# 4-20mA Analouge input

Discussion in 'Electronic Design' started by AJ, Feb 18, 2007.

1. ### AJGuest

Hi,

I have been asked to make a 4-20mA input and im not 100% how to go about
this. Im sure there is a standard im meant to be following but im not sure
what it is. Can anyone help point me in the right direction? Im imagining
a sense resistor and an amplifier???

Best regards

AJ

2. ### LeftyGuest

4-20 dc ma is a standard industrial process control interface signal
between field sensors (pressure,temp,flow,etc) and process control
electronics in the control house. This 'loop' is generally powered
from a 24vdc power supply. The advantage, of course, is that there is
no error induced because the length (resistance) of the signal wire
pair that can be hundreds to a few thousands feet long if needed. The
loop voltage also 'powers' the field sensor device. Most input
circuits just wire the 4-20 in series with a 250 ohm resistor to
measure the 1-5vdc voltage drop using a diff input voltage amplifier
to further process or convert it with a A/D convertor. The reason the
4ma = 0% level (live zero) is to have a means of detecting an open
loop condition and of course to have a small amount of power for the
field device to consume independent of the measurement value. Works
very well and has been in use for decades.

Lefty

3. ### JamieGuest

Yes, you supply a 24 volt current source from your input to the
transmitting device.
Normally, you do that via a 24 volt supply through a resistor
where at which point, you measure the return voltage and do the
math./.
just subtract what ever is coming back and use that to do the
basic ohms law of the source resistor you're using.
just put this into a op-amp as a voltage follower.

4. ### JamieGuest

and remember, that in cases where you have multiple devices connected
in series, each device will attempt to maintain 20 ma on the TX in the
loop as an off condition so that the other devices in the loop can lift
the loop to 4 ma so that a change in condition can be seen at the
receiver. This is mostly for SERIAL communications.
Just thought i would through that in!

5. ### LeftyGuest

I'm not sure at all what you are describing but it's not the
industrial standard instrumentation 4-20ma interface. This is strickly
a DC analog measurement standard and not a digital communication
interface. There can be digital communications superimposed by
injecting a small ac fsk signal onto the DC measurement (HART
protocol) but the basic 4-20ma standard is to support one field device
hardwired to a central control point and was in use well before
digital electronics came onto the industrial control world.

Lefty

6. ### Roger HamlettGuest

You need to know a little more about what you are connecting to.
4-20mA, 'does what it says on the tin', delivering a varying current,
dependant on some signal that wants to be transmitted. Now it is common on
some stuff, for the source to provide isolation, and to power the loop.
However on other areas using the same interface, the receiver powers the
loop. Even worse, there are some sensors, which require receiver
isolation...
So you need to know, which end of the loop is going to supply power, and
if the other end is isolated or not (if not, then you should provide
isolation). Some applications will require both ends to provide isolation.
AN54, from Siemens, shows how to use their IL300 analog opto-coupler, to
provide a line powered isolated interface. TI (Burr Brown), do an op-amp,
which provides an isolation barrier in the amplifier, as an alternative
way of doing this.

Best Wishes

7. ### AJGuest

Thanks heaps Lefty, Jamie and Roger, Been a great help.

Best regards

AJ