[REQ] Advice about 4-20mA Loop

[sam_des]

Hi,
A Part of my new project requires 4-20mA or 0-10VDC output. Let me say this is my first design with 4-20mA output. Some broad specifications by the customer are,
1) 16-BIT resolution
2) Galvanic isolation
3) Field Scalable
4) Load upto 500ohm
5) Hardware selectable 4-20mA or 0-10VDC output

I've been going through datasheets of various devices and their app-notes from Texas, Analog Dev. and Maxim. I've nearly finalized, AD5422+ADuM1400/01 combination.

Since time-schedule for the project is extremely tight, I may not afford to devote as much time as I would like to(as always customer have added it to feature list nearly at the end).  

So,  I would like to hear some sound practical advice about the AD devices I've selected(or any other better alternative) and also about 4-20mA loop design in general.

One specific question I have is about testing the 16-BIT resolution of current loop as Customer doesn't have the receiver ready yet. Do I need something like RCV420 + 24-BIT Sigma-Delta  or something else ?

Thanks in advance,
sam_des

[solutions]

Current loops are SERIAL, so you can have a bazillion bits of resolution in the datastream.  

It's unclear to me if you are sending 16 bit DAC/ADC data or not.  If so, determine your (start/stop bits, pad, protocol overhead, resolution) and multiply by sample rate and from that you get your baud rate. The baud rate determines your distance.

FWIW, did you notice that 20mA into 500 ohms is 10V?

[sam_des]

@solutions,
Thanks for the reply.

Current loops are SERIAL, so you can have a bazillion bits of resolution in the datastream.  

It's unclear to me if you are sending 16 bit DAC/ADC data or not.  If so, determine your (start/stop bits, pad, protocol overhead, resolution) and multiply by sample rate and from that you get your baud rate. The baud rate determines your distance.

As far as I know, for 4-20mA Current loop is generally scaled as- 0 Reading of sensor = 4mA & Full scale reading of sensor = 20mA. Thus we've 16mA for entire range. Thus for 16-BITs we have 16mA/65535 per LSB.
At the receiver end, we have high precision/high stability resistor which gives the voltage proportional to current. This voltage is further must be scaled according to the ADC used to read the value transmitted. My concern is measuring 16mA/65535=0.244uA.

FWIW, did you notice that 20mA into 500 ohms is 10V?

Yes, I've noticed that. But drivers & receiver are capable of handling voltages upto +/-40V

regards,
sam_des

[solutions]

No - on a current loop you DO NOT encode current magnitude. You transport current on or off, nothing more and these are serial bits. The voltage accommodates the resistance of the loop, which varies with length.

Your client's notion of using a 16 bit ADC is uninformed, naive, and will FAIL due to environmental noise, not to mention the EMI that'll be radiated, failing class B emissions. Your NEXT will be horrible if you crank the magnitude up.

What exactly are they trying to do and how far does it need to go?

[pickit2]

do a google on printers and telytypes, 20ma loop is easier to use in noisy environment, than rs232.
www.samson.de/pdf_en/l153en.pdf

[sam_des]

Hi,
This is confusing me   I think I wasn't able communicate properly. I've attached a simple description of 4-20mA loop I'm trying to implement.

No - on a current loop you DO NOT encode current magnitude. You transport current on or off, nothing more and these are serial bits. The voltage accommodates the resistance of the loop, which varies with length.

0mA current on a 4-20mA current loop usually gets identified as "loop break", isn't it ?

Your client's notion of using a 16 bit ADC is uninformed, naive, and will FAIL due to environmental noise, not to mention the EMI that'll be radiated, failing class B emissions. Your NEXT will be horrible if you crank the magnitude up.

What exactly are they trying to do and how far does it need to go?

Here is the basic idea of what we've been trying to achieve...

At the sensor end, Strain Gauge Bridge->24-BIT Sigma-Delta ADC->uC->2.5kV Galvanic Isolation->16-BIT 4-20mA Transmitter
At the Controller End, High Precision Resistor->4-20mA Receiver->24-BIT sigma-delta ADC->2.5kV Galvanic Isolation->Controller->Plant Control Loop

4-20mA Transmitter, such as AD421, AD5420, has 16-BIT DAC + Voltage to Current Converter which regulates the current with help of a internal/external BJT/FET. The value of the current is decided by the 16-bit digital data fed to DAC over SPI bus.

4-20mA Receiver, such as RCV420, has a high precision resistor + Current to Voltage Converter, which converts current being circulated on loop to a voltage suitable for ADC being used, usually 4-20mA scales to 1 to 5VDC.

In entire loop, voltage source driving the loop must have value greater than all the voltage drops, including drop of transmission wires.

As far as I know there is one protocol for 4-20mA current loop - HART(Highway Addressable Remote Transducer). This is FSK, which superimposes digital communication on 4-20mA. In this current in the loop is sine-modulated as, 1200Hz = Binary 0, & 2200Hz = Binary 1. No DC is added to current value by HART, so a simple LPF is required at traditional receiver can receive it as normal 4-20mA signal.

regards,
sam_des