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INA128: Ultrasnoic reciever constant noise !!

Talha Qureshi
Prodigy 20 points
Part Number: INA128

Hi, i'm currently working on a personal project where I have to use ultrasonic transmitters and recievers. For the reciever circuitry I choose INA128 as it provides sufficiant gain in a single stage. In the datasheet (page number 14.15) of INA128 it is told to use a resistor as an input bias current return path. There is no information on how to choose the size of the resistors  I am currently using 10Mohm because anything less gives a constant 0 output. When I do not use the resistors I get random noise and when i do, The range is then limmited to a few inches. Can someone help me with this. I am using Jameco 40T/R-12 ultrasnoic pair and the INA is operated at 9v

  • 0
    Guru 140200 points
    Hi Talha,

    the INA128 is no rail to rail OPAmp. Input voltage of INA128 must be 2V higher than the V- supply voltage and 2V lower than the V+ supply voltage. So, you would need to power the INA128 with +/-9V and not +9V/0V.

    Kai
  • 0 in reply to kai klaas69
    Prodigy 20 points

    I don't think this answers my question regarding the choice of the input biased current return path resistors and the unwanted noise !! And my signal is purely of the TTL level and the ina128 is working good except for the noise but I will try to work on it using the +/-9V.

  • 0 in reply to Talha Qureshi
    Guru 140200 points
    Hi Talha,

    datasheet of ultra sonic pair says, that a load between 1k and 100k shall be connected from the output to GND, or by other words across the pins of receiver.

    If you connect the receiver to an OPAmp with a single supply, then this OPAmp must be able to handle input voltages down to 0V and even input voltages which go below the negative rail, because the receiver is producing an AC voltage! A single supplied INA128 is not able to do this. As datasheet says the common mode voltage range begins at 2V above the negative rail.

    Kai
  • 0 in reply to kai klaas69
    TI__Genius 11135 points
    Talha

    We haven't heard back from you so we assume this resolved your issue. If not, post a reply below, or create a new thread if this one has timed-out.

    Thanks
    Dennis