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INA331 Cell load

Leandro Aguierre
Prodigy 30 points

Other Parts Discussed in Thread: INA331, INA326, INA125

Hi, I'm using the INA331 as amplifier for load cell (wheastong bridge, 2000kg, 5VDC exitation, I/0 resistence aroung 350 ohms) with 3mV/V, so it has 15 mV at full scale.

The cell load works ok, however I have some problems with INA331. First at all the offset, that it's hugh. For tested it, I configurated INA for G = 5, and I have 30mV offset!!! The cell load has <<.05mV offset, so the arithmetic disagree.

I need a G = 315 (R1 = 620k and R2 = 10k), this will be connected to ADC. With this configuration with cell load connected, I have .9V offset!!!!!!! However, when I charged with my weight (aroung 80kg, around .6mV) and in the output of INA has .9V offset + 20mV of my weight. It´s imposible!!!!

Could someone help with this. Thank you!

  • 0
    Expert 1220 points

    A schematic is most helpful to highlight power supply connections and voltages along with any biasing on the inputs.

    You can always connect a DC supply to both inputs and measure the output voltage in the mean time. It should read close to the datasheet typical offset presuming there is no issue with common-mode or power supply noise inducing offsets.

    -Ken 

  • 0
    TI__Genius 12375 points

    Leandro, 

    The output voltage swing to the power supply rails on the INA331 is specified in the datasheet as 25mV typical, 50mV max with a 10k load. 

    When you supply the part with 5V and Ground, the INA331 will only be able to output voltages within 25 to 50mV of either 5V or Ground. This is where the 30mV offset that you are seeing comes from. When configuring the INA331 for higher gains, the 25-50mV rail limitation is then amplified by the output amplifier A3. Therefore, if will be difficult to use this amplifier on a single supply, in high gains if you need to output 0V. Alternatively, take a look at my post here: http://e2e.ti.com/support/amplifiers/precision_amplifiers/f/14/p/296369/1034180.aspx#1034180 on the INA326 which has a differrent topology and will allow better performance on a single supply. 

  • 0 in reply to John Caldwell
    Prodigy 30 points

    Dear John,

    Thank you for the info, but I´m in trouble yet, because I did the hardware and as lead time to my client is finished.The next time I will test the INA326. What do you think about INA125? I attached the curve (0028.Ch1.xlsx)

    So, I try another solution (the problem exits tooI I "understand" the nonlinearity in the output). I increased the voltage to INA331 to 7V (max 7.5V) and load cell, so I have 40% in full scale (21mV). But, in the output has the almost .9V offset and is nonlinearity until 5mV in differencial mode at INA331. So, I lost a lot of data and critical for my aplication.

    If you have another idea, please let's me know. Thank in advance

  • 0 in reply to Leandro Aguierre
    TI__Genius 12375 points

    Because the internal amplifier is saturated at the negative rail, increasing the positive power supply voltage will not affect this. One option would be to add a negative power supply to the board.