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INA317: Gain Issue

Part Number: INA317

Hi team,

  We have set  Gain = 33 by using (R173)3.16K resistor across 1 and 8 th pin of the amplifier. But we observe that the gain of the amplifier is around 2 only. 

  We also tried placing different gain resistors like 1K,300 but we observe that the gain remain as 2 only. Please give us your suggestions.

  • Hi Diwakar,

    INA317 is an instrumentation amplifier. It amplifiers the difference of inputs between Vin+ and Vin- and output Vout = Gain(Vin+ - Vin-) + Vref, where Vref =0 in your circuit. Vcm input requirements and output requirements are shown below. Your circuit does not meet input Vcm requirement, and has issues with Vout with Vref = 0. Furthermore, as gain increases in instrumentation amplifier, the BW response decreases as shown below, which is likely too low for your application, I believe. 

    With step input of 100mV, Gain=33V/V and Vref=0, your output will be near 3.3V, which it will not work with input Vcm and Vocm requirements per the datasheet. 

    Your circuit has 3 pulse inputs with 0-30V, 0-10mV or 0-100mV (2 gain selections are for 10mV and 100mV pulse input), if I understand your design correctly. Pulsein_ampin node at pin3 of INA317 is clamped at 3.0V, if input pulse is 0-30V. If the input pulse is 0-10mV or 0-100mV, then you want to amplifier the input signal via INA317, turn on BJT Q6 and trigger the opto U3. Pin2 in Vin- is connected to isolated ground. I think that you want to turn on bjt Q6 and trigger the opto U3 in all three conditions. Since input is pulse signal, the BW response of the op amp is important (I assume). 

    I was wondering what if you are using a selectable op amp comparator for the application. TI has a very fast TLV3501 comparator that will meet the application. You can use single rail of 3.3V or 5V and compare the incoming pulse with a known triggered reference. Once TLV3501 is triggered, it will turn on bjt Q6 and opto U3 for the event. 

    If the response BW pulse signal is not critical, you can use an op amp instead of INA317 to accomplish the same task. 

    Enclosed is INA317 instrumentation amplifier example for your reference. 


    If you have any questions, please let us know.