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Digital Potentiometer Net Seminar Part I

Introduction and Basic Applications

Alan Li

alan.li@analog.com

Jan 2003

2

Also Known As

Digital PotDigit PotRDACE2POTDCPVRVariable ResistorProgrammable Resistor

3

What Is Digital Potentiometer ?

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

B

A

Digital Code

W

4

What Is Digital Potentiometer ? - Continue

B

A

RWA(D)

RWB(D)

Digital Code

W =

Rs (TC1)

B

SW1

ADDRESSDECODER

A

WX =

Digital Code

B

A

W

RS

5

REPLACES

Why Should You Consider Digital Pot?

High Resolution

Remote Controllable

Minimum Drift

No Mechanical Wear-out

Insensitive to Vibration

High Density Multi-Channels

Daisy Chainable

Less Bulky

Fast Adjustment Time

Ease of Layout

Scalable Resistance and Resolution

Permanent Settings and Additional Information Can be Saved In EEMEM *

Make Automation Possible

It Saves “System” Cost in Most Applications

* Applies to Nonvolatile Pot Only

6

Where are They Used?

Motor Speed Control LCD Projector Keystone

Correction System Offset Trimming Sensor Calibration Gain and Offset Control Frequency Tuning And Lot More……………

RF Power Amp Biasing

LCD Brightness and Contrast Control

Laser Diode Bias and Modulation

Control

Programmable Power Supply

7

How are They Used?

Rheostat Mode( 2-Terminal Variable Resistor)

Potentiometer Mode(3-Terminal Voltage Divider)

WRABRN

DN

DWAR

WRABRN

DDWBR

2

2)(

2)(

D = Decimal Equivalent of Data Bit2N = Number of Positions

AVNDDWV

VND

AVNDDWV B

2)(

2

)1(

2)(

(If VB is grounded)

8

Volatile(no memory)

Nonvolatile*

(with memory)

Present Next Power On

OTP(one time

Programmable)

Can be changed dynamically

Cannot be changed

Power up at random state

* The terms Nonvolatile Memory, EEMEM, E2PROM, and Flash are used interchangeably

Types of Memory

9

M anualU p/D ow n

SPI

I2C

C ontro l In terface

Volatile M em ory(N o M em ory)

SPI

I2C

C ontro l In terface

EEM EM(U nlim ited T im es)

M anualU p/D ow n

SPI

I2C

C ontro l In terface

O TP(O ne T im e Program m able)

N onvolatile M em ory(W ith M em ory)

M em ory

O perating Voltage

R esistance

N o. of C hannels

R esolution

Digital Pot Selection Tree

1, 2, 3, 4, 6 Channels

32, 64, 128, 256, 512, 1024 steps

1k, 10k, 20k, 50k, 100k, 200k, 250k, 1M

+5V, ±2.5V, ±5V, +15V, +30V

10

Wiper Resistance

Q: Why do I care?A: Because it’s the major source of error

25oC

70oC

Rs (TC1)

B

SW1

ADDRESSDECODER

A

WX

1

2

ADD

W

Qp

Qn

X

Rw (TC2)

WX=

4

VX - V

Rw(TC2)

ADDR

Rs

Rw

RS

RW

11

Wiper Resistance Effects - Absolute Accuracy

D

1.0 2.0 4.0 6.0 8.0 10 12 20 40 60 80 100 200 300V(I1:-)/I(I1) V(I2:-)/I(I2)

10

20

40

60

80

100

200

400

600

800

1.0K

2.0K

4.0K

6.0K

8.0K

10KRWB in Ohm

D

0 64 128 192 256V(I2:-)/I(I2) V(I1:-)/I(I1)

0

2K

4K

6K

8K

10KRWB in Ohm

Linear Scale

Log Scale

Ideal

RWB Dominated by RW

Actual

12 255

RWB

RWB

Code

12

Wiper Resistance Effects - Temperature Coefficient

Rheostat Mode Tempco Potentiometer Mode Tempco

0

500

1000

1500

2000

2500

3000

0 128 256 384 512 640 768 896 1024

CODE - Decimal

RH

EOS

TA

T M

OD

E T

EMP

CO

- p

pm

/oC

VDD = 5.5V, VSS = 0V

TA = -40oC/+85oC

-20

0

20

40

60

80

100

0 128 256 384 512 640 768 896 1024

CODE - Decimal

PO

TEN

TIO

MET

ER M

OD

E T

EMP

CO

- p

pm

/oC VDD = 5.5V, VSS = 0V

TA = -40oC/+85oC

VA = 2.00V

VB = 0V

Dominated by RW Tempco

Dominated by RS Tempco

ADI offers both thin film and poly resistor versions of digital pots and the tempco of the thin film parts are 10X better than its poly resistor counterparts

