Initial disclaimer - I am still getting my feet wet with arduino programming and circuit design.
I am working on putting together a pressure monitoring system based on this publication:
I have assembled the circuit and been able to successfully get pressure readings through a simple arduino sketch, though it seems that I am not able to read negative pressures (it will just stop at zero when a vacuum is pulled on the sensor).
I believe this is because I am not supplying true negative voltage to the V- pin of the INA122P amplifier, but rather connecting it to the common ground rail.
Do the capacitors in the attached circuit diagram (from the publication) actually serve to create a negative voltage to supply the V- pin on the INA122P? I tried putting them in the circuit (and maybe I just didn't do it correctly) and it didn't seem to behave any differently than when they were omitted. If they are not for generating a negative voltage, what are they there for?
If you want and need a negative voltage for the IA, the easiest thing to do is to buy a dual output +12/-12 power supply. There are chips you can buy and build a negative voltage generator, but why buy a WHOLE bunch of parts when you can just but it all done for you.
The single supply circuit is fine as a strain guage outputs are mid-rail. The ref input to the INA sets the reference voltage for the output to the analog pin.
If you want to read both positive and negative offsets on the inputs, connect to ref pin to a 1:1 potential divider
betweej Vcc and gnd.
The reason you might want a negative supply is if the incoming signal was AC centred about gnd.
And the opposite of Vcc is Vee. Just as Vdd is the opposite of Vss. They are names, not voltage values, so a sign is meaningless. (e for emitter, c for collector, s for source, d for drain)
But both are wrong as CMOS is symmetric anyway. V+ and GND will do just fine.
If you apply a vacuum, the pressure becomes near zero. In space it's as close as absolute zero pressure you can practically get.
If you have a differential pressure sensor, where the measured pressure is referenced against another (e.g. ambient), the measured pressure may be a negative number indicating it's lower than the reference.
A quick skim through the unlinked article didn't show me a schematic. If there is one, do post it here, together with all other relevant information.
Forget the INA setup, and use a HX711 breakout board.
Assuming the 'pressure sensor' will work on the ~4.3volt excitation the HX711 provides.
The digital-out HX711, despite of the low excitation voltage, will have a much higher resolution than that Arduino A/D setup.
No 12volt or dual rail supply needed either.
Leo..
If you apply a vacuum, the pressure becomes near zero. In space it's as close as absolute zero pressure you can practically get.
If you have a differential pressure sensor, where the measured pressure is referenced against another (e.g. ambient), the measured pressure may be a negative number indicating it's lower than the reference.
This is correct - I am working with a differential pressure sensor:
adwsystems:
I also suggest you would need a dual supply IA. The INA122 is single supply.
opamps are never "single supply" or "dual supply"(*), they are all capable of both, what "single supply"
actually means is that the inputs and output work all the way down to the -ve supply, so that signal ground can
be at the same voltage as the -ve supply, so that a virtual ground doesn't have to be generated if used single-supply.
(*) Actually some have a ground input, but that's used as a reference for the enable pin, not anything
to do with the amplifier function.
