current output schematic

Hi Masa,

Do you know what application the customer is using this circuit in?  If this circuit is intended to create industrial current outputs, the customer could consider the XTR300, XTR111, or XTR110.  The XTR116 probably won't work because it is a 2-wire transmitter and the customer circuit requires a local GND which moves them to a 3-wire solution.

While this circuit looks complex at first glance, it can be simplified into an op-amp with a current boost transistor circuit on the output which is being used in a Howland Current Pump configuration.  Components R7, R8, R9, R10, R11, TR1, and TR2 form the output current boost circuit.  Components R1, R2, R3, R4, R5 and R6 form the Howland current pump.

Here is the original circuit and output for a 0-10V input voltage.  0-10V input creates a 0-21.77mA output current:

The circuit can be redrawn and simplified by removing the output current boost circuitry and by combining R4 and R6 into a single resistor.  This is shown below.  Notice that the output current is exactly the same as the previous circuit for a 0-10V input:

The transfer function for the Howland current pump is shown below.  The top line is the un-simplified transfer function for the circuit above.  If R1 is set equal to R3 and R4 is set equal to R2+RS then the equation simplifies to what's shown at the bottom where Iout = Vin*(R2+RS)/R1*RS:

Therefore the load resistance does not affect the output current as long as the condition where R1=R2 and R3+R5 = R4+R6.  This is shown below by sweeping the load resistance and monitoring the output current of the original circuit.  Notice that while the output voltage changes, the output current is constant at 19.59mA for a 9V input:

So, in summary this circuit looks like it's designed to create a roughly 0-22mA output.  Because the output current requirements are not that high, this circuit could be simplified by selecting an op amp that can directly drive the load such as the OPA192.  Is it possible the customer uses this circuit to create higher output current levels as well which is why the transistor current boost circuitry was included?  If so, we can look into replacing the ADI device with the TI equivalent TL082 or TL084. 

Provide a little feedback on the questions above and we'll work on selecting the best device for this application.