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Load Cell with INA122P Full Range issue

Igor Ramos
Prodigy 85 points
Other Parts Discussed in Thread: INA122, INA333, TPL0401A-10

Hello friends,

I am new to load cells and instrumentation amps, so I want your opinion if the results below are typical or I'm doing something wrong.

As you can see in the graph below, I could not find gain & polarity combination that would work in the full weight range. It seems like one polarity works best for higher weights (over 400g) and the opposite polarity works best for less weight (under 300g). Is this the nature of the load cell or I should tweak something?

here my circuit

 for your reference here's additional project details: 

https://sites.google.com/site/isramos/home/load-cell

thank you!

  • 0
    Guru 21975 points

    Hi Igor,

    Thanks for providing good information to help answer your question.

    It appears that the load cell has some initial offset that requires adjustment. This is common. Here is a simple way to trim the offset of the system:

    This trim circuit should be connected to one of the output terminals of the bridge (load cell). The value of R1 can be changed to alter the adjustment range of the circuit. P1 is adjusted to the desired zero value. R13 and R14 can be eliminated because the load cell provides an input bias current path.

    Note that the output voltage swing limitations of the INA122 will limit the range near 0V and 5V. The INA333 may be a better choice as it can swing very close to 0V and 5V.

    The lower gain, 4882 looks to be near the needed value, once the offset is properly adjusted.

    Regards, Bruce.

  • 0 in reply to Bruce Trump
    Prodigy 85 points

    Hi Bruce,

    Thanks for helpful answer. I was able to remove the offset with your suggestion (see graph). Good tip on removing R13 and R14. The INA333 has max gain of 1000 so that may not be a good option.

    One more question. What would be an alternate way to interface with that same load cell that does not require a mechanical trim pot? Would you recommend using a digital pot such as TPL0401A-10 to  remove the offset?

    thank you

  • 0 in reply to Igor Ramos
    Guru 21975 points

    Igor,

    I don't know any reason why the INA333 could not be used in a higher gain. The maximum gain number is pretty arbitrary. Though there is some loss of accuracy in higher gains, the degradation is minimal and is very unlikely to be seen at the 10-bit level.

    You could do offset trimming with an electronic pot or with a D/A converter. I'm guessing that you are using a uC with a D/A output. Connect the D/A output to the 20k resistor instead of the pot.

    Regards, Bruce

  • 0 in reply to Bruce Trump
    TI__Intellectual 860 points

    If you are using a microcontroller you can sometimes just subtract the offset in software. 

    To do this you need to make sure the signals always fall into the useable range of the amp and ADC and you would also need some kind of zeroing algorithm.

  • 0 in reply to Tom Mathews
    TI__Genius 12325 points

    Just to throw another option in there: If your microcontroller does not include a D/A converter you can use a PWM output that is low-pass filtered as a D/A converter and trim the duty cycle of this PWM waveform to correct for your offset.

  • 0 in reply to Bruce Trump
    Prodigy 85 points

    Bruce,

    I was taking the max gain stated in the datasheet literally.  Since I'm using a 10-bit A/D would the INA333 still be accurate at G=5000 or G=10000? Which datasheet parameter would tell me that?

    thank you

  • 0 in reply to Igor Ramos
    Guru 21975 points

    Igor,

    I understand what the data sheet says. For some reason I don't fully understand, it long ago became customary to show a useful gain range for instrumentation amplifier. While the minimum number is determined by the topology and is quite fixed, the maximum number is pretty arbitrary. The op amps in the INA333 have a typical open loop gain of approximately 3000000, leaving plenty of loop gain for good accuracy in a gain of 5000 or even more. I expect the overall performance of the INA333 to be better than the INA122 in a gain of 5000.

    Regards, Bruce.

  • 0 in reply to John Caldwell
    Prodigy 85 points

    John,

    that is an excellent idea. I understand the concept, but could not get it working. Could you provide a circuit sketch showing how it would interface to the bridge and op amp? 

    thank you,

  • 0 in reply to Bruce Trump
    Prodigy 20 points
    Can you provide final circuit with update (include above circuit) ?
    I am not getting that where to put this circuit.