How to use a digital potentiometer to smooth out voltage changes

Overview

Both mechanical and digital potentiometers have uncertain end-to-end tolerances. Maxim's digital potentiometers typically have an end-to-end resistance error of 20% to 30%. When a digital potentiometer is connected in series with other resistors to form a voltage divider network , this resistance deviation may cause some problems, causing the voltage change to exceed the allowable error range.

This application note discusses a ratiometric circuit design method that converts resistance deviations into acceptable current changes, effectively canceling voltage changes. In the circuit presented here, the voltage output depends on the ratio of the potentiometers, and the temperature coefficient can also be well controlled in the design.

Ratio Circuit Design

The immediate problem facing this design is that a 3% error can result in a voltage variation between 3V and 4.5V. Using the block diagram in Figure 1, a basic calculation can be performed. The digital potentiometer is 50kΩ (25% tolerance), R1 is 16.5K (1%), and R2 is 100K (1%). The 25% tolerance of the potentiometer end-to-end resistance is the largest source of error in the design.

Now consider the same calculation with different wiper resistors. If the potentiometer is 37.5kΩ, the top voltage is 4.46V and the bottom voltage is 3.25V. If the potentiometer is 62.5kΩ, the top voltage is 4.54V and the bottom voltage is 2.79V. In this circuit, due to the large end-to-end resistance deviation of the potentiometer, this basic architecture cannot be used to solve the voltage variation problem.

Two voltage references are introduced into the circuit to control the error and temperature coefficient. The end-to-end absolute deviation of the digital potentiometer will change the loop current but will not affect the voltage. The output voltage changes proportionally and depends only on the resistance ratio of the potentiometer wiper position.

Both references control the output voltage through feedback, and R2 (25K to 50K) determines the source current of the two references. The bypass capacitor is discussed in the data sheet of the MAXIM digital potentiometer, and the capacitor can be added according to the layout.