Understanding Digitally Programmable Potentiometers Fundamentals of Digitally Programmable Potentiometers (DPP) and design ideas for applications 1 • DPP Tutorial • Aug-11 Architectural Overview • The digital potentiometer is a mixed signal device designed as an electronic replacement for mechanical potentiometers. The function of the potentiometer section of the electronic potentiometer is the same as the mechanical version. In both cases, the potentiometer or pot is a three terminal device. • Between two of the terminals there is a resistive element. The third terminal called the wiper is connected to various points along this resistive element. 2 • DPP Tutorial • Aug-11 Digital Potentiometers (DPPs) Mechanical Potentiometer Digital Potentiometer • Consists of a resistor and a third terminal called the wiper • The wiper divides the resistor • The position of the wiper is adjusted mechanically – e.g. using a screwdriver • Fulfills the same function • The position of the wiper is adjusted via serial interface • The wiper can move through a discrete number of steps (taps) 3 • DPP Tutorial • Aug-11 Potentiometer Basic Functionality High Resistance Low Resistance High Wiper Position High Voltage Low Wiper Position Low Voltage Applications • Volume Adjustment • Voltage Trimming • Frequency Attenuation • Joysticks • Brightness Adjustment • Motor control • Contrast Adjustment • Automated calibration 4 • DPP Tutorial • Aug-11 Mechanical Potentiometers vs. DPPs Mechanical Potentiometers DPPs Negligible wiper resistance Wiper resistance ~100 Ω Well controlled end-to-end resistance End to end resistance processdependent: ±20% tolerance • BUT ratio between wiper positions constant Cumbersome mechanical interface Can be controlled by microprocessors or push buttons via standard serial interfaces: • I2 C • SPI • Up/Down • Inc/Dec 5 • DPP Tutorial • Aug-11 Main Distinguishing Features Memory • Volatile DPP resets the wiper at mid-scale on power-on. Although they don’t have internal non-volatile storage, volatile DPP provides a cost-effective solution by using the storage capability already existent within the application. • The non-volatile DPP has an EEPROM for wiper storage, thus recalling the wiper position at power-on. This feature simplifies applications that require the wiper position to be automatically saved (for example, saving the last user setting). Control Interface • Most common asynchronous bus is the increment/decrement interface U/D INC CS Up Down 6 • DPP Tutorial • Aug-11 • The most common synchronous buse is I²C. SCL Up/Down Counter SDA I2C Bus Interface Up/Down Counter CS SCK SI SPI Bus Interface CAT5128 SO Adjustable Gain Circuit with Rheostat 7 • DPP Tutorial • Aug-11 Adjustable Gain Circuit with Voltage Divider 8 • DPP Tutorial • Aug-11 Positive LCD Bias Control 9 • DPP Tutorial • Aug-11 Programmable Instrumentation Amplifier 10 • DPP Tutorial • Aug-11 Programmable Square Wave Oscillator 11 • DPP Tutorial • Aug-11 Programmable Voltage Regulator 12 • DPP Tutorial • Aug-11 Sensor Auto Referencing Circuit 13 • DPP Tutorial • Aug-11 Programmable I to V Converter 14 • DPP Tutorial • Aug-11 Automatic Gain Control 15 • DPP Tutorial • Aug-11 Programmable Current Source/Sink 16 • DPP Tutorial • Aug-11 For More Information • View the extensive ON Semiconductor portfolio of products for energy efficient electronics at www.onsemi.com. 17 • DPP Tutorial • Aug-11