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INA326 Instrumentation Amp - Error in functioning

Atul Bhakay
Genius 3205 points
Other Parts Discussed in Thread: INA326

Please refer the connections of INA326 which I did in circuit. Initially I have assembled this much circuit only to check the performance of INA 326 and it is working fine.

Now I have integrated power supply sections(IC- TPS 5430 and IC- LM 2830) and 4-20 mA section(IC- XTR 111) along with this INA 326. All sections are working fine except INA 326.

I have Gas sensor whose operating voltage is 2.4V. So my common mode voltage becomes 1.2V. The input signal to INA 326 is around 120-160mV and I have to amplify it over 0-5V range.

The reference input of INA 326 is adjusted by potentiometer(1k).When both the input terminals are at same potential, the output should be zero ideally (practically 50-60mV). But I am getting around 600 to 700 mV output when the bridge is balanced and that  output voltage is not changing as expected after unbalancing the bridge (by applying gas to gas sensor). There is very small change in output voltage.

 Kindly advise

Atul Bhakay

Please advice with your suggestions.4064.INA 326.pdf

  • 0
    TI__Mastermind 25835 points

    Hello Atul,

    A few quick observations and questions:

    a) Have you tried removing the sensor and shorting the input pins? If
    so, what voltage is measured at pins 5 and 6?

    b) When you are reading an output voltage of 600-700mV, what voltages
    are measured at pins 2, 3, and 5?

    c) Have you tried removing the potentiometer and replacing it with  
    surface mount resistors?

    d) One observation is that you state "...the reference input of the
    INA326 is adjusted by potentiometer...". The attached schematic shows
    the potentiometer is connected to pin 3, which is not the reference
    input. In order to add a reference voltage to pedestal the output you
    should use pin 5 as shown in Figure 6 of the data sheet.

    e) Can you please give us the part number and/or data sheet for the gas sensor you're using?

  • 0 in reply to Pete%20Semig
    Genius 3205 points

    8524.3 wire_INA326_ rev1.pdf

    Hi Pete Semig,  Thanks for your prompt reply. Given below are my answers to your qeries:-

    a)      Have you tried removing the sensor and shorting the input pins? If
    so, what voltage is measured at pins 5 and 6? – As suggested removed the sensor and shorted the input pins and observed the following voltages.

    Voltage @ pin no.2— 1.238mV

    Voltage @ pin no.3— 1.238mV

    Voltage @ pin no.5— 291mV

    Voltage @ pin no.6— 300mV

     

    b)      When you are reading an output voltage of 600-700mV, what voltages
    are measured at pins 2, 3, and 5?  Following voltages are observed(sensor and potentiometer connected in circuit)

    Voltage @ pin no.2— 1.242mV

    Voltage @ pin no.3— 1.242mV

    Voltage @ pin no.5— 535mV

    Voltage @ pin no.6— 563mV

     

    c)      Have you tried removing the potentiometer and replacing it with  
    surface mount resistors? Removed the potentiometer and sensor and observed same voltages as case b)

     

    d)     One observation is that you state "...the reference input of the
    INA326 is adjusted by potentiometer...". The attached schematic shows
    the potentiometer is connected to pin 3, which is not the reference
    input. In order to add a reference voltage to pedestal the output you
    should use pin 5 as shown in Figure 6 of the data sheet.

    I have stated it wrongly. The reference voltage is 0V. The potentiometer is connected to non inverting pin of INA 326 to adjust the input voltage to get zero or few mV when the bridge is balanced. (It is used for zero setting) . I do not require any reference voltage in this case.

     

    e)      Can you please give us the part number and/or data sheet for the gas sensor you're using?

    We are testing this with Gas Sensor KGS 601. Please find below mentioned link for more details.

    http://newcera.en.ec21.com/Gas_Sensor_KGS_601--641976_642938.html

     

    I have attached complete schematic for your reference. Please have a look and suggest.

  • 0 in reply to Atul Bhakay
    TI__Mastermind 25835 points

    Hello Atul,

    The original schematic had only 1 potentiometer.  The most recent schematic shows a 2nd potentiometer (P1) connected between pins 1 and 8 of the INA326.  The INA326's gain pins are particularly sensitive to capacitance.  Please try replacing P1 with a surface mount resistor and ensure nothing is connected to pins 3, 5, and 7 of header J2.