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Unexpected behaviour of ina 128

fikret kus
Prodigy 20 points
Other Parts Discussed in Thread: LM393, INA128

Hi,

  I am using ina 128 as a comperator in my project. I have used the basic connection that showed in datasheet in page 13. The gain resistor is 2k. I have applied the signal to Vin-(pin 2) input and the reference to the Vin+ (pin 3) input. I have noticed that when there is no signal in pin 2 and I have applied 4.2V to the pin 3, I get 940 mV in the other input (pin 2). Why this is happen? Is it normal? I am using this setup in very precision instrument so this make me fear about unexpected results. 

  • 0
    TI__Expert 3126 points

    Hi fikret , 

    The instrumentation amplifier is used to amplify the differential signal at the + and - pins 

    i.e the output of the INA will be = Gain X ( Vin+   -    Vin-)

    I would not recommend that part for your application 

    I believe you are looking for a comperator  , for comparing your reference signal at 4.2V 

    please find the circuit below with LM393 which is a open collector comparator and would be right fit for your application

  • 0 in reply to Shreenidhi Patil
    Prodigy 20 points
    Hi Shreenidhi,
    Thanks for your reply. I don't use it as a comperator actualy. I use it as a part of PID converter. It serves as proportional part of the PID. It works good but I don't understand how Vin+ input effects the other input. Is this possible? This can couse to a kind of instability.
  • 0 in reply to fikret kus
    TI__Expert 3126 points

    Hi fikret, 

    Thats good to hear   , yes you can use it as  proportional part of PID

    I believe you have kept the (Vin -) pin floating , (that is not connected to any source)

    The Instrumentation amplifier is not to be configured with any input pins as  floating

    Hence the 940mV that you get on the (V -) pin doesn't say anything about the stability or accuracy of the INA128