Use AD8295 for Loadcell Amplifier Board

Hello, I'm making a loadcell amplifier board. Input from -18mVDC to +18mVDC. Output +-2.5V. Input signal frequency from 0 to 14Hz. I have made a design as the attach. The total gain changes from about 140 to 50 according to real condition.

- First, I don't know why the output is very fluctuated and gaining is wrong.

- Second, I intend changing the low-pass filter to pre-amp + filter. The IA will became to post-amp.

Anyone help me what is the wrong in my design and how to solve it.

Thanks,

Anhcao

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  • 0
    •  Analog Employees 
    on Jan 23, 2015 2:27 AM

    Hi Ahncao,

    I'm sorry for the delay. The AD5259 is a 5.5V maximum device. Solid-state switches can't pass signals beyond their supplies. You can see from the data sheet that the Voltage Range at A, B, and W is 0 to Vdd.

    The AD8295 in-amp, on the other hand, has Superbeta NPN transistors at the inputs, meaning that the voltage at the Rg pin is equal to the voltage at the corresponding input minus about 0.5V. If your bridge is perfectly balanced, the common-mode voltage is 6V and your Rg pins each have about 5.5V. If this is beyond the AD5259 supply, fault current will flow and change the output of the AD8295.

    There are a few more common problems with a circuit like this as well. Have a look at this article for more info:

    Use a PGIA to avoid getting burned by switch parasitics

    If you could run your bridge on lower voltage, something like AD8556 or AD8557 programmable bridge sensor amplifiers might make a lot of sense.

    Otherwise, I believe the best solution is to take the minimum gain that you will need with the AD8295 instrumentation amplifier, and implement the PGA with the uncommitted op-amp, taking into account the voltage range and suggestions from MT-088 (specifically figure 28).

    I hope this helps.

    Best regards,

    Scott

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  • 0
    •  Analog Employees 
    on Jan 23, 2015 2:27 AM

    Hi Ahncao,

    I'm sorry for the delay. The AD5259 is a 5.5V maximum device. Solid-state switches can't pass signals beyond their supplies. You can see from the data sheet that the Voltage Range at A, B, and W is 0 to Vdd.

    The AD8295 in-amp, on the other hand, has Superbeta NPN transistors at the inputs, meaning that the voltage at the Rg pin is equal to the voltage at the corresponding input minus about 0.5V. If your bridge is perfectly balanced, the common-mode voltage is 6V and your Rg pins each have about 5.5V. If this is beyond the AD5259 supply, fault current will flow and change the output of the AD8295.

    There are a few more common problems with a circuit like this as well. Have a look at this article for more info:

    Use a PGIA to avoid getting burned by switch parasitics

    If you could run your bridge on lower voltage, something like AD8556 or AD8557 programmable bridge sensor amplifiers might make a lot of sense.

    Otherwise, I believe the best solution is to take the minimum gain that you will need with the AD8295 instrumentation amplifier, and implement the PGA with the uncommitted op-amp, taking into account the voltage range and suggestions from MT-088 (specifically figure 28).

    I hope this helps.

    Best regards,

    Scott

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