How to design output module plc in three-wire constant current source driver circuit

output module plc due to the temperature of the integrated operational amplifier parameters that affect not as a transistor or FET parameters affect significantly the integrated amplifier composed of a constant current plc communication modules  source with the stability better, more constant performance advantages. In particular, the need to ground at the load end of the occasion, received a wide range of applications. Dual op amp constant current source. UA1 amplifier output module plc which constitute adder, 1) load can be grounded; 2) When the op amp is a dual power supply, the output current is bipolar; 3) constant current size adjusted by changing the input reference datum reference resistor Rref0 size to achieve, it is easy to obtain a stable small current calibration and compensation.
Since the resistance of the mismatch, the  plc power supply module voltage across the reference resistor output module plc Rref0 will affect the load end of the driving voltage Vb. The same time as a constant current source, and will definitely change with load changes, which will affect the stability of the constant current source. Obviously, this high-precision constant current source is unacceptable. So R1, R2, R3, R4 selection principle which four resistors is to try a small mismatch, and every size of resistor mismatch direction to be consistent. In practice, a lot of the same batch can be screened precision resistors, where resistance is close to elect four resistors.