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The World Leader in High-Performance Signal Processing Solutions
Digital Potentiometer Net Seminar Part I
Introduction and Basic Applications
Alan Li
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
23
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
The World Leader in High-Performance Signal Processing Solutions
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