Fluctuating ADC Reading for 4-20mA Pressure transducer input

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bhargav shankhalpara

Joined May 9, 2010
19
Hello...

I am working on one project of water pressure measurement, in which i used pressure transducer which gives me output between 4-20mA Current form.

I am using one 250 ohm resistance to convert current into voltage, and than i give this voltage to ADC of arduino UNO channel 0 (A0).

ADC reading is continuously fluctuating up to 10 decimal numbers.

I have also check by connecting multi-meter and measure sensor current reading, for particular pressure. but current reading is stable (even not fluctuate 1 or 2 points.)

also i have checked voltage value after 250 Ohms +Ve and ground. converted voltage is also showing very stable.

but why ADC reading is fluctuating..?

i am using arduino uno, i doesn't have any changes in AREF, AVCC etc...

please suggest effective solution,

Thank in advance.
 

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qrb14143

Joined Mar 6, 2017
112
I'm not very clued up on Arduino but it may help others here if you post the code you use to read the ADC.
In my experience with other ADCs it is normal for the Least Significant Bits (LSBs) to fluctuate depending on the input signal. If your Arduino converts the binary output of the ADC into a decimal value, then if the 3 LSBs are fluctuating this would cause a change of up to 15 in decimal form.
This may give you something to go on until more skilled Arduino gurus come along :)
 

danadak

Joined Mar 10, 2018
4,057
Averaging helps a lot in that problem to enhance sinad.

See attached ap notes.

Use scope on infinite persistence and look at signal and supply rails for noise.

Lastly not all caps are equal performers in bypass applications -



Regards, Dana.
 

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dendad

Joined Feb 20, 2016
3,602
The Arduino is using its Vcc as the reference for the ADC.
A better voltage reference than the power is one way to go. Circuit layout can cause trouble too.
Make sure the only current flowing in the analog in circuitry is the 4-20mA signal.
Do you have any R/C filtering on the analog input?
Can you post a circuit and picture of your Arduino setup?
 

Kermit2

Joined Feb 5, 2010
4,162
I had a customer sending 4-20 signal from a yokogawa pressure transducer to a national instruments DA converter. It was doing what you describe. Fluctuations in the signal prevented a steady pressure reading on the NI equipment.
I rewired the setup with shielded cable and a good earth ground.
 

dendad

Joined Feb 20, 2016
3,602
Also, a lot depends on where the cable is run. We had an installation where cables were laid in the same duct as those between a VFD and the motor. Lots of problems with induction until the cables were rerouted away from the noise.
 
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