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.