dwarf your measurement range, so our CSA parts may not be a good fit. However, depending on the supply rail you need to monitor and available sources on your boards, an op amp or instrumentation amp may be a good choice. I've contacted a colleague who may be able to help. If you could provide a few more details about your application, or perhaps a prelim schematic, that would be excellent.
  • 0 in reply to Jason Bridgmon
    TI__Guru 64520 points

    Aman,

    You have not provided any details about the power supply, input common-mode voltage and the required speed, thus I made my own assumptions.  

    One of the simpliest solution would involve using INA321 instrumentation amplifier together with 100k shunt resistor which in Gain of 5 with Vout referenced to 1.25V would result in 1.27V (Vout=1.25*5*0.04uA*100k) for 0.04uA minimum current and 3.25V (Vout=1.25*5*4uA*100k) for 4uA maximum current - see below.

    Below please see valid Vcm vs Vout ranges for given supply voltage, gain and reference voltage:

    I also attached Tina-TI schematic for you own simulations:

    4073.INA321.TSC

  • 0 in reply to Marek Lis
    Prodigy 55 points

    Hey marek,

    Thanks a lot for the suggestion. 5V and 3V are the available power supply and I am not sure of the available Vcm as I have the values of the varying currrent only. I am attaching the circuit below . Also, I was wondering if instead of attaching the current sensing amplifier, I could measure the voltage values across the 100k resistor using a ADC?

    (P.S. - I am still a novice in electronic design so pardon any errors in understanding).

    Also would it be possible to use INA199 as the amplifier ? I had that available on hand .

    Thanks again.

  • 0 in reply to aman garg1
    TI__Guru 64520 points

    Aman,

    INA199 is a Current-Shunt Monitor that is meant to be used to measure very high current levels at tens of amperes, or higher, in which case its input bias current, IB, of 28uA may be neglected - see below.  

    However, since in your case you attempt to measure 0.04uA to 4uA current levels, the error due to INA199 IB would completely dominate your actual measurement and for that reason, you cannot use it - see INA199 simplified schematic below.

    With 5V power supply available, I would recommend you to use INA321 as shown in my previous post. However, you will also need a reference voltage to make the accurate  measurement.  It is not clear from your schematic whether you try to measure high or low side current but as long as the input common-mode is between 0.55V and 4V (see below in blue ) my previous schematic will do the job.