Strain Gauge Integration circuit

Thread Starter

AtomicFlea69

Joined Mar 23, 2022
17
Hello forum, for the past few days I have been trying to construct a circuit that differentiates, amplifies, and triggers an output given by a Wheatstone bridge with a strain gauge. I am using an LM358 op-amp for all three processes. Can you guys suggest a circuit that can do the following? Thank you in advance.StrainGaugeProcess.jpeg
 

ericgibbs

Joined Jan 29, 2010
15,342
hi,
The AD623 is a low cost Inst Amp IC.
You can construct an IA amp using regular OPA's [3 off] but you have to use high precision resistors for the best results.

The LM358 can be used to make a low specification IA amp.
E

Post details of your Bridge circuit.
 

Thread Starter

AtomicFlea69

Joined Mar 23, 2022
17
So my bridge circuit is basically a strain gauge whose resistance is 350 ohms ideally. I use a 2k ohm resistor across the strain gauge and its mirror circuit is two 1k resistors in series. I have attached the image below

1652456429035.png
 

Irving

Joined Jan 30, 2016
2,531
Do you have any idea what the actual strain-related output is going to be? You need an instrumentation amp with a very low offset voltage, uV rather than mV. Building this from discrete parts is going to be frustrating...
 

Thread Starter

AtomicFlea69

Joined Mar 23, 2022
17
Do you have any idea what the actual strain-related output is going to be? You need an instrumentation amp with a very low offset voltage, uV rather than mV. Building this from discrete parts is going to be frustrating...
This is for my project where I basically replace the functionality of the hx711 module and build a circuit on my own!
 

Irving

Joined Jan 30, 2016
2,531
Yes, you can build something with LM358s but I don't see the point except as an exercise in demonstrating why general purpose op-amps just don't cut it in such circuits. With an input offset of 2mV typ, 4-7mV max (depending on which datasheet you look at, and for comparison HX711 is ~20uV) the error at 0.2% strain on a typical strain gauge with a gauge factor of 2 will be significant as will its temperature stability. Input noise, without any input bandwidth limitation is around 10uV rms per stage, so at the comparator input U3 you have almost 4mV of noise, coupled with its 2mV+ input offset, means your likely repeatability of switching point is going to be poor.

If you really must use LM358s at least use the poor man's IA configuration shown in the Diodes Inc datasheet, preferably with R1 = o/c for a stage gain of 1 to minimise noise generation and all resistors 0.1% or better to get best CMRR. A single chip IA will be matched to 0.01% or better

1652530625729.png
 

Reloadron

Joined Jan 15, 2015
6,516
Something on a side note to consider. When building your bridge is you want a very stable excitation voltage applied to the bridge. Also, per Eric's circuit in post #13 you want the bridge resistors (350 Ohm) to be 1% precision resistors.

Ron
 
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