The World Leader in High-Performance Signal Processing Solutions

The World Leader in High-Performance Signal Processing Solutions Digital Potentiometer Net Seminar Part I Introduction and Basic Applications Alan Li alan. li@analog. com Jan 2003

Also Known As u Digital Pot u Digit Pot u RDAC u E 2 POT u DCP u VR u Variable Resistor u Programmable Resistor 2

What Is Digital Potentiometer ? u It is Simply a 3 -Terminal Programmable Resistors u Complementary Resistors, RWA and RWB, are Functions of Code u It is a D/A Converter with Resistance Output u It can be Converted Easily to Voltage and Current Outputs A W B 3 Digital Code

What Is Digital Potentiometer ? - Continue A A RWA(D) RS = W B Digital Code 4 = W RWB(D) Digital Code B

Why Should You Consider Digital Pot? REPLACES u High Resolution u Fast Adjustment Time u Remote Controllable u Ease of Layout u Minimum Drift u u No Mechanical Wear-out Scalable Resistance and Resolution u Insensitive to Vibration u u High Density Multi-Channels Permanent Settings and Additional Information Can be Saved In EEMEM * u Daisy Chainable u Make Automation Possible u Less Bulky It Saves “System” Cost in Most Applications 5 * Applies to Nonvolatile Pot Only

Where are They Used? LCD Brightness and Contrast Control Programmable Power Supply u LCD Projector Keystone Correction System Offset Trimming Sensor Calibration u 6 Motor Speed Control u RF Power Amp Biasing u u Laser Diode Bias and Modulation Control Gain and Offset Control u Frequency Tuning u And Lot More……………

How are They Used? u Rheostat Mode ( 2 -Terminal Variable Resistor) u Potentiometer Mode (3 -Terminal Voltage Divider) (If VB is grounded) 7 D = Decimal Equivalent of Data Bit 2 N = Number of Positions

Types of Memory Present Volatile Next Power On Power up at random state (no memory) Nonvolatile* Can be changed dynamically (with memory) OTP Cannot be changed (one time Programmable) * The 8 terms Nonvolatile Memory, EEMEM, E 2 PROM, and Flash are used interchangeably

Digital Pot Selection Tree 32, 64, 128, 256, 512, 1024 steps 1, 2, 3, 4, 6 Channels 1 k, 10 k, 20 k, 50 k, 100 k, 250 k, 1 MW +5 V, ± 2. 5 V, ± 5 V, +15 V, +30 V 9

Wiper Resistance = Rw(TC 2) Rw RW Rs S R ADDR 70 o. C 25 o. C 4 V -V Q: Why do I care? A: Because it’s the major source of error X 10

Wiper Resistance Effects - Absolute Accuracy RWB Linear Scale RWB Log Scale Actual Ideal RWB Dominated by RW 11 12 Code 255

Wiper Resistance Effects - Temperature Coefficient Dominated by RW Tempco Dominated by RS Tempco Rheostat Mode Tempco >> Potentiometer Mode Tempco 12 ADI offers both thin film and poly resistor versions of digital pots and the tempco of the thin film parts are 10 X better than its poly resistor counterparts

Tolerance Inherent from Process Limitations, Si Resistor Thickness Variations Dictates Digital Pot Tolerance to +/-30%. u Large Numbers of Steps Adjustments Compensates the Limitation. u Tolerance is Much Tighter in Potentiometer Mode Operation Due to Tolerances Tracking Between RWA and RWB. u Tolerance Enhancement is Also Possible as Shown u 13

Operating Voltage Most Digital Pots Limit to 5 V Operations Because High Voltage Parts Require Larger Silicon Areas. u Terminal Voltages Must be Less Than or Equal to VDD and VSS. u Terminal Voltages Have No Polarity Constraints. u 14

Maximum Current u IWBmax and IWAmax are resistance dependants bounded by maximum allowable operating voltage. u Internal switches also limit maximum allowable current u Adding proper Power MOSFET can boost to any desirable currents 15

Bandwidth Buffered Potentiometer Mode Frequency Response 55 p. F 25 p. F Bandwidth is code dependant at a given RAB. Bandwidth is dynamic that it should be modeled in SPICE 16 *AD 5273 64 -Step Digital Pot. PARAM D=64, RDAC=10 E 3 *. SUBCKT DPOT (A, W, B) * CA A RWA A CW W RWB W CB B *. ENDS DPOT 0 W 0 B 0 25 E-12 {(1 -D/64)*RDAC+50} 55 E-12 {D/64*RDAC+50} 25 E-12

Programming Settling Time Code = Midscale VW = 1 V/DIV CS = 5 V/DIV Note: Nonvolatile Memory Restore Time Also Falls into ms Range Typically 17

