High-precision programmable current source based on 14-bit D/A converter

3.2 Principle of programmable current source
In the figure above, the current at the D/A output terminal IOUT is
,
where R0 is the output impedance of the ladder resistor network.
Since IRFB is the current at the feedback terminal RFB of the reference voltage VREF through the feedback resistor R1, its value is
, the current
IDS flowing through the junction field effect transistor is
,
where VD is the sum of the forward voltage drops of diodes D1 to D4, and VA is the voltage at point A in Figure 3. The current output by the current source is
,
according to Figure 3, VA=VB, IOUT=IDS, and the solutions (6) and (7)


are the output currents of the current source when the D/A converter input D is all 0. Adjust R1 so that the current source is 4mA when the digital input D is all 0; adjust resistor R3 so that the current source is 20mA when the digital input D is all 1.
3.3 Software Implementation Method
According to the above analysis, when designing the software of the intelligent measuring instrument for electro-hydraulic actuators, the digital output D corresponding to each step is calculated according to the stepping mode of the measuring instrument, and the microcontroller sends the high 6 bits and low 8 bits to MAX7534 twice to start the conversion. In this way, a high-precision programmable current source is obtained. 

The author used the MAX7534 chip to design and implement a high-precision programmable current source as a given signal source for the electro-hydraulic actuator in the DZ-1 electro-hydraulic actuator intelligent measuring instrument, which met the system's 1.6‰ accuracy requirement and achieved good results.