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.

 

hi A69,
Do you have any Instrumentation amplifiers in your bench stock.?
We could then suggest a circuit.

E

 

If you can suggest a circuit with a good in-amp then we can maybe try to procure it.

 

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.

 

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

 

hi atom,
Those bridge resistors will give a very large offset voltage.
Change R2 to 350R 0.01%
E

 

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...

 

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!

 

hi atom,
Those bridge resistors will give a very large offset voltage.
Change R2 to 350R 0.01%
E

Will note that down. Can you suggest to me a circuit according to block diagram?

 

hi atom,
This is a basic Bridge > Differential amp > Amplifier > Comparator.

Amps operate from a dual voltage supply approx +/-5V and the Comp a single supplt.

Uses LM358's

E

 

hi atom,
This is a basic Bridge > Differential amp > Amplifier > Comparator.

Amps operate from a dual voltage supply approx +/-5V and the Comp a single supplt.

Uses LM358's

E
View attachment 267178

Thank you for giving me this circuit @ericgibbs. Will try it out and let you know!

 

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

 

Hi,
Already created a LM358 IA version
But the TS as, asked for a Differential circuit using LM358's
E

 

Hmmm, isn't that still differential, conceptually? At R13 = o/c, G=1 its two unity voltage followers driving a differential amp...

 

hi,
Read post #10. ie: Diff.

Post #13 IA was added to explain post #4.
E

 

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