A Primer on Digitally-Controlled Potentiometers ® Application Note June 7, 2005 AN124.0 Author: Chuck Wojslaw The objective of this technical note is to provide the design engineer with the fundamentals of the operation and application of digitally-controlled potentiometers. The potentiometer is a three terminal variable, resistancelike device whose schematic symbol is shown in Figure 1. CW (VH or RH) Wiper (VW or RW) CCW (VL or RL) FIGURE 1. SCHEMATIC SYMBOL OF THE POTENTIOMETER There are two types of potentiometers; mechanical and electronic. The terminals of the mechanical potentiometer are called CW (clockwise), CCW (counter clockwise), and wiper. The corresponding names or designations for the terminals of the electronic version are VH or RH, VL or RL, and the wiper VW or RW. The mechanical pot is a three terminal device while the electronic pot is an integrated circuit with a minimum of eight terminals. Intersil’s digitally-controlled potentiometer (XDCP) is an electronic potentiometer whose wiper position is computer or digitally controlled. The electronic version of the potentiometer also has memory where wiper settings and/or data can be stored. and (3) SPI (Serial Peripheral Interface). The control signals for the 3-wire bus are Up/Down, Increment, and Device Select. The Up/Down control input is a level sensitive signal which establishes the direction of the movement of the wiper. The wiper is moved on the falling edge of the Increment control input in the direction established by the Up/Down signal. The Device Select control input is like an address line and enables or disables the device. The control inputs for the 2-wire bus are Clock (SCL), a bidirectional Serial Data line (SDA), and Address lines (ADDR). The control inputs for the SPI bus are Clock (SCK), Serial In (SI) and Serial Out (SO) data lines, and address lines (ADDR). The 2-wire and SPI serial interfaces have protocols that are explained in the data sheets. The digitally-controlled potentiometer is an integrated circuit whose implementation is shown in Figure 3. Polycrystalline resistors are connected in series between the RH and RL terminals and solid state switches implemented by nMOS or CMOS transistors are connected at each end of this resistor array and between the resistors. The switches are equivalent to a single pole, single throw switch. One end of all the switches are tied together and are connected to the wiper terminal. Only one switch will be closed at a time connecting a node in the series resistor array to the wiper. The resistors are polycrystalline silicon deposited on a oxide layer to insulate them from the other circuitry. VH/RH The digitally-controlled potentiometer (XDCP) is a system level control device performing a component level function. The block diagram of a typical digitally-controlled potentiometer is shown in Figure 2. VCC (Supply Voltage) Up/Down (U/D) Increment (INC) Device Select (CS) VH/RH Control and Memory VW/RW VW/RW VL/RL VSS (Ground) FIGURE 2. BLOCK DIAGRAM OF THE DIGITALLYCONTROLLED POTENTIOMETER VL/RL The control and memory section of the device is implemented in CMOS and biased with a 3V or 5V digital or logic supply. The device is controlled through one of three different serial buses; (1) 3-wire, (2) 2-wire similar to I2C, 1 FIGURE 3. IMPLEMENTATION OF THE ELECTRONIC POTENTIOMETER CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures. 1-888-INTERSIL or 1-888-468-3774 | Intersil (and design) is a registered trademark of Intersil Americas Inc. Copyright Intersil Americas Inc. 2005. All Rights Reserved All other trademarks mentioned are the property of their respective owners. Application Note 124 The potentiometer can be used in application circuits as a three terminal device or as a two terminal device. The most common way to use the potentiometer as a three terminal device, Figure 4, is as a voltage divider circuit. Plus and/or minus voltages are connected across the potentiometer and the wiper goes from one voltage limit to the other as the wiper is moved from the low to high terminals. In many applications, this circuit can be substituted for a digital-to-analog convertor since it performs a digital in, analog out function. SCL (Clock) R0 SDA (Data) INTERFACE 8 AND A0 CONTROL Data R2 A1 R1 R3 VH / RH WIPER COUNTER REGISTER (WCR) VL/ RL A2 A3 VW/ RW (Address) FIGURE 6. XDCP SYSTEM CONTROL AND REGISTERS +5V -5V →+5V ⇓ Programmable Voltage -5V FIGURE 4. THREE TERMINAL CONFIGURATION AND APPLICATION The second fundamental way of using the XDCP is as a two terminal, variable resistance. A simple application illustrating this configuration is shown in Figure 5 where the potentiometer functions as a variable resistor and, in essence, varies the current through the LED since the voltage across the potentiometer is relatively constant. The XDCPs with a 2-wire (I2C) or SPI serial bus have an instruction set. The typical instructions are shown in Figure 7 along with the typical bit structure in Figure 8. The instructions control the flow of data internally or through the bus, the increment decrement feature, and some specialized commands. The global transfer command transfers data between registers in devices with a multiple number of potentiometers and the write in progress (WIP) instruction monitors the completion of the nonvolatile write process. Read/Write Wiper Counter Register (WCR) Read/Write Data Registers (D) Transfer WCR and D Global Transfer WCR and D A2 A3 Increment/Decrement Wiper +5V SCL SDA A0 A1 INTERFACE AND CONTROL Read “WIP” status bit example: X9408 ⇒ FIGURE 7. INSTRUCTION SET Programmable Current FIGURE 5. TWO TERMINAL CONFIGURATION AND APPLICATION The two basic applications illustrate the use of the digitally controlled potentiometer in a