How to Eliminate Voltage Variations Using Ratiometric Circuit Design-EEWORLD

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Mechanical and digital potentiometers have uncertain end-to-end tolerances, with Maxim's digital potentiometers typically having an end-to-end resistance tolerance of 20% to 30%. This resistance variation can cause problems when the digital potentiometer is connected in series with other resistors to form a voltage divider network, causing voltage changes that exceed the allowable error range.

This application note discusses a ratiometric circuit design method that converts the resistance variation into an acceptable current change, which effectively cancels the voltage change. In the circuit shown here, the voltage output is determined by the ratio of the potentiometer, and the temperature coefficient can also be well controlled in the design.

Ratio Circuit Design

The immediate problem with this design is that a 3% error can cause the voltage to vary 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 end-to-end resistance of the potentiometer is the largest source of error in the design.

Figure 1. Basic Block Diagram

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

The circuit in Figure 2 simply uses the resistance ratio of the potentiometer to divide the voltage.

Figure 2. Alternative design using two voltage references.

Figure 2. Alternative design using two voltage references.

Figure 2. Alternative design with two voltage references

The introduction of two voltage references in the circuit allows the error and temperature coefficient to be controlled. The absolute end-to-end deviation of the digital potentiometer will change the loop current but not the voltage. The output voltage changes proportionally, depending only on the resistor 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 Maxim digital potentiometers, and the capacitor can be added according to the layout.

Reference address：How to Eliminate Voltage Variations Using Ratiometric Circuit Design

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