ADC voltage fluctuating with 4-20mA pressure sensor

Hi

I wonder if someone has some ideas to help?

I have a simple circuit with a Shelly Uni to measure depth of a water tank using a 2 wire 4-20mA pressure sensor.

I cannot get the ADC reading stable although the ADC is stable when measuring other things (VCC itself or a battery source).

The pressure sensor has stable current (to 0.1mA) when measured with a multimeter.

The pressure sensor shows a stable voltage (to 0.01V) across R1 with a multimeter.

The Problem: Voltages measured by the ADC are in the 4-5V range but are bouncing around by 0.5 to 1V. No two readings are the same.

I am using a 12V DC 2A power supply and it remains stable when measured with a multimeter.

My thoughts:

- ground loop? (I can't find it)

- damaged ADC on the Shelly Uni (it is new and has not had any other use).

Does the ADC have any special requirements such as pull-up resistor?

Thanks for your help.

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Thanks for your reply thgoebel.

I don't have a 24V supply but I did just try a 19.5V DC supply with 2A output and the result is the same.

The sensor I have is this one ....

Water Level Sensor, 4-20mA Throw in Type Water Level Sensors, Anti Bending 0 5m Range Depth Input Liquid Level Transmitter, 6m/19.7ft, 316 Stainless Steel, with Waterproof Cable: Automotive: Amazon.com.au

Some of the description says 24V, some 36V but also "9-36V Wide Voltage Power Supply".

I may yet try it with the 36V battery from my e-bike.

I reconfirmed that at the same time that the ADC of the Shelly UNI is fluctuating the multimeter is quite stable.

I wonder if I need to add a low pass filter (resistor with cap to ground) on the input?

I will look into that further and report back if I have any luck.

Still using the 19.5V PSU, I tried the filter. It made zero difference - volts read by ADC are still fluctuating wildly. Filter was a 1k ohm resistor in the line between the pressure sensor minus and a 47nF monolithic capacitor between ADC and Sensor GND.