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10V Load Cell to INA125 to MCU

Prodigy 50 points

Replies: 10

Views: 4905

Hi,

I am trying to read a load cell's value with a microcontroller and use the INA125 as an interface between the load cell and MCU. I have built the Basic Connection schematic on the datasheet of INA125. The supply voltage is 12V. The gain resistor is 200Ohm which will a gain of 304.This is all good in theory, but when I test the output with an oscilloscope, I get a steady voltage.

Is there anything I need to add to the schematic to make this work?

Thanks

10 Replies

  • Fardin,

    How do you have the sleep mode pin connected and what is the constant output voltage that you are reading? Have you viewed the output of the load cell directly with an oscilloscope to ensure that it is functioning properly? 

    John Caldwell

    Product Line Manager - High Speed Amplifiers

    The New OPA2834: Wide-Bandwidth, Low-Distortion, only 170uA per Channel! http://www.ti.com/product/OPA2834 

  • In reply to John Caldwell:

    Hi,

    The sleep pin is connected to V+ (12V), as we are not using the shutdown mode. I have not actually tested the load cell with an oscilloscope (its brand new Omega part).

    There is something that I have noticed which I am not sure if it is a problem. When the load cell is not connected there is 10V at pin 16 (Obviously), and pin 16 is connected to pin 4 (VrefOut). But when I connect the Load cell, this voltage drops to around 5V. Is this a problem?

    Thanks

  • In reply to Fardin Barekat:

    Also here is the Load cell that I am using:

    http://www.omega.ca/shop/pptsc.asp?ref=lcm101_lcm111&Nav=pref05

  • In reply to Fardin Barekat:

    Fardin,

    I believe the reference voltage is collapsing to 5V because the INA125 is unable to provide enough current to maintain 10V across the load cell. A 10V excitation across a 350 ohm load cell will draw 28.6mA which is above the output current capability of the reference output. Figure 4 in the applications section of the datasheet shows a method to boost the output current using an external transistor. 

    John Caldwell

    Product Line Manager - High Speed Amplifiers

    The New OPA2834: Wide-Bandwidth, Low-Distortion, only 170uA per Channel! http://www.ti.com/product/OPA2834 

  • In reply to John Caldwell:

    Hi John,

    Thank you for your reply. I modified the schematic as follows:

    I added a 2n4400 NPN transistor to boost the current. The collector is connected to power, base is connected directly to pin 4, and emitter is connected to pin 16 (10V ref) and the input of my load cell. I am getting 10V out of the emitter and the voltage on pin 16 is 10V as well. However I am still getting the same results: steady voltage on the output.

    Please address this problem,

    Thank you,

  • In reply to Fardin Barekat:

    Fardin,

    Have you measured the output voltage of the load cell with a voltmeter? First I would ensure that the polarity of the load cell is connected such that it produces a positive differential input voltage. It would be helpful if you could include a schematic of your circuit and elaborate on what the fixed output voltage is. Is it 0V? 12V?

    John Caldwell

    Product Line Manager - High Speed Amplifiers

    The New OPA2834: Wide-Bandwidth, Low-Distortion, only 170uA per Channel! http://www.ti.com/product/OPA2834 

  • In reply to John Caldwell:

    Hello John,

    Thank you for your reply.

    My assumption that the Load cell actually worked was incorrect. After some troubleshooting (measuring resistances, output voltage) I found out that the load cell was wired wrong internally. Looking at figure 4-5 of this document as a visual reference, R2 and R3 are swapped and I was getting 220 mV output. That is too high.

    Going further, the serial number on the load cell body, did not match the serial number on the calibration sheet.

    I would have never thought that the problem was the load cell and here I am trying to fix the amplifier circuit.

    Thank you,

  • In reply to Fardin Barekat:

    hi  john,

    I had purchase INA125P amplifier . I tried to produce the 100 gain when I applied to Ov to across v+ and v- i got output from pin 10 as 120.57 mV but according to datasheet   it as to be around 250 uV. 

    Please help me it reducing the offset to 250 uV

    Thanks and Regards,

    Anavaratha Selvan

  • In reply to Anavaratha Selvan:

    Please mention Output offset of INA15P suppose I short the V+ and V- to GND

  • In reply to Anavaratha Selvan:

    Anavaratha,

    What power supply voltages are you using with the INA125? A schematic of how the part is configured and how you are testing it would be extremely useful in helping to solve this issue. 

    John Caldwell

    Product Line Manager - High Speed Amplifiers

    The New OPA2834: Wide-Bandwidth, Low-Distortion, only 170uA per Channel! http://www.ti.com/product/OPA2834 

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