I have a problem making a circuit like this working
I got it from this TI application note. For testing purposes I have set:
\$R4=R2=R3=R1=47 \Omega\$
\$ Rf = 10 k\Omega\$
\$Rg= 26.70\Omega\$
The op amp \$V_S=+5V\$. This should give a gain of 750 having \$V_{SIG}\$ as 0.8E-4V/kg.
I agreed that 80kg would be enough for full scale. The problem is that this setup doesn't show any variations when i load the strain gauge. It just stays at 2.40V which doesn't make much sense since it doesn't reach near \$V_S\$. I am using the TL064. I have tried a lot and think my wiring is correct. Do you have any suggestions?
Update:
I came up with the 750 of gain by assuming that:
\$R4=R2 <=> \dfrac{R4}{R2}=1\$
\$Gain=\dfrac{2Rf}{Rg}+1\$
\$(Sig_+)-(Sig_-) = V_{IN} => V_O=V_{IN} \times 1 \times Gain <=>\$
\$Gain = \dfrac{V_O}{V_{IN}} <=> \dfrac{2Rf}{Rg}+1 = \dfrac{V_O}{V_{IN}}.\$
\$V_{IN}/kg = 0.8E-4V/kg\$
I also i assumed that my absolute maximum load would be 80kg. So
\$V_{IN(MAX)} = 0.8E-4 \times 80 = 6.4E-3\$ V
So we can determine the needed gain for
\$V_{OUT}=5V. G=\dfrac{V_{OUT}}{V_{IN}}=\dfrac{4.8}{6.4E-3}=750\$
Taking the previous equations:
\$ \dfrac{2Rf}{Rg}+1 = 750 <=> 2Rf = 749 Rg <=> Rg = \dfrac{Rf}{374.5}\$
Knowing this and the resistors I had available I chose to set Rg as a function of Rf because i have to arrange the resistors(series or parallels to approximately get the desired gain) I chose:
Edit:
Corrected Equation mistake but still no results
\$Rf=10 k\Omega => Rg = 10E3/374.5 = 26.70 \Omega\$
Considering I don't have this precise value I came up with
\$Rg = 47 || 47 = 23.5\$ which is close to \$26.70 \Omega\$
Hope everything is clear for everybody, otherwise please ask for clarifications
Thank you
Best Answer
You've made an error in the amplification.
With the values you're using this gives
If you want a \$G\$ of 750 you'll have to make \$R_G\$ = 26.7\$\Omega\$, so that