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I'm trying to get a reading from a Digi-Star load cell. I'm using the INA122 instrumentation amplifier, the schematic is attached.

Nothing complicated, I connect the cell output directly to the ina122 inputs, and I use a low gain (10K Rg). Single power supply, 5V, to power the amplifier.

The problem is that at the output I have about the supply voltage.. about 4,8 V.

I used the same schematic as the one in the datasheet.. I don't understand why this behavior.. Any idea? Thanks load cell

EDIT: problem solved! I used the same voltage for the amplifier and the load cell, and I added a potentiometer to control Vref. Thanks to Andy for his advices! enter image description here

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The regulator is the problem - you are exciting the bridge with 12 volts and this means that the typical voltage on the bridge output (to ground) is half this value at 6V.

You are running the chip from 5V and so I suggest you feed the bridge power connection from 5V. The next problem you will have is that the offset (reference) pin of the INA122 is connected to ground and this should ideally be connected to half the power rail of the INA122. This then accommodates a bipolar input range from the bridge. See figure 5 of the data sheet.

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  • \$\begingroup\$ Thank you Andy! The load cell needs 12 V as power supply, so maybe should I use the same one also for the INA122. The datasheet says that the maximum value is 36V, so it should work.. \$\endgroup\$ – user53026 Feb 2 '15 at 11:40
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    \$\begingroup\$ This should also work but check the data sheet on where the ref pin should be tied to for correct bipolar operation. If you only need cone-sided weight measurement then it can be connected to 0V but you may need to swap/reverse the bridge output wires. \$\endgroup\$ – Andy aka Feb 2 '15 at 11:59
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    \$\begingroup\$ A load cell is just a resistor bridge; nothing bad will happen if you give it a lower excitation voltage except that the output differential voltage will be smaller. \$\endgroup\$ – Nick Johnson Feb 2 '15 at 12:23
  • \$\begingroup\$ I'm not completely sure if the cell is only a resistor bridge. The supplier didn't provide the internal schematic... @Andy: why you say "bipolar"? I have single supply.. I'm going to try with 12 V for both the load cell and the amplifier... \$\endgroup\$ – user53026 Feb 2 '15 at 12:49
  • \$\begingroup\$ @user53026 bipolar in this context means works with negative weight as well as positive weight. \$\endgroup\$ – Andy aka Feb 2 '15 at 13:05

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