Analog voltage signal transmission over long distances due to signal source resistance and cable resistance voltage drop generated by the letter ended input impedance , the lower the greater the relative pressure drop , the greater the error . To send a high-precision voltage signal , the voltage signal to current signal into the first , namely, the current transmission . It is a constant current output circuit . Suppose cable impedance 100O, because of the current transmission circuit equal , there is no current consumption , will not produce voltage error , generally the voltage into a 4mA (0%) and 20mA (100%) current is transmitted . Specific circuit as shown below. A2 as a result of the op amp constant current output circuit , current is converted to voltage with a resistor R11, the last voltage 0.4 ~ 2.0V. A2 input has two parts , 4mA drift and 0 ~-1.6V input signal . no input signal, to increase the bias current of 4mA , for R8 = 10R4, therefore , PR2 output voltage adjusted to -4V. Drift voltage of 0.4V with the input signal voltage 0 ~-1.6V, the total input signal voltage of 0 ~-2.0V. R11 on the voltage set large, the voltage margin Decrease , so set 2Vmax this work in the attenuation of the op amp A1 status , gain of 0.16 .