Another 4-20mA to 0.6-3.0V conversion problem

[mpathma]

4 20ma conversion

Hi, Guy,

I need to design an input circuit to convert the 4-20mA to 0.6 to 3.0 Volt.

I have constructed a circuit using IN326, I set the gain to 150.

The current loop 4-20mA will be feed to 75+1ohm resistor, the voltage drop across the 1ohm will be feed to IN326. The 75ohm is connected to have 75ohm input impedance for the 4-20mA current loop.

The circuit works if the IN326 is powered from Battery and the 4-20mA is source from portable current injector (which is also battery powered)

MY PROBLEM;
1. the moment I connect the scope to the ground pin of IN326, the output started to fluctuate.

2. if I the input from a bench-top current injector, the problem happened.

I think I have a problem with common mode. But do not know how to resolve it.

My Question;
1. Should I used the IN326. I wanted to protect my ADC chip from input, so, the OP was used.

2. is there any other PROVEN circuit that can do the task needed

I need to used this into industrial environment


PLEASE HELP ME. I HAVE BURN MANY CANDLES.

 

[biff44]

how to scope 4-20ma

what is an IN326? Do you mean LM326?

 

[mpathma]

4-20ma input circuit

Hi,

Sorry, its should been INA326.

here is the details https://focus.ti.com/docs/prod/folders/print/ina326.html

Basically it's a Instrument Amplifier with Single Supply and Rail to Rail.

 

[biff44]

4 20 ma current schematic

I do not see any reason that it can not work. Are you using a schematic as shown in figure 8? I would try that one, but with the power supply supplied thru the dotted line, and the connection between Rs and Pin 7 broken. I would use a couple capacitors on pin 7 (.1 uF and maybe 4.7 uF to start). Sometimes the wire length to reach a bench power supply has enough inductance to make an op amp oscillate.

In that schematic Rs would be 1 ohm, and Rl would be 75.

Rich