rprodrigues
Member
Hi all,
I'm working in a project where I have do read from one 4-20mA output sensor (LU-20 - Flowline LU20 EchoTouch™ Two-Wire Ultrasonic Level Transmitter) by the MCP3208 ADC (using PIC18F2620 and others stuffs also).
I built the signal conditioning circuit by means of LM358 OPAMP with differential inputs (see input.jpg) and one 250 Ohm resistor (to get 0-5V from 4-20mA).
During some tests I applied 0-5V through the 250 Ohm circuit input resistor and everything worked fine and the ADC outputs were as they were supposed to be, but when I put the circuit input (its 250 Ohm resistor) in the 4-20mA sensor closed loop, I noticed that the ADC outputs began to fluctuate around the values they should be.
Even using a single power supply (sensor and circuit in the same supply with same ground for both) or using two power supplies (one for the sensor and another one for the circuit), the ADC output fluctuation still exist.
I have already spent a good amount of time trying to solve this problem with no success.
Does anyone have any idea?
Any help would be appreciated.
Thank you,
Rodrigo
I'm working in a project where I have do read from one 4-20mA output sensor (LU-20 - Flowline LU20 EchoTouch™ Two-Wire Ultrasonic Level Transmitter) by the MCP3208 ADC (using PIC18F2620 and others stuffs also).
I built the signal conditioning circuit by means of LM358 OPAMP with differential inputs (see input.jpg) and one 250 Ohm resistor (to get 0-5V from 4-20mA).
During some tests I applied 0-5V through the 250 Ohm circuit input resistor and everything worked fine and the ADC outputs were as they were supposed to be, but when I put the circuit input (its 250 Ohm resistor) in the 4-20mA sensor closed loop, I noticed that the ADC outputs began to fluctuate around the values they should be.
Even using a single power supply (sensor and circuit in the same supply with same ground for both) or using two power supplies (one for the sensor and another one for the circuit), the ADC output fluctuation still exist.
I have already spent a good amount of time trying to solve this problem with no success.
Does anyone have any idea?
Any help would be appreciated.
Thank you,
Rodrigo
Attachments
Last edited: