RashmiMhapsekar
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High Precision AD/DA board ADS1256

Thu Dec 07, 2017 4:19 am

I am using ADS1256 for ADC conversion which is sit on RPi3.
ADS1256 have 24 bit resolution.
I am using the Potentiometer which is on the ADS1256. The potentiometer is connected to channel 0.
I had run the ADS1256 application.
Observed that for fixed input, ADC count is varying for last 12 bit.
For example
Suppose input 4.846V ADC count = 0x7B7F10
The last F10 bits are changing for fixed input.

Could you please help me to understand above statement.

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OutoftheBOTS
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Re: High Precision AD/DA board ADS1256

Thu Dec 07, 2017 10:36 am

24 bit resolution is very fine. R u shore that your input is that stable at that a fine resolution. You will find that voltage just floats around in the air and inducts onto wires in small amount constantly. This is why they normally put a decoupling capacitor on the voltage supply of a chip right next to the chip.

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Burngate
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Re: High Precision AD/DA board ADS1256

Thu Dec 07, 2017 11:46 am

The first 12 bits represent 1 part in 4k; the next hexadecimal digit takes it to 64k.
So if your input range is 2.5V your input could be changing by (of the order of) 40uV
Unless your reference voltage is changing by that much. Are you sure it isn't?

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scruss
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Re: High Precision AD/DA board ADS1256

Thu Dec 07, 2017 4:18 pm

Potentiometers also tend to be really noisy: you can see noise even in the 10-bit ADCs in Arduinos.
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RashmiMhapsekar
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Re: High Precision AD/DA board ADS1256

Fri Dec 08, 2017 6:42 am

OutoftheBOTS wrote:
Thu Dec 07, 2017 10:36 am
24 bit resolution is very fine. R u shore that your input is that stable at that a fine resolution. You will find that voltage just floats around in the air and inducts onto wires in small amount constantly. This is why they normally put a decoupling capacitor on the voltage supply of a chip right next to the chip.
Reference voltage is 5 V which is given by Rpi. But when we measured reference voltage by multimeter it was 5.33V.
You mean to say we need to use decoupling capacitor for reference voltage.
Please confirm.

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flatmax
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Re: High Precision AD/DA board ADS1256

Fri Dec 08, 2017 7:08 am

your setup is like so :


5v =====> pot =====> ADC input ===>
..................||
..................||
.................GND

What people are saying is that the 5v reference will have analog signal NOISE. This noise is measured by your ADC ... from what you say, your noise is showing in the last 8 to 12 bits are showing noise. This means that you will have to use the top 12 to 16 bits to make your measurements.

As mentioned, the potentiometers are typically very noisy ... however it is a nice simple approach you have here.

I recommend trying to reduce noise by using a low pass filter (in software) ... as you have DC on your pot. it will have a lot of noise when changing the position ... however you need DC so that you know the average value of the pot.

My recommendation is that you do the following :
5v ==> pot ==> ADC ==> low pass filter in software ==>
.............||
.............||
..........GND

Another way to do this is to use a capacitor as you suggested in parallel with the ADC input - however you will limit the maximum frequency through the system ... however as you are using a pot - the maximum frequency someone can move it with will be very slow !

Matt
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joan
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Re: High Precision AD/DA board ADS1256

Fri Dec 08, 2017 11:10 am

The Pi's 5V supply is unregulated. Unless you have found a way to make it stable it will be going up and down. You also need to check if the output pin of the ADC is also at 5V - that could harm the Pi.

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davidcoton
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Re: High Precision AD/DA board ADS1256

Fri Dec 08, 2017 2:02 pm

Go back to basics. From the example you quote, full scale input voltage range is about 10V. This represents the differential range +/-5V, with +5V giving 7FFFFF, 0V gives 000000, and -5V gives 800000.
Each step is therefore 10/2^24 = 0.6uV.

You need to get a very stable reference voltage, derived from a stable regulated supply independent of any other load.
So run a 3V3 regulator from the Pi's 5V supply, add a high quality capacitor, then add a precision voltage regulator, one specified to sub-microvolt stability.
The absolute accuracy implied by 24 bits can only be achieved by calibration against a suitable voltage standard.

Noise will be a problem. You will need to decouple all your supplies, again making them independent of other loads. Everything will need to be screened very effectively.

Read the datasheet and the application notes for ideas about how to make it all work.

Last, but not least: What are you measuring that requires 24 bit accuracy? Are you being realistic about what you can achieve? How many bits are actually significant in your application?
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