I have a pressure sensor (MSP40 - GDF) that i want to use in my project. The pressure sensor outputs a max voltage of 75mV. I know I need to boost this to voltage measurable by arduino (Nano). I have choosen LM358 for this task. But Something isn't clear to me:
First Question:
From my understanding of voltage measurement in arduino:
0v = 0
5v = 1023 (integer rep)
if I boost the voltage from my sensor by a factor of 50 then the Max voltage will be 3.75v. This would be 767.25 (max).
I would love to achieve "1023", Do I need to sellect resistors combinations such that I would obtain max 5v in the output of the Op-Amp?
Second Question
LM358 is an IC with two Op-Amps. How do I configure the two Op-Amp to output 5v max with a max input of 75mA?
Ignore one, choose feedback resistors to give a voltage gain of 5/0.075 around the other. However if you want to get a range of 0 - 5v on the op amp output then the LM358 probably isn't suitable, you would need a "rail to rail" op amp. Google is your friend - find tutorials on opamp circuits and data sheets.
A Wheatstone Bridge needs a Intrumentation Amplifier.
There are a number of ways to measure pressure. You have a pressure sensor with a so called "Wheatstone Bridge". That is why the sensor has two outputs: +Output and -Output. There are two -Outputs, you can tie those together.
A normal circuit with a OpAmp has a certain input impedance. A "Instrumenation Amplifier" has a high input impedance, so it does not disturb the Wheatstone Bridge.
You could also forget this sensor and buy a sensor with a normal output, so you don't have this trouble.
The Arduino Uno has a differential amplifier with gain (and also with a high input impedance). That might work, but it is hard to understand and almost no one uses it.
For a test, you could connect +Output to a analog input and -Output to a other analog input. In the sketch you can calculate the difference.
Thank you all for your quick reply. I am happy with your recommendations. But, Since I can get a 5v/0.075v then what advantage will a rail to rail Op-amp be bringing? (just asking for more understanding). I forgot to mention that the min Output voltage of the sensor is 0.001v (which I would love to appear as 0v on the arduino's input).
What rail to rail op-amp will you recommend for this purpose?
OpAmps typically have a restricted output voltage range, like 1V to 4V from a 5V supply. Only rail-to-rail opamps can output their full supply voltage range.
You can use an MCP6002 with R1=1k and R2=62k which will give you a gain of ABOUT 63;
so 0V in will give vout = 0V approx and
75mV in will give 4.725V out
While you COULD trim the values to get close to 0V - 5V out that would not be a good idea as it allows no "headroom". And even true "rail-rail" op amps can not maintain linearity when being worked exactly to their supply voltage limits.
Post the schematic of the part.
If a single opamp is going to be used in a differential amplifier configuration the resistance of the bridge arms will have to be known to calculate the needed gain resistors.
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Are you sure about that.
The wheatstone bridge of the sensor could output 2.5volt without pressure,
which is out of range for 1.1volt Aref.
OP shouldn't have bought a bare sensor.
These sensors come for a few dollars more on a breakout board that has an instrumentation amp and a 24-bit A/D included. Yep, a poor man always buys twice.
Leo..
A wheatstone bridge, powered by 5volt, has 2.5volt on it's two outputs.
Pressure generates a differential voltage between those two outputs, which is 0-75mV.
Leo..
