AND8420/D Former Catalyst Document Number DN4 Improving the Resolution of Digital Potentiometer (POT) Applications http://onsemi.com Resolution is the ability to discern one of many. In application circuits using digital POTs, resolution is both a function of the potentiometer and the circuit design. The objective of this application note is to illustrate a few basic device and circuit ideas on resolution. This application note focuses on resolving voltage. There are millions of applications of the potentiometer, however, the most common one is the programmable voltage divider shown in Figure 1. For a potentiometer with n taps (remember two of the taps are used for the bottom and top of the resistive array), the resolution of the potentiometer is given as: V OUT (smallest) + APPLICATION NOTE If the range of the programmed voltage can be restricted in the application, the pot can be inserted in a voltage divider as shown in Figure 2. Resistors R1, R2, and the end-to-end resistance of the potentiometer, RPOT, define the voltage range and the number of pot taps defines the voltage resolution within this range. V OUT (smallest) + V REF For example, if the voltage range is restricted from 0.601 V to 0.701 V, the resolution is for the tap pot 1.01 mV over this range. 0 V v V OUT v V REF The output of the circuit is the wiper voltage which may be buffered (VOUT‘) or not (VOUT). The amount of loading of the wiper will determine whether to use the buffered or unbuffered version of the digital POT. For example, the resolution is 50.5 mV for a 100 tap pot and a 5 V reference. The number of taps of ON Semiconductor’s digital POTs vary from 32 to 256. VREF R2 (432kW) V 2 (0.701V) VREF (5V) RPOT (10kW) V1 (0.600V) VOUT (50.5mV) (CAT5113) + − R1 (60.4kW) Semiconductor Components Industries, LLC, 2013 VOUT(1.01mV) + − VOUT’(1.01mV) VOUT’ (50.5mV) (CAT5111) Figure 2. Differential Voltage Divider Figure 1. Programmable Voltage Divider July, 2013 − Rev. 1 n * 1 R POT ) R 1 ) R 2 V 1 v V OUT v V 2 n*1 RPOT R POT V REF 1 Publication Order Number: AND8420/D AND8420/D An amplifier and a few discrete resistors can extend the resolution to satisfy any application requirement. The circuit in Figure 3 consists of two buffered digitally controlled, 100 tap, potentiometers and a summing amplifier circuit with weighted-value, input resistors. For this circuit, V OUT + 99 V 1 100 ) VREF (+5V) Digital POT 1 + − V2 V1 100 R V 1,2 1 V OUT (smallest) + n * 1 100 + − VREF (+5V) The output of this circuit example varies from 0 to 5 V in half-millivolt steps and is the equivalent to a 13+ binary bit DAC (with memory!!). The use of summing amplifier (inverting and noninverting) circuits, weighted-value input resistors, and digital POTs can extend the resolution to any required level. Digital POT 2 99R + − VOUT (0.505mV) V2 Figure 3. 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