13

Tolerance

Inherent from Process Limitations, Si Resistor Thickness Variations Dictates Digital Pot Tolerance to +/-30%.

Large Numbers of Steps Adjustments Compensates the Limitation. Tolerance is Much Tighter in Potentiometer Mode Operation Due to Tolerances

Tracking Between RWA and RWB. Tolerance Enhancement is Also Possible as Shown

14

Operating Voltage

Most Digital Pots Limit to 5V Operations Because High Voltage Parts Require Larger Silicon Areas.

Terminal Voltages Must be Less Than or Equal to VDD and VSS. Terminal Voltages Have No Polarity Constraints.

15

Maximum Current

IWBmax and IWAmax are resistance dependants bounded by maximum allowable operating voltage.

Internal switches also limit maximum allowable current Adding proper Power MOSFET can boost to any desirable currents

16

Bandwidth

25pF25pF55pF

Bandwidth is code dependant at a given RAB . Bandwidth is dynamic that it should be modeled in SPICE

*AD5273 64-Step Digital Pot.PARAM D=64, RDAC=10E3*.SUBCKT DPOT (A,W,B)*CA A 0 25E-12RWA A W {(1-D/64)*RDAC+50}CW W 0 55E-12RWB W B {D/64*RDAC+50}CB B 0 25E-12*.ENDS DPOT

Buffered Potentiometer Mode Frequency Response

17

Programming Settling Time

Code = Midscale

CS = 5V/DIV

VW = 1V/DIV

Note: Nonvolatile Memory Restore Time Also Falls into s Range Typically

18

MR RESET

GND

VCC

ADM812

Manual Up/Down Control with De-bounce Circuit

How to Control It – Manual Up/Down Control

19

How to Control It – Manual Rotary Control

Rotate Clockwise for A Leads B and Therefore Increment

Rotate Counter-clockwise for A Lags B and Therefore Decrement

RE11CT-V1Y12-EF2CS

20

CS = Chip SelectCLK = ClockSDI = Data Bits

CS

CLK

SDIB

A

W

Generated by Micro-Controller Micro-Processor DSP FPGA CPLD PC Discrete Logics

SPI Interface

How to Control It – Digital Control

21

Built-in Increment/Decrement Controls with ADI’s Nonvolatile Memory Digital Pots

Increment one step Increment all one step Increment 6dB Increment all 6dBDecrement one stepDecrement all one stepDecrement 6dBDecrement all 6dB

G N D

R D A C 2

R D A C 1

P W R O N P R E S E T

S E R I A L I N P U T

R E G I S T E R

R D A C 1 R E G I S T E R

P R

W P

C S

G N D

V D D

W 1 A 1

B 1

V S S

A D D R E S S D E C O D E

R D Y

C L K

E E M E M 1

W 2 A 2

B 2 E E M E M 2 E E M E M C O N T R O L

R D A C 2 R E G I S T E R

S D I S D O

2 6 B Y T E S U S E R E E M E M

& Built-in Increment/Decrement Functions

AD5231/AD5232/AD5233/AD5235AD5255/ADN2850/ADN2860

22

Multi-Parts Operation

SDO

VDD

R4 R3 R2 R1

U4

SDI SDI SDO

U3

SDI SDO

U2

SDI SDO

U1

SDISPI* Interface

S D A S C L

A D 5 2 8 2 A D 1 A D 0

M A S T E R S D A

S C L

R p R p

+ 5 V

S D A S C L

A D 5 2 8 2 A D 1 A D 0

S D A S C L

A D 5 2 8 2 A D 1 A D 0

S D A S C L

A D 5 2 8 2 A D 1 A D 0

+ 5D V + 5 V + 5 V

I2C* Interface

* Compatible

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Design Considerations Summary

Most Digital Pots Limited to 5V ADI is currently the only company makes +15V (±5V), +30V (±15V) Digital

Pots Wiper Resistance

Wiper resistance affects DC accuracy and tempco. ADI Digital Pots have the lowest Rw, 50 typical, in the industry

Tolerance Large numbers of steps adjustments compensates the effect

Temperature Coefficient Tempco are functions of code, operation mode, and resistance type. ADI

offers the lowest tempco, 35ppm/oC, Nonvolatile Digital Pots in the market Bandwidth

Generally limited to 1MHz applications. BW is dynamic and is function of codes and rated RAB

Low Current Generally less than 5mA DC. There are workaround solutions to meet

current requirements

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Basic Applications

25

System Parameters Settings and Adjustments

Temperature ControllerLaser Diode DriverLCD ControllerMotor Controller……..etc

ABNSET RD

R 2

where D is digital code in decimal 2N is numbers of steps

26

Buffered Output for Level Setting

ANo VD

V 2

27

Fine Adjustment

R1 and R2 >> RAB

28

Programmable Trip Point

V_Vi

0V 0.5V 1.0V 1.5V 2.0V 2.5V 3.0V 3.5V 4.0V 4.5V 5.0VV(Vo)

0V

1.0V

2.0V

3.0V

4.0V

5.0V

6.0V

D = 64

128

192

0 5VVi

0

6V

Vo

comparator

29

Programmable Current Source

SET

WBL R

VI

IL

* Decoupling caps are omitted for clarity

30

Opamp Offset Adjustment

Non-inverting Inverting

31

1R

R

V

V WB

i

o

Normalized Digital Pot Setting

* Compensation and supply decoupling caps are omitted for clarity

AD5207

Linear Gain Control

32

WA

WB

i

o

R

R

V

V

Normalized Digital Pot Setting

* Compensation and supply decoupling caps are omitted for clarity

AD5207

Pseudo Log Taper Gain Control

33

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

0 0.2 0.4 0.6 0.8 1

Digital POT Setting

Ga

in (

Vo/V

i)

Bipolar Output With +5V Digital Pot

1

2

WAWB

WAWB

)R(R bottom at theFor wiper

1

)R(R topat theFor wiper

R

R

V

V

V

V

i

o

i

o

11

2 i

o

V

V

R

R

(x100%)

AD5273

Normalized Digital Pot Setting

* Compensation and supply decoupling caps are omitted for clarity

OP1177

34

Programmable REF with Boosted Current Capability

Wo VR

RV )1(

1

2

35

Programmable Transimpedance Amplifier

DIR

RWBR

RWBR

oV

112

1

ID

36

Precision Reference Trimming

REF02

4

4.2

4.4

4.6

4.8

5

5.2

5.4

0 0.2 0.4 0.6 0.8 1

Digital POT Setting

Vo (

V)

Rtx = 0: Trim Range ~ 4.15 to 5.35V

Rtx = 100k: Trim Range ~ 4.83 to 5.05V

Rtx = 0

Rtx = 100k

ADR02

37

Programmable Power Supply

Linear Regulator Switching Regulator

38

Volume Control

Full Scale, Gain = 11

Mid-Scale, Gain = 5.5

Vi

Vo

Vi

Vo

* Decoupling caps are omitted for clarity

39

Tone Control

RDAC2: Boost TREBLE Cut

RDAC1: Boost BASS Cut

fBASSfTREBLE

20dB

10dB

0

-10dB

-20dB

* Decoupling caps are omitted for clarity

40

Programmable Phase Shifter

41

ADI Digital Pot Market Position

Broadest Portfolio Resolution, No of Channels, Operating Voltage, Resistance

Options, Interfaces, Volatile Memory, Nonvolatile Memory, One Time Programmable

Highest ResolutionCompact Packaging

SC-70, SOT-23, SOIC-8, LFCSP 4x4 mm2

Lowest Temperature CoefficientCost Competitive

The World Leader in High-Performance Signal Processing Solutions

Digital Potentiometer Net Seminar Part II

Advance Applications and Optimization (TBD)

43

ADI Digital Pot Web Site

http://www.analog.com/digitalpotentiometers