Greetings,

I am working on my first analog design project which is to build a two-wire current loop interface (preferably 4-20ma) for transmitting the output of an instrumentation amplifier measuring the voltage across a solenoid. An important feature of the project is that the two wires transmit the appropriate interface current and also provide power to the instrumentation circuit and necessary voltage reference.

So far, my setup is the following:

• A 2 op-amp (single supply) instrumentation amplifier with referenced
  inputs, who's output is represented by V_SIG
• A 5V reference used to provide the in-amp reference
• A voltage controlled current source driven by V_SIG

I was hoping to use op-amps with low operating currents such that the idle
current (V_SIG = 0V) is lower than 4ma, the additional ~16mA would be
drawn by the VCCS. For testing purposes I have been using a signal
generator to simulate the in-amp output but the rest of the circuit is
shown below.

Recently, I added the load resistor (used for measuring the interface
current) as shown in the schematic but found that it drastically
attenuated my AC amplitude which was previously 8mA, while the DC level
remained at 12mA. What could explain this behavior?

Any additional comments or recommendations are much appreciated since I am
not sure if this is the best approach for designing my interface.

 

What is the AC signal frequency?

You do know there are commercial ICs that are optimized for 4mA-20mA current loops?

 

I've been testing at 1 kHz but the bandwidth of the final product should be on the order of 50 kHz.

Yes, I am aware of that but the maximum operating temperature for my design should be ~185° and I have not found any commercially available current loop ICs rated at this temperature. My plan is to first design my own loop interface using low temp. components, then use high temp. components and if need be, add temp. compensation to the final design.