digital-to-analog voltage circuit and in a digital-to-analog current circuit. Figure 6 expands the block diagram of the XDCP to include the internal registers and data paths. The WCR or Wiper Counter Register is a volatile register whose output determines the position of the wiper by closing a switch connecting the wiper terminal to a point in the resistor array. For the XDCPs with a 2-wire or SPI bus, there are also four nonvolatile Data Registers (R0-3 ) which can be used to store data or additional wiper settings. The wiper counter and data registers can be programmed from the bus or data can be transferred between the registers through the device’s instruction set. Normally, data register R0 is used to store the wiper setting for the power-up condition. For the XDCPs with a 3-wire interface, there is only one internal nonvolatile register per potentiometer, which stores the wiper setting for restoration during the power-up condition. 2 Intersil digitally-controlled potentiometers also come with active devices like operational amplifiers and comparators. The block diagram of the dual pot-dual op amp device is shown in Figure 9. VCC SCL SDA A3 A2 A1 A0 WP RH(0,1) VNI(0,1) V+ ACR (0,1) Control and Memory + – VOUT (0,1) WCR (0,1) VSS RL(0,1) RW(0,1) VINV(0,1) V– FIGURE 8. POTENTIOMETER AND OPERATIONAL AMPLIFIER The terminals of both the potentiometer and the op amp are brought out to accommodate all possible configurations. The op amp is powered or biased with the externally connected V+ and V- analog voltage supplies. The WCR programs the location of the wiper of the potentiometer and the Analog Control Register (ACR) programs features of the operational amplifier. AN124.0 June 7, 2005 Application Note 124 3mA, limits the maximum amount of current allowed through the wiper switches. Absolute linearity describes the actual versus expected value of the potentiometer when used as a divider and is guaranteed to be accurate within one least significant bit or minimum increment (MI). Relative linearity describes the tap to tap accuracy and is guaranteed to be 0.2 of an LSB or MI. The potentiometer has a maximum power rating between 10mW and 50mW. Two parameters describe the temperature dependence of RTOTAL and the resistances in the series array. RTOTAL TC (temperature coefficient) is a nominal 300ppm/°C and the ratiometric TC is guaranteed to be within 20ppm/°C. The analog data sheet parameters reflect the limitations of the digitally-controlled potentiometer. The key analog data sheet parameters are number of taps, end to end resistance, maximum voltages on the potentiometer pins, wiper resistance and current, power rating, resolution, noise, linearity, and temperature coefficient. Figure 10 lists the data sheet parameters and their values for a typical XDCP. The number of taps in a potentiometer varies from 16 to 256 and reflects the resolution of the device or its ability to discern 1 of n. The end to end resistance (RH to RL) of the potentiometer is RTOTAL and comes in 1k¾ to 1M¾ values. The voltage VCC, 3 to 5V, provides the voltage biasing for the digital control and memory section and V+ and V- provide the voltage biasing for the analog section. The voltages VTERMINAL are the maximum voltages, ±5V to +15V, that can be applied to the potentiometer pins in their application. Wiper resistance, nominally 40W, models the resistance rds (on) of the MOS switches used to connect the wiper terminal to a node in the resistor array. The wiper current spec, 1- While the data sheet parameters reflect the performance limitations of the digitally controlled potentiometer, there are a large number of circuit techniques that minimize these limitations. Intersil application notes and technical briefs describe these techniques and are available at the Intersil’s website www.Intersil.com In addition, these publications describe the myriad of possible applications. Write Wiper Counter Register (WCR) S T A R T device type identifier 0 1 0 1 device addresses A 3 A 2 A 1 Byte 1 Two-wire Interface A 0 S A C K instruction opcode 1 0 1 wiper addresses 0 0 0 S A C P1 P0 K Byte 2 wiper position (sent by master on SDA) 0 0 S A W W W W W W C P P P P P P K 5 4 3 2 1 0 S T O P Byte 3 Slave Acknowledge FIGURE 9. BIT STRUCTURE 3 AN124.0 June 7, 2005 Application Note 124 ANALOG CHARACTERISTICS (Over recommended operating conditions unless otherwise stated.) Limits Symbol RTOTAL Parameter Min. End to End Resistance Typ. -20 Power Rating IW Wiper Current -3 RW Wiper Resistance Vv+ Voltage on V+ Pin Vv- Voltage on V- Pin VTERM Max. +20 % 50 mW +3 mA 40 100 150 250 Ω X9410 +4.5 +5.5 X9410-2.7 +2.7 +5.5 X9410 -5.5 -4.5 X9410-2.7 -5.5 -2.7 V- V+ Voltage on any VH or VL Pin Noise Resolution (4) Absolute Linearity (1) Linearity (2) Relative Temperature Coefficient of RTOTAL Ratiometric Temp. Coefficient Units Test Conditions 25°C, each pot VCC = 5V, IW=±1mA VCC = 2.7-5.5V, IW=±1mA V V V -140 dBV 1.6 % Ref: 1kHz -1 +1 MI(3) Vw(n)(actual) – Vw(n)(expected) -0.2 +0.2 MI(3) Vw(n + 1) – [Vw(n) + MI] ±300 ppm/°C 20 ppm/°C Notes: (1) Absolute Linearity is utilized to determine actual wiper voltage versus expected voltage as determined by wiper position when used as a potentiometer. (2) Relative Linearity is utilized to determine the actual change in voltage between two successive tap positions when used as a potentiometer. It is a measure of the error in step size. (3) MI = RTOT/63 or (VH – VL)/63, single pot (4) Iindividual array resolutions. FIGURE 10. Analog Data Sheet Parameters Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is cautioned to verify that the Application Note or Technical Brief is current before proceeding. For information regarding Intersil Corporation and its products, see www.intersil.com 4 AN124.0 June 7, 2005