The precise, programmable current sink of Figure 1 requires only a few electronic components.
Figure 1: This circuit controls load current with 8-bit resolution, over the range 0A to 1A.
(Click on image to enlarge)
Load current flowing through a precision resistor with predefined value (R1) produces a voltage that is compared with a given high-accuracy reference voltage (VR ). Then, a feedback loop constantly adjusts the output (load) current to insure that the two voltages are equal.
Loop accuracy depends on the precision of the resistor divider (IC3). Using two separate discrete resistors of 1% tolerance, for example, gives a maximum output error of 2%, which is equivalent to 6-bit resolution. Or, you might choose a precise 0.05% divider like the MAX5490, whose accuracy is equivalent to 11-bit resolution.
For the goal of 8-bit resolution in Figure 1, we need about 0.1% tolerance on the current-sensing resistor (R1), along with the precision resistor divider (IC3), the dual rail-to-rail op amp (IC2), and the low-cost 8-bit DAC (IC1), which is controlled by an I2 C interface.
Shunt resistor R1 produces about 1V for the maximum load current (1A), and the DAC’s internal reference is exactly 4V. To insure an optimum full-scale use of the DAC, the divider is chosen so the lower op amp’s output is also 4V at the maximum load current. The lower op amp must therefore have an accurate gain of four.
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