How to Control It – Manual Up/Down Control VCC MR RESET ADM 812 GND Manual Up/Down Control with De-bounce Circuit 18

How to Control It – Manual Rotary Control RE 11 CT-V 1 Y 12 -EF 2 CS Rotate Clockwise for A Leads B and Therefore Increment 19 Rotate Counter-clockwise for A Lags B and Therefore Decrement

How to Control It – Digital Control Generated by n Micro-Controller n Micro-Processor n DSP n FPGA n CPLD n PC n Discrete Logics A CS CLK SDI SPI Interface CS = Chip Select CLK = Clock SDI = Data Bits 20 W B

Built-in Increment/Decrement Controls with ADI’s Nonvolatile Memory Digital Pots u Increment one step u Increment all one step u Increment 6 d. B u Increment all 6 d. B u Decrement one step u Decrement all one step u Decrement 6 d. B u Decrement all 6 d. B 21 AD 5231/AD 5232/AD 5233/AD 5235 AD 5255/ADN 2850/ADN 2860

Multi-Parts Operation VDD R 4 SPI* Interface SDI SDO U 4 I 2 C* Interface * Compatible 22 SDI R 3 SDO U 3 SDI R 2 SDO U 2 R 1 SDI SDO U 1

Design Considerations Summary u Most l Digital Pots Limited to 5 V ADI is currently the only company makes +15 V (± 5 V), +30 V (± 15 V) Digital Pots u Wiper l Resistance Wiper resistance affects DC accuracy and tempco. ADI Digital Pots have the lowest Rw, 50 W typical, in the industry u Tolerance l Large numbers of steps adjustments compensates the effect u Temperature l Coefficient Tempco are functions of code, operation mode, and resistance type. ADI offers the lowest tempco, 35 ppm/o. C, Nonvolatile Digital Pots in the market u Bandwidth l Generally limited to 1 MHz applications. BW is dynamic and is function of codes and rated RAB u Low l 23 Current Generally less than 5 m. A DC. There are workaround solutions to meet current requirements

The World Leader in High-Performance Signal Processing Solutions Basic Applications

System Parameters Settings and Adjustments Temperature Controller Laser Diode Driver LCD Controller Motor Controller ……. . etc 25 where D is digital code in decimal 2 N is numbers of steps

Buffered Output for Level Setting 26

Fine Adjustment R 1 and R 2 >> RAB 27

Programmable Trip Point 6 V D = 64 Vo 128 192 comparator 0 0 28 Vi 5 V

Programmable Current Source IL * Decoupling caps are omitted for clarity 29

Opamp Offset Adjustment Non-inverting 30 Inverting

Linear Gain Control AD 5207 Normalized Digital Pot Setting * Compensation and supply decoupling caps are omitted for clarity 31

Pseudo Log Taper Gain Control AD 5207 Normalized Digital Pot Setting * Compensation and supply decoupling caps are omitted for clarity 32

Bipolar Output With +5 V Digital Pot OP 1177 AD 5273 Normalized Digital Pot (x 100%) Setting * Compensation and supply decoupling caps are omitted for clarity 33

Programmable REF with Boosted Current Capability 34

Programmable Transimpedance Amplifier ID 35

Precision Reference Trimming ADR 02 Rtx = 100 k Rtx = 0: Trim Range ~ 4. 15 to 5. 35 V Rtx = 100 k. W: Trim Range ~ 4. 83 to 5. 05 V 36

Programmable Power Supply Linear Regulator 37 Switching Regulator

Volume Control Mid-Scale, Gain = 5. 5 Vi Vo Full Scale, Gain = 11 Vi Vo 38 * Decoupling caps are omitted for clarity

Tone Control RDAC 1: Boost ¬ BASS ® Cut 20 d. B 10 d. B 0 -10 d. B f. BASS -20 d. B RDAC 2: Boost ¬ TREBLE ® Cut 39 * Decoupling caps are omitted for clarity f. TREBLE

Programmable Phase Shifter 40

ADI Digital Pot Market Position u Broadest l Portfolio Resolution, No of Channels, Operating Voltage, Resistance Options, Interfaces, Volatile Memory, Nonvolatile Memory, One Time Programmable u Highest Resolution u Compact Packaging l SC-70, SOT-23, m. SOIC-8, LFCSP 4 x 4 mm 2 u Lowest Temperature Coefficient u Cost Competitive 41

The World Leader in High-Performance Signal Processing Solutions Digital Potentiometer Net Seminar Part II Advance Applications and Optimization (TBD)

ADI Digital Pot Web Site http: //www. analog. com/digitalpotentiometers 43