Amplifying strain gauges on an arduino

Question

I have 4 standard mild steel foil strain gauges bonded to a steel bar in a wheatstone bridge arrangement. They are implemented into a circuit with the INA 125P amplifier. I only manage to use about a fifth of the actual arduino resolution (200 of 1023), and I am assuming that this is because the signal is amplified to around 1 volt or less, where the arduino supplies 5V to the circuit.

I tried swapping arbitrary resistors to set the gain for the INA 125P amplifier but they either failed to make a difference or completely removed any signal to be read on the PC from the COM.

The question is, what else could I try to do in order to get a better (higher) signal?

Answer 1

Resistor \$R_G\$, between pins 8 and 9 of the INA125, sets the gain of the instrumentation amplifier, which is given by:

$$ G=4+\dfrac{60,000}{R_G}. $$

If your output range is only one volt, and you want 5 V, reduce \$R_G\$ according to that equation, until G is 5 times higher.

If you want the details:

$$ k=\dfrac{G_{new}}{G_{old}} $$$$ R_{Gnew}=\dfrac{15000·R_{Gold}}{15000 k - R_{Gold} + k R_{Gold}} $$

The INA125's output is not rail to rail, so either use a supply > 5 V, to get a 5 V output range, or use a +5 V supply, and get an output range smaller than 5 V. Even if you adjust AREF of the Arduino to match your highest voltage, you will still lose range near the 0 V level.

Answer 2

analogReference(type)

Configures the reference voltage used for analog input (i.e. the value used as the top of the input range).