Hi
I would like to know why my real time measurements & calculation don`t match up?
I have a load & I amplify the difference between 2 wires with an op amp & 1Mohm resistor. Load cell has 4 resistors. You can see the values in the image below.
In real time I measure Vout as 1.6V but with calculations I get very strange results. (see the image please). Why ?
If I use multimeter to measure voltage between Green & white wire I get 2mV
The resistances in your strain gauge circuit don't add up. The resistances between red & white and red & green wires are 841R so R1 & R3 = 841. The resistances between white & black and green & black are 750 so R2 & R4 = 750R.
The resistance between red and black should be equal to (R1 + R2) and (R3 + R4) parallel but it isn't.
R1 + R2 = 841 + 750 = 1591
R3 + R4 = 1591
Since both the resistances are the same, we get half the resistance when they're connected in parallel (easier than doing product over sum):
R (between the red & black wires) = 1591/2 = 795.5
But you measured 1091 which can't possibly be right.
Did you measure them with anything else in the circuit?
It is possible to use the resistance of the strain gauge as part of the differential amplifier circuit but R1 & R2 become the equivalent of the top and bottom resistances in parallel and it's important the bridge is well balanced. I would use R1 & R2 in the differential amplifier circuit and add the strain gauge resistance on to them, when performing the calculation. I'll post an example later.
https://datasheets.maximintegrated.com/en/ds/MAX4238-MAX4239.pdfSee attached. The circuit has a gain of 100. The high and low side resistors in parallel equal to 500R which is added to the values of R1 & R2 to make 2k7.