Dear all,
I need an analogue digital converter of 22-bits for my project, and I was wondering how it would be used in an arduino,
Installation, board compatibility, etc.....
Any advice would be highly recommended.
Thank you..
Be aware you need a very stable power supply
if you google for 22 bit ADC you get many hits including - Sample code for 22-bit ADC: Microchip MCP3550 - Sensors - Arduino Forum -
Can you please explain what do you mean by a very stable power supply ?
Thank you !
If you only need one channel and can live with 80Hz sample rate, why not just use an HX711 24-bit ADC? They're less than US$5 and there's lots of information on the web and this forum about using them with the Arduino.
Qutadah:
Can you please explain what do you mean by a very stable power supply ?Thank you !
ADC's tend to be affected by their power supply on which the reference voltages are based.
If the voltage drops the readings will vary, and the more bits an ADC has the more it will drop.
What kind of signal do you want to measure as 22 bits is quite a bit..
Well, I want to measure a sensor signal, it is in the range of a 6-11 mV, and the readings are just too close. Its a load cell, wheatstone bridge configuration.
Are the reading stable ? Or will they flunctuate !? If the flunctuation exceeds the readings difference between two slots, say 500-501, that would produce erroneous results to the signal reading.
Thanks,
Hi,
What about amplifing the signal?
Regards
vffgaston:
What about amplifing the signal?
+1
A HX711 breakout board is made for load cells.
Voltage reference and 128x gain.
Leo..
Well I cant amplify the signal, it would give only part of the range I need to measure.
The difference between each slot allocated after amplification to the 0-5 Volts range will be around 4.8810-3 mV which is around 9 times my desired difference between slots. (Desired= 0.55944410-3 mV).
So Amplification will not work in my case.
But thanks, 
Hi,
Can you be a little more explicit?; i.e.: a graph Weigth - Volts (mW) will help (hand made does).
Regards.
Qutadah:
Well I cant amplify the signal, it would give only part of the range I need to measure.The difference between each slot allocated after amplification to the 0-5 Volts range will be around 4.8810-3 mV which is around 9 times my desired difference between slots. (Desired= 0.55944410-3 mV).
So Amplification will not work in my case.
But thanks,
Practically, e.g. with an INA125, you have to be happy with 9-bits with Arduino's A/D (~500 values).
A HX711 (inbuild 24-bit A/D converter) will practically be ~13-bits (~5000 values).
Maybe one bit more, depending on your building/powering/shielding/grounding skills.
Leo..
vffgaston:
Hi,Can you be a little more explicit?; i.e.: a graph Weigth - Volts (mW) will help (hand made does).
Regards.
Well to be honest, the graph is unclear.
I am using a Platform Weight Scale intended to measure up to 1500 Kgs.
The problem is that the load cells sensitivity is 3.0028 mV/kg (manufacturer), there are four sensors on each side of the platform, and they are all connected to the circuit. but measured from the signal output a 1 Kg difference induces a voltage difference of merely (0.06-0.065) mV/ Kg !!
This is what is confusing me...
Do you think there is something missing, perhaps. Why exactly is there a variation between measured output signal, and the manufacturers specification for the load cell used.
Thank you !
Could you post a link to the load cell.
Are they 4-wire.
How are they wired.
Leo..
Wawa:
Could you post a link to the load cell.
Are they 4-wire.
How are they wired.
Leo..
Well the specifications of the VMC-A100 load cell used is as follows:
Sensitivity: 3.0028mV/V
Capacity: 4 Klb
the excitation voltage measured by a voltmeter is 9.67 V
The images attached are a layout and soldering of the circuit.
Thanks !
What are the potentiometers for? Calibration?
The HX711 boards I know work with an excitation of 4.3volt.
That drops sensitivity of the load cells to 43% of the datasheet (10volt specs).
That should not be a problem.
I don't know if the excitation voltage of a HX711 board is able to supply four load cells.
That voltage must be measured when four cells are connected.
You must use the excitation voltage of the HX711 board to stay within the dynamic range of the HX711.
9.67volt won't work for a chip that runs on a 5volt supply.
Chips like the INA125 can work with 10volt.
Problem is that they need a bipolar supply and you still need an external A/D.
Leo..
Johnny010:
What are the potentiometers for? Calibration?
Seems they control the sensitivity of the individual load cells by finetuning the excitation voltages.
Leo..
Wawa:
You must use the excitation voltage of the HX711 board to stay within the dynamic range of the HX711.
9.67volt won't work for a chip that runs on a 5volt supply.Chips like the INA125 can work with 10volt.
Problem is that they need a bipolar supply and you still need an external A/D.
Leo..
I dont understand the part where the 9.67 volts will not work for a chip that runs on 5Volts supply, Because we are only concerned with the signal output, and it is only few millivolts....
If i use a 22 ADC how do i connect it with the arduino so that it will be function properly ? and what 22 ADC do you recommend?
The load cell I am using has minimum excitation voltage of 10 Volts.
Qutadah:
I dont understand the part where the 9.67 volts will not work for a chip that runs on 5Volts supply, Because we are only concerned with the signal output, and it is only few millivolts....
ADCs have limits on the common mode voltage, and the maximum common mode voltage is usually less than the ADC supply voltage. For example, the maximum common mode voltage for the 24-bit HX711 is the supply voltage minus 1.3 volts, or 3.7 volts, if powered with 5 volts. So if you used 10 volts excitation, the common mode voltage would be 5 volts, which is no good because it exceeds the maximum of 3.7 volts. Suggested google search terms: common mode voltage load cell
ADCs also have limits on the differential input range. If you find an ADC that can handle 5v common mode and use 9.67 volts excitation, then with a load cell with 3 mV/V sensitivity, your max input would be 29 mV. That has to be within the ADC's differential input capabilities. Suggest looking at an ADC datasheet. See the HX711, for example, whose data sheet is about a simple as they come. Or, check out the sheet for the AD7780 for lots of good self-education reading.
Qutadah:
If i use a 22 ADC how do i connect it with the arduino so that it will be function properly ? and what 22 ADC do you recommend?
Why are you stuck on 22 bits? Why not 24? And what sample rate do you need? As previously mentioned, if you can live with 80 Hz or 10 Hz sample rate, then the 24 bit HX711 could be a good choice, because there is a LOT of information on the web and this forum about connecting it to the Arduino so that it will function properly.
Qutadah:
The load cell I am using has minimum excitation voltage of 10 Volts.
I suspect that you could use an excitation voltage of 5 volts, or less, and it would work fine; the load cell's output differential voltage would be smaller, but still usable (and within the max input range of the HX711). Seems like it would be extremely simple to test. Why don't you just buy a HX711 and give it a go? As previously mentioned, they don't cost very much.