Amplifying Load Cell Output - What Am I Doing Wrong?

  • Thread starter corona7w
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In summary, the user is experiencing issues with amplifying the signal from a load cell using a non-inverting amplifier. They are advised to use a differential amplifier instead, and to check the values of their DC power supplies to ensure they are not limiting the output voltage. Additionally, they should make sure they are using the appropriate type of amplifier for their specific type of load cell and to calculate the expected change in voltage from the applied force. Finally, they are reminded to be cautious of reaching the maximum output voltage for the amplifier.
  • #1
corona7w
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So I have a load cell that outputs a + and - signal in the range of 0 to 30 mV depending on the force applied. I fed the +signal through a non-inverting amplifier that bumps up its value to about 4 V. However, when I applied force onto the load cell, the voltage range did not get amplified. The signal now varies from 4 V to ~4.03 V. Can anyone tell me what I am doing wrong here?
 
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  • #2
I've never used a load cell but I'm guessing that you probably need a differential amplifier rather than a simple non-inverting amplifier.
 
  • #3
What are the values for the DC power supplies you are using for the op amp? Because it could be the supplies are limiting your output voltage, depending on the output range of the supplies. If the supplies for your op amp are only capable of sourcing 4 V, you can't output higher than 4 V with your op amp. This is a saturation effect where the op amp stops behaving linearly and doesn't continue to amplify the signal beyond the DC supply levels.
 
  • #4
What type of load cell is it? For example is it of the strain gauge Wheatstone bridge type, the piezoelectric type, or some other type? You should use a differential amplifier (such as an instrumentation amplifier) for the strain gauge wheatstone bridge type. Let me know if you want me to explain the the problem with using a non-inverting amplifier with a Wheatstone bridge.

I'm less experienced with the other types, but an instrumentation amplifier should be appropriate for them as well.

What are the forces that correspond to the range of 0 to 30 mV output? How much force did you apply? What is the gain of your amplifier?

The manufacturer/provider of the load cell likely provided information on how to convert voltage into force. For the strain gauge type this information may be at a specific voltage or current powering the bridge; if this is the case, make sure that this voltage or current is correctly applied to the load cell.

It makes sense to calculate the expected change in voltage from the applied force using the provided infomation and the amplifier gain. You may also want to make sure the offset (when no force is applied) is within reasonable bounds, based the manufacturer specified offset for the cell and the amplifier gain and offset. It may not be a bad idea to test the amplifier with a known signal.

And, as artikk mentioned, make sure you do not get output voltage close the the maximum output voltage for the amplifier, which typically depends on the power supply voltages.
 
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1. How does an op-amp work?

An op-amp, short for operational amplifier, is a type of electronic circuit component that amplifies an input signal to a higher output signal. It typically consists of two inputs, an inverting and non-inverting input, and a single output. The output is the difference between the two inputs, multiplied by a very high gain. This gain allows the op-amp to amplify the input signal significantly.

2. What are the basic components needed to amplify with an op-amp?

The basic components needed to amplify with an op-amp are an op-amp IC, a power supply, input and output resistors, and a feedback resistor. The power supply provides the necessary voltage for the op-amp to operate, while the resistors are used to control the gain and the input/output signals. The feedback resistor is connected between the output and the inverting input, and it helps to stabilize the output signal.

3. How do I choose the right op-amp for my application?

Choosing the right op-amp for your application depends on several factors, such as the required gain, bandwidth, input/output voltage range, and power supply voltage. It is essential to carefully consider these specifications and select an op-amp that meets your requirements. Additionally, you should also consider the cost, availability, and reliability of the op-amp.

4. What is the difference between inverting and non-inverting amplifiers?

In an inverting amplifier, the input signal is connected to the inverting input of the op-amp, and the output signal is the inverted version of the input signal. In a non-inverting amplifier, the input signal is connected to the non-inverting input of the op-amp, and the output signal is amplified but not inverted. The choice between the two types depends on the desired output signal and the input signal source.

5. How do I ensure stability when amplifying with an op-amp?

To ensure stability when amplifying with an op-amp, you should carefully select the feedback resistor and the input and output resistors. The values of these resistors determine the gain of the amplifier and can affect the stability of the op-amp. Additionally, you can also use decoupling capacitors and bypass capacitors to reduce noise and improve stability. It is also crucial to follow the recommended layout guidelines provided by the op-amp manufacturer.

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