Hi,
I'm trying to amply the output voltage from the pressure sensor by a factor of 10.
https://www.nxp.com/files/sensors/doc/data_sheet/MPXV7002.pdf
I'm using a LM324 amplifier.
Should I use an inverting or non-inverting method?
Thanks
Do you want an inverted or a non-inverted output?
Typically with the Arduino you don't want an inverting amplifier....
If you feed a positive voltage into an inverting amplifier, obviously a negative voltage comes-out.
Negative voltages can damage the Arduino, or if the op-amp doesn't have a negative power supply it will saturate as low as it can go (as close as it can get to zero-volts), which means it's no longer operating as a linear amplifier.
You would do better with a rail to rail amp like MCP6002 or equiv., the 324 can only get to about 3.6 volt output on a 5V supply.
And a non-inverting config.
The output of that sensor has a zero offset of 2.5V. Amplify by 10 and you will have 25V.
Please tell us what you REALLY want to do -- that sensor may not work for your application.
Sensors like this have a ratiometric output.
Sensor supply has to be linked to Vref if you want to keep this.
An opamp could upset things.
Maybe wiser to use a 16-bit A/D.
You can also gain some resolution by taking multiple samples, and smoothing/averaging.
Note that these sensors have a ~2.5% accuracy, which you also will be amplifying.
Leo..
jremington:
The output of that sensor has a zero offset of 2.5V. Amplify by 10 and you will have 25V.Please tell us what you REALLY want to do -- that sensor may not work for your application.
That would point to using an offset of 2.5V.
But yes, to the OP, we need to know what you are trying to do.
The inverting circuit has a finite input impedance (that of the input resistor),
the non-invering circuit has a very high input impedance (dependent on the opamp input),
which might affect your decision...
I would suggest using a non-inverting configuration since that will work whatever the
the output impedance of the sensor is (the datasheet seems vauge). It would be referenced
to the mid-rail voltage - for instance two 2k resistors, one to GND, one to +5V, and a 9k feedback
resistor from the op-amp output. the 2k resistors together behave like a 1k resistor from a 2.5V rail,
9k feedback gives you a gain of 10 ((9k+1k) / 1k)
The datasheet mentions "an output source current of 100uA at full scale output"
Sensor output could be connected to one analogue input, and the opamp output to another analogue input.
Auto-ranging 1x or 10x in software.
Leo..
Based on the output source current at full scale of 100 uA, if you put a 25 k ohm resistor in series, you should get 2.5V at full scale.
@Wawa,
Doesn't that make sense ?
A cow is an animal, so... an animal must be a cow.
I think it just means that you have to keep load current under 0.1mA.
Leo..
A cow is an animal, so... an animal must be a cow.
VFSO @Vcc=5Vdc = 4.5Vdc
Why would he need to amplify the output unless the pressure he want to measure is < 1/10th of FS ?
Did you see anywhere where the OP states the pressure range HE wants to measure ?
I'm trying to amply the output voltage from the pressure sensor by a factor of 10.
The sensor range is -2kPa to +2 kPa (-2000 Pa - +2000 Pa)
If he wants to amplify it by a factor of 10 then he must be using the wrong sensor because he is trying to measure -200 Pa to +200 Pa.
Is the cow flying yet ?
In post#5 I suggested a 16-bit A/D.
upto 64x amplification (of noise and inaccuracy).
Arduino cats can fly too.
http://www.hitnow.net/wp-content/uploads/2015/10/Famous-men-created-the-flying-cat-now-plans-to-produce-helicopters-flying-cows-again.jpg
Leo..
Arduino cats can fly too.
That looks like fun... ;D
Hi,
Can you post a link to spec/data of your sensor please?
Thanks.. Tom.. 
Hi all,
I have this sensor inside a face mask trying to measure a persons breathing (detecting regular v irregular breathing). I have it attached to an oscilloscope and the amplitude is not very large. Therefore, I thought amplifying it would be a good idea.
The signal oscillates above and below 0 on the oscilloscope. Negative pressure/positive pressure breathing.
This is the sensor again that I'm using.
https://www.nxp.com/files/sensors/doc/data_sheet/MPXV7002.pdf
So I shouldn't won't use an inverting amp.
Could you suggest any other things that I should do?
Thanks
If you just want to detect breathing, you could amplify the AC component only.
Sensor output to the +input of a rail2rail out opamp.
10k resistor from opamp output to -input.
1k resistor from -input to the + of an electrolytic cap (47uF?), and negative of the cap to ground.
Leo..
Hi Wana,
Thanks for your advice. I only have a LM324 but I'll get a rail2rail out opamp. What is the advantage of one of these?
Also, could you explain the reason for amplifying the AC component only? Why would this work?
Thanks, I'm relativity new to electronics.
The LM324 will work, but the output can't swing the full 5volt on a 5volt supply.
Maybe ~0.1volt to 3.5volt.
That might be enough to reliably detect breathing.
You avoid stability problems by just amplifying the breathing part and not the 2.5volt offset.
With the part values given, breating pressure difference is amplified 11x, while static pressure is not amplified (1x).
Leo..
transcendglue:
I have this sensor inside a face mask trying to measure a persons breathing (detecting regular v irregular breathing). I have it attached to an oscilloscope and the amplitude is not very large. Therefore, I thought amplifying it would be a good idea.
If you're "measuring" pressure, then amplification wouldn't give much if any benefit. Normal breathing already uses a good portion of the sensor's range. Amplification could easily clip the signal and cause greater inaccuracies. This abstract lists differential pressure results: The pressure required to force liquid through breathing system filters. Some readings here (min-max kPa) would use more than the full range of Arduino's ADC.
This shows about 2kPa differential for normal breathing: Teacher’s notes 49 Breathing patterns
From the datasheet, ±1kpa would cause ±1V change in signal which is ±208 ADC value. This 416 ADC span uses 40% of the ADC range.
transcendglue:
The signal oscillates above and below 0 on the oscilloscope. Negative pressure/positive pressure breathing.
Where and how are you taking this measurement? The sensor's output signal should be centered at 2.5V.
If you're just "detecting" differential pressure, then some amplification could provide benefits.