4 to 20mA reading circuit.

[pedrovdsc]

Hi! 
I'm developing a circuit to read current from a flow meter and his output is the typical 4 to 20mA. I'm thinking use a precision 250 ohm resistor - maybe 4 1k precision resistors in parallel - in series with the current source and read the drop across the resistor with an ADC. The voltage drop will vary between 1 and 5 volts.
Are there any problem doing that? Have a better way to do that?
Thanks!!

[lwatts666]

That's the way its done in the industry. Usually 250 Ohm resistor, giving 1-5V.

There are two gotcha's:

1) There will be a limit to how many 250 ohm devices can be in series (eg. Control System input, Local Display, etc., plus your input). Flow meter spec should define maximum output voltage it can drive, or the maximum resistance. If there is a problem, use a lower value resistor and amplify the signal accordingly.

2) You have to use a differential input ADC, and it must be able to withstand the open circuit voltage of the flow meter.

[retrolefty]

Hi! 
I'm developing a circuit to read current from a flow meter and his output is the typical 4 to 20mA. I'm thinking use a precision 250 ohm resistor - maybe 4 1k precision resistors in parallel - in series with the current source and read the drop across the resistor with an ADC. The voltage drop will vary between 1 and 5 volts.
Are there any problem doing that? Have a better way to do that?
Thanks!!

 That's the way it is mostly done. Just be sure about the loop supply polarity and how you will reference the loop common to your measurement common. Also be sure how you will mediate/protect your input circuitry what happens if your dropping resistor ever is opened (usually due to human brain fart with loop powered up) that will impress full loop voltage to your measurement input pin, usually 12-24vdc.

[Jeroen3]

You can use specialised current shunts amplifiers. You'll need an amp anyway.

If you need isolation, take a look at TI's AMC1200. It will be ideal for this since you'll only need the resistor and an dc/dc brick.
The real challange comes in designing such circuit that it can either do +-10V or +-20mA by selecting a dip switch.