3.2 Wheatstone Bridge (contd.)
The figure shows a Wheatstone bridge with resistor
(R2) replaced by a strain gage variable resistance Rg +
DR. Rg is the resistance of the gage at zero
strain, DR is
the additional resistance due to strain given by the expression
DR/Rg =
Fe
where e
is the strain and the constant of proportionality F is called the
gage factor. Using the above analysis the relationship between vout
and DR is
vout/vs = R3 / ( R3 + Rg +
DR ) -
R4 / ( R4 + R1 )
The strain can therefore be measured as a voltage at the output.
The sensitivity of the circuit is defined as
d(vout)/d(DR) =
-R3 / ( R3 + Rg + DR )2
As you can see this only depends on R3.