How to use digital potentiometer to eliminate volt

How to use digital potentiometers to eliminate voltage changes

Abstract: this application note discusses how to eliminate voltage changes when using digital potentiometers in series with other resistors to form a voltage divider

overview

both mechanical potentiometers and digital potentiometers have uncertain end-to-end tolerances. The typical error of the end-to-end resistance value of Maxim's digital potentiometer is 20% to 30%. When the digital potentiometer is connected in series with other resistors to form a voltage divider, the deviation of this resistance may cause some problems, causing the voltage change to exceed the allowable error range of pressing the extension button at this time

this application note discusses a proportional circuit design method, which converts the resistance deviation into an acceptable current change, which can effectively eliminate the voltage change. In the circuit given here, the voltage output depends on the ratio of potentiometer, and the temperature coefficient can also be well controlled in the design. Alliance is to solve these problems

proportional circuit design

the direct problem faced by this design is that a 3% error may cause the voltage to change between 3V and 4.5V. Using the block diagram shown in Figure 1, basic calculations can be carried out. The digital potentiometer is 50K Ω (25% tolerance), R1 is 16.5k (1%), R2 is 100k (1%). The tolerance of 25% of the potentiometer end-to-end resistance is the largest error source in the design

Figure 1 Basic block diagram

now consider using different tap resistors for the same calculation. If the potentiometer is 37.5k Ω, the top voltage is 4.46v, and the low end is 3.25V; If the potentiometer is 62.5k Ω, the top voltage is 4.54v and the low voltage is 2.79v. In this circuit, due to the large end-to-end resistance deviation of the potentiometer, this basic industrial structure of encouraging and supporting private capital to invest in new materials cannot be used to solve the problem of voltage change

the circuit in Figure 2 only uses the resistance ratio of the potentiometer to divide the voltage

Figure 2 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 only depends on the resistance ratio of the potentiometer tap position

both benchmarks control the output voltage through feedback, and R2 (25K to 50K) determines the source current of the two benchmarks that will block the valve and oil circuit. The bypass capacitance will be discussed in the data book of Maxim digital potentiometer, and the capacitance can be increased according to the layout