FEATURES
Dual-channel, 1024-position resolution
25 kΩ, 250 kΩ nominal resistance
Maximum ±8% nominal resistor tolerance error
Low temperature coefficient: 35 ppm/°C
2.7 V to 5 V single supply or ±2.5 V dual supply
SPI-compatible serial interface
Nonvolatile memory stores wiper settings
Power-on refreshed with EEMEM settings
Permanent memory write protection
Resistance tolerance stored in EEMEM
26 bytes extra nonvolatile memory for user-defined
information
1M programming cycles
100-year typical data retention
APPLICATIONS
DWDM laser diode driver, optical supervisory systems
Mechanical potentiometer replacement
Instrumentation: gain, offset adjustment
Programmable voltage-to-current conversion
Programmable filters, delays, time constants
Programmable power supply
Low resolution DAC replacement
Sensor calibration
GENERAL DESCRIPTION
The AD5235 is a dual-channel, nonvolatile memory,1 digitally
controlled potentiometer2 with 1024-step resolution, offering
guaranteed maximum low resistor tolerance error of ±8%.
The device performs the same electronic adjustment function
as a mechanical potentiometer with enhanced resolution, solid
state reliability, and superior low temperature coefficient performance. The versatile programming of the AD5235 via an
SPI®-compatible serial interface allows 16 modes of operation
and adjustment including scratchpad programming, memory
storing and restoring, increment/decrement, ±6 dB/step log taper
adjustment, wiper setting readback, and extra EEMEM1 for userdefined information such as memory data for other components,
look-up table, or system identification information.
1
2
FUNCTIONAL BLOCK DIAGRAM
CS
AD5235
ADDR
DECODE
RDAC1
REGISTER
A1
W1
CLK
SDI
SERIAL
INTERFACE
SDO
PR
WP
RDY
EEMEM1
POWER-ON
RESET
VDD
RDAC1
RDAC2
REGISTER
B1
A2
W2
EEMEM
CONTROL
B2
EEMEM2
RTOL*
26 BYTES
USER EEMEM
RDAC2
VSS
GND
02816-001
Data Sheet
Nonvolatile