I'm doing a course in which I need to connect a wheatstone bridge (which contains a light sensor) to an op amp.
I'm using the same devices and voltage supply (9V) as the course, still, when I measure the voltage in the nodes I'm supposed to connect to the op amp I obtain a voltage that (as far as I understand) is too high for the op amp, let me explain:
This is the circuit schema:
The op. amp. datasheet (
OPA344PA) is here op. amp. datasheet
From the datasheet,I can see that the
input voltage range (
VCM) must be between -0.3 and -1.8 V, right?
Now, according to this document Input and output limitations of an op. amp.,
(VIN+ + VIN-) / 2.
VIN- are the two nodes of
Vout in the whatstone bridge. One of them is fixed, the other comes from a sensor, so it varies.
I have measured both of them with respect to ground (in the case of the sensor, I have taking the maximum voltage that I can get with the current illumination). These lectures are: 1.651V and 2V.
Applying the formula above, I obtain
1.8075. First of all, that value is positive, when the datasheet states that the input voltage should be negative. Even ignoring that, that value is a bit higher than 1.8V, so I guess it's not safe to connect the current circuit to the op amp (if some light is turned on and the sensor voltage becomes higher, it will get much worse)
My question is: Is the above reasoning right? If so, I guess I should use smaller resistors for
R4, so the voltage drop is greater through
R3. Is this correct?