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INA118: INA118 ampr issue

Gaurav Valecha2
Prodigy 90 points
Part Number: INA118

Hello , 

We are currently using the said part for sensing wide range of voltage , please find  the schematic in pdf & its LTspice equivalent ( just for pictorial representation of single channel ) .

Problem statement : we have design the said circuit for gain =1 . However some strange pattern( offset )  is been observed on all the INA ( we are using 7 such INA in the design ) . Please find reading for single channel . 

The offset appears to be consistent in all INA . 

Can you please comment on the same . 

Gaurav  4263.pdf

  • 0
    TI__Mastermind 48951 points

    Hi Gaurav,

    Q: we have design the said circuit for gain =1 . However some strange pattern( offset )  is been observed on all the INA 

    I am going to comment on the schematic in the pdf file. 

    From the table and schematic you provided, Vin+ > Vin- at the input and the calculate output should be positive. (The output will be negative, if Vin+ < Vin-). The observed output is off significantly from the input, and it does not seem to have any output-to-input correlations. Have you checked out the schematic traces in the PCB? My guess is that there are mistakes in layout, connection and/or soldering issues in the PCB. 

    BTW, this is not INA118's Vos or voltage offset issue, which it should be in uV to mV range (Vref = 0V in INA118 configuration). 

    Below is the simulation in INA118, as you can see that it is a linear relationship when input is from 0-6Vdc and output follows the input with Gain = 1 V/V. 

    INA118 E2E 08242021.TSC

    If you have additional questions, please let me know. 

    Best,

    Raymond

  • 0
    Guru 140200 points

    Hi Gaurav,

    provided the right components are mounted, the layout is ok and the supply voltage is clean, stable and nose-free, the offset could be caused by oscillation. Connecting a voltmeter directly to the output can result in instability due to the high capacitive load of voltmeter input. Best you repeat the measurement after the insertion of a 100...250R isolation resistor which you optimally mount very close to the output pin of INA118.

    Kai