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Mechanical potentiometers and digital potentiometers both have uncertain end-to-end tolerances. The typical end-to-end resistance error for digital potentiometers is 20% to 30%. When digital potentiometers are used in series with other resistors to form a voltage divider network, this resistance deviation may cause some issues, leading to voltage changes exceeding the allowable tolerance range.

This application note discusses a proportional circuit design method that converts resistance deviations into acceptable current variations, effectively eliminating voltage fluctuations.

In this circuit presented here, the voltage output depends on the potentiometer's ratio, and the temperature coefficient can also be well controlled in the design. The direct issue faced by the proportional circuit design is that a 3% error could result in the voltage fluctuating between 3V and 4.5V. Basic calculations can be made using the block diagram shown in Figure 1. The digital potentiometer is 50kΩ (25% tolerance), R1 is 16.5K (1%), and R2 is 100K (1%). The 25% tolerance of the potentiometer's end-to-end resistance is a source of error in the design.

Now considering the same calculations with different tap resistors, if the potentiometer is 37.5kΩ, the top voltage is 4.46V, and the bottom voltage is 3.25V; for a potentiometer of 62.5kΩ, the top voltage is 4.54V, and the bottom voltage is 2.79V. In this circuit, due to the significant variation in the end-to-end resistance of the potentiometer, this basic architecture cannot be used to solve the voltage change issue.

Two voltage references are introduced into the circuit, controlling errors and temperature coefficients. The end-to-end deviation of the digital potentiometer alters the loop current, yet does not affect the voltage. The output voltage varies proportionally, depending solely on the resistance ratio of the potentiometer tap position.

Both references control the output voltage via feedback; R2 (25K to 50K) determines the sourcing current of the two references. The MAXIM digital potentiometer's data sheet discusses bypass capacitors, which can be added according to the board layout.