[SOLVED] Strain gauge and instrumental amplifier

[igru25]

Hello!

I am beginner in electronics and i try to make instrumental amplifier for my strain gauge.

I have already made following circuit based on LM358, but i am not sure if it is OK.


I apply multimeter between Vref (LM336) and output and see voltage jumping up and down - from 4mV to 1.2V even if no force to strain gauge applied.

My circuit is a compilation of everything i could find in internet for single supply.

Can someone tell me if it looks ok or not? Or how could i debug circuit having old 20MHz oscilloscope?

Some data:
Strain gauge - 120 Ohm. I want to analyze small finger press force (measured bridge output voltage from 0mV to 1mV).

Rgain = 200Ohm (Gain = 400).


Thanks in advance.

 

[Kral]

Igru25,
You have implemented the classic 3 Op Amp instrementation amplifier. It is very difficult to obtain accurate resistor ratios with this circuit using discrete resistors. Your problem may be that ripple on the 5V excitation to the strain gauge bridge appears as a common mode voltage in the input amplifiers. Because of mismatched resistors, this common mode voltage may be converted to a differential voltage (due to the resistor mismatch) and amplified with the gain of 400. You are much better off using an IC instrumentation amplifier such as the Analog Devices AD624.
.

 

[igru25]

Your problem may be that ripple on the 5V excitation to the strain gauge bridge appears as a common mode voltage in the input amplifiers. Because of mismatched resistors, this common mode voltage may be converted to a differential voltage (due to the resistor mismatch) and amplified with the gain of 400. You are much better off using an IC instrumentation amplifier such as the Analog Devices AD624.
.

I have used 0.1% tolerance resistors.

Question - it is possible to avoid possible excitation ripple using battery supply (such as Lithium coin cells CR3032)?


I will play a little bit with circuit... and if it won't work - i will use InAmp.

 

[keith1200rs]

You cannot use the LM358 to make a decent instrumentation amplifier. The offset is far too high. I am sure you would have bias current /bias current offset / offset drift / bias current drift problems as well. You could do what you want using high quality opamps but it would be easier just to buy an instrumentation amplifier.

Keith