Strain gauge output amplification using LM358P

Thread Starter

AtomicFlea69

Joined Mar 23, 2022
17
Hello community, I am stuck on something. I am currently working on a project which requires the output of a strain gauge. Since its deflection is very small, I thought of amplifying the output using a LM358P Op-Amp in Non - Inverting mode. I use a setup which is a simple voltage divider circuit as per attached in the image. The output of the circuit is given to the non - inverting setup whose circuit is also attached in the image. Now the voltage is getting amplified, but since the deflection is very small, the amplified output is of no use. Can anyone help me out with the following:
i) Is the above process correct for the amplification of a strain gauge?
ii) if no, can anyone help me through the correct steps and process to be followed.

This is the first post in this forum, so kindly bare with my errors in asking the question and a BIG Thanks in advance!
 

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ericgibbs

Joined Jan 29, 2010
15,547
Hi AF,
Welcome to AAC.
The problem with your circuit, you have a high offset voltage on the input of the OPA.
What is the specification of the gauge. ie: working voltage and mV/V and maximum weight.?
E
 
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crutschow

Joined Mar 14, 2008
29,832
Often the strain gauge is put in 4-resistor bridge circuit to cancel the DC offset.
This can consist of three resistors plus the strain-gauge, each with a resistance equal to the strain-gauge resistance under no load,
You then differentially amplify the voltage across the bridge (a differential instrumentation IC amp generally works best for that).
 
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Ian0

Joined Aug 7, 2020
5,193
Don’t think of amplifying anything with an LM358. Buy an op-amp that isn’t 50 years old and doesn’t have a huge input offset voltage.
Microchip do some nice precision low-voltage op amps, with part numbers like MCP6051, MCP6061, MCP6071 etc.
 

bassbindevil

Joined Jan 23, 2014
422
As far as I could tell, I got away with using TL084 op-amps wired as an instrumentation amplifier for a couple of strain gauge sensor applications. I matched the resistors to the limits of my ohmmeter. But for a more recent strain gauge project, I used an INA instrumentation amp since I didn't need to build it on an existing board that had been laid out for quad op-amps.
 

Thread Starter

AtomicFlea69

Joined Mar 23, 2022
17
Hi AF,
Welcome to AAC.
The problem with your circuit, you have a high offset voltage on the input of the OPA.
What is the specification of the gauge. ie: working voltage and mV/V and maximum weight.?
E
Hello gibbs, I bought the strain gauge from a local vendor, I only know its resistance which is 120 ohms.
 

Thread Starter

AtomicFlea69

Joined Mar 23, 2022
17
Often the strain gauge is put in 4-resistor bridge circuit to cancel the DC offset.
This can consist of three resistors plus the strain-gauge, each with a resistance equal to the strain-gauge resistance under no load,
You then differentially amplify the voltage across the bridge (a differential instrumentation IC amp generally works best for that).
Hello @crutschow, I was struggling with a voltage divider network, now the wheatstone bridge works perfectly. I just require to select a Op-Amp.
 

Thread Starter

AtomicFlea69

Joined Mar 23, 2022
17
Don’t think of amplifying anything with an LM358. Buy an op-amp that isn’t 50 years old and doesn’t have a huge input offset voltage.
Microchip do some nice precision low-voltage op amps, with part numbers like MCP6051, MCP6061, MCP6071 etc.
Hello @Ian0 , I would like to know a website or a software where I can simulate and see the IC's that you have given above.
 

Thread Starter

AtomicFlea69

Joined Mar 23, 2022
17
I forgot to mention, I use a Labjack U6 to get a accurate reading of the changes. The readings I saw where having large deflections when I used the LM358. Can the offset voltage be the cause of these deflections?
 

Thread Starter

AtomicFlea69

Joined Mar 23, 2022
17
As far as I could tell, I got away with using TL084 op-amps wired as an instrumentation amplifier for a couple of strain gauge sensor applications. I matched the resistors to the limits of my ohmmeter. But for a more recent strain gauge project, I used an INA instrumentation amp since I didn't need to build it on an existing board that had been laid out for quad op-amps.
Hello @bassbindevil , can you mention the series of the INA InAmp which you used?
 

crutschow

Joined Mar 14, 2008
29,832
I only know its resistance which is 120 ohms.
So how much resistance change with load do you expect?
Need that to determine the required circuit gain.
I would like to know a website or a software where I can simulate and see the IC's that you have given above.
Many, including myself, use the free LTspice simulator from Analog Devices.
 
Last edited:

Ian0

Joined Aug 7, 2020
5,193
Assuming you don't need much bandwidth - have a look at the Microchip parts MCP60*1 where * can be 2, 5, 6 or 7 depending on how much speed you need.
All are very low offset.
 

Thread Starter

AtomicFlea69

Joined Mar 23, 2022
17
So how much resistance change with load do you expect?
Need that to determine the required circuit gain.
Many, including myself, use the free LTspice simulator from Analog Devices.
So according to the circuit I constructed It gives me a change from 120 ohms to 120.3 - 120.4 ohms change which is good enough for me!
 
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