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XTR106: Adjust Load Cell ZERO and SPAN

MARCIO CABRINI
Prodigy 40 points
Part Number: XTR106

Hello,

 

I am designing a circuit for load cells using XRT106. The circuit is based on the FIGURE 1. - Basic Bridge Measurement Circuit with Linearization from the XRT106 datasheet.

 

The resistors R1 and R2 form bridge trim circuit to compensate for the initial accuracy of the bridge, what means it adjusts 4mA @ 0.0000mV/V (or unbalanced load cell ZERO).

R1 is a fixed value resistor and R2 a trimmmer resistor (trimpot).

Vin is connected before R1, what means R1 does not affect the load cell SPAN.

 

For example, a standard 1kg load cell with 2mV/V output: zero=0.0110mV/V and Span 1.9890 mV/V. The ZERO value can be adjusted using R1 and R2 resistors to balance the bridge, to result in 4mA output. What means that 0kg on the load cell is 4mA (0.0110 mV/V adjusted to 0.0000mV/V).

 

I did not figure out how to adjust the Span for any type of load cell. For this specific load cell, I can calculate RG for VFS 1.9890mV/V to give me 20mA @ 1kg of mass?

 

I would like to use another trimmmer resistor to adjust SPAN (20mA). Would I use a trimmer resistor on RG? What would be the best technique to adjust the SPAN?

 

 

Thanks in advance,

Márcio Cabrini

  • 0
    Guru 140200 points

    Hi Marcio,

    I think the best way is to replace Rg by a series circuit of a bigger fixed resistor and a smaller trimmer. Of course, you can also use a parallel circuit of a smaller fixed resistor and a bigger trimmer. Use a precision and stable multiturn cermet trimmer with a low temperatue coefficient. And choose an as small as possible adjustment window. This guarantees you the best precision and stability.

    Kai

  • 0 in reply to kai klaas69
    TI__Guru* 76801 points

    Hi Marcio,

    Thanks for Kai's great suggestions. 

    Some application or designers do not like to use or prohibit to use trim pot in a design, I often parallel precision resistors on top of it to refine the circuit, until errors are below what your requirements. It is a bit more work, you have to calculate desired values, and find the closest ones from the desired values, then solder them onto the PCB. This is alternative ways to further reduce tolerances of precision resistors. 

    Best,

    Raymond

  • 0 in reply to kai klaas69
    Prodigy 40 points

    Hi Kai,

    Good suggestion, I will follow it.

    Thanks a lot!

    Regards,

    Marcio

  • 0 in reply to Raymond Zhang1
    Prodigy 40 points

    Hello Raymond,

    Agree, trimpot can compromise stability. But the board will be used internally on load cells, and some clients need to perform a final adjustment of the output after they install it.

    For high precision load cells, we normally use a DAC to generate the 4-20mA. We precisely adjust the output via software.

    Thanks a lot for your suggestion.

     

    Regards,

    Marcio

  • 0 in reply to MARCIO CABRINI
    Guru 140200 points

    You are welcome :-)

    Good luck!

    Kai