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Issue with INA168 at low current measurements

Juan Ocampo
Intellectual 385 points
Other Parts Discussed in Thread: INA168

I had a question about the INA168(Current Shunt Monitor).  I have Rs=0.1ohms(0.1%), RL=50k(0.1%), and the Is(the current across Rs) should be very close to 0A when measured with a multimeter. For some reason, I keep getting a voltage of 0.4V on RL, which would mean that my current(Is) is 0.4A. Am I thinking of the math incorrectly?

  • 0
    TI__Expert 6930 points

    Hi Juan,

    Thank you for the question regarding INA168.  Your math seems correct, however you seem to also indicate a problem with the circuit, expecting perhaps a much lower output voltage.  If you need additional help please feel free to reply with a schematic and upon that schematic please indicate the voltages you measure at each point?  Can you tell me any more about your application?  What is the end equipment?   What are the environmetal conditions of the application?

  • 0 in reply to Edward Mullins
    Intellectual 385 points

    Hello, 

    Thank you very much for your answer. The voltages measured are as follows:

    -INA168.pin3 = 3.36V 

    -INA168.pin4 = 3.36V 

    -INA168.pin1 = 0.17V (yesterday I was measuring 0.4V) 

    The INA168 is used to measure the supply current of a laser diode that is triggered with a pulse width of 100nS and a period of 100uS. We have a 5mA fast blow fuse, which means the current is not going over 5mA yet the INA168 is saying that the current is 170mA.

    Thank you!

  • 0 in reply to Juan Ocampo
    TI__Expert 6930 points

    Hi Juan,

    Thank you for the additional information.  While I do not have the exact answer regarding the amount of error you see at the output (seems a bit high to me given everything you have provided) I have another concern.  You describe that your input signal that you are trying to measure is 100nS in width.  The INA168 is configured in a gain of 10 by virtue of the 50k Ohm resistor connected at the output.  There is a graph in the data sheet on page 5 that shows the transient response of the INA168 when configuerd in a gain of 10...as you can see from this graph the rise time is on the order of 10uS.  The signal will have come and gone by then so I think this circuit requires a much faster currrent sensor.

    Is it possible for you to measure the current on the low side using a very fast opamp circuit?

  • 0 in reply to Edward Mullins
    Intellectual 385 points

    Hey Edward, 

    We ended up changing the laser for a continuous laser which can draw up to 1A when triggered. For some reason, when the laser is not triggered and the current measured should be 0, the INA168 is getting the following:

    INA168.pin1 = 158.7mV

    INA168.pin2 = 3.9mV

    INA168.pin3 = 4.98V

    INA168.pin4 = 4.98V

    INA168.pin5 = 3.296V

    Thank you,