Correct! Weiht of LSBit of the 10-bit ADC of UNOR3 = 5000 mV/1023 ~= 0.004 V.
If Vref voltae is taken as 1.1 V, then the resolution is: 1.1/1023 = 1 mV.
With a 5V reference the step size is 5V/1024 = 4.88mV
If you use an external reference of 4.096V, the resolution will be exactly 4mV. For your 0-4V input signal you will be using close to the maximum range of the ADC.
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Is it possible to apply an external reference of 1.024 V to achieve a resolution of 1 mV?
Using the 5V reference, as you mentioned, leads to step size 5V/1024 = 4.88 mV.
For the operating range I require, this will output a range of 0 mV to 40mV.
If not amplified, 40 mV/4.88 mV/step = 8.2 ADC steps for that range.
As already mentioned, save yourself a lot of trouble, and get vastly increased resolution by using the HX711 instead.
That is very close to the Vref min of 1V, so you need to make it is accurate and stable.
I had a long desire to know the minimum allowable value of Vref for the ADC. Now I understand that it can be as low as 1 V. Is there any reference to this in the datasheet or elsewhere?
DS40002061B-page 319
+1 for the reference datasheet-2020.
Pressure sensor with external instrumantation amp.
HX710 on that board (little sister of the HX711)
A mashup of AD8221 and Arduino A/D is a dumb idea.
Leo..
I am considering using an ADS1115 module.
From what I have read it seems like that will bypass many of my issues as I won't have to rely on an amplifier and can simply just communicate my reading to the Arduino.
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Wrong choice again.
FWIW I've been using some NovaSensor I2C pressure sensors that work very nicely and avoid the whole analog interfacing issues. The ones I'm using are 14 bit 30psi units bit I think they also have some in your desired range.
Only problem is that they have a fixed address so you can only have one on the bus.
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I looked into your previous suggestion of the HX711 and it seems like it is not at all meant for my type of sensor - for load cells with 4 inputs? Also, the pressure sensor you suggested does not work with the pressure ranges I will be using. It would be great if instead of simply stating I made the wrong choice, maybe explain why it wouldn't work.
I have just found a video on youtube of someone using the ADS1115 module with the exact sensor I have.
You forgot to tell us which pressure sensor you have.
A gas/liquid sensor that requires an instrumentation amp usually has four wires,
the four wires of the internal Wheatstone bridge. Two signal wires and two power wires.
Leo..
I have an MPX2200GP sensor. It has 4 pins: Vs, GND, +Vout, -Vout. It also requires a source voltage of 10V.
That's an older (bare/basic) series of sensors.
Newer versions (higher first number, like the MPX4200 (obsolete)) have built-in instrumentation amps and some even have built-in A/D converters (with I2C output).
You don't have to run this sensor on 10volt.
It's just a Wheatstone bridge. It just needs more amplification on a lower excitation voltage.
The regulated 4.3volt that a HX711 provides is just fine, and no doubt the best solution for that sensor.
Leo..
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It’s not clear from the datasheet but for that sensor the output voltage is actually offset by half of the supply voltage. If you supply 10V with zero pressure applied and measure the voltage on the + and – output pins with respect to ground, they will measure about 5V.
It is the difference between the + and – outputs that have the range of 0 to 40mV. So you may see the – pin at 5V and the + pin going from 5V to 5.040V, the difference being 5.040V - 5V = 0.04V
It seems like the offset is very small (as far as I can tell), and in the operating characteristics table (for Vs = 10V, 25°C) this offset ranges from -1mV to +1mV
I have already purchased this pressure sensor, and as a mechanical engineer I am frankly a bit overwhelmed with the available options.
I will give the HX711 a shot and update next week how it goes. Thank you for the continued help.
I disagree but if that is indeed true then it will not work with an HX711 because it outside its common mode voltage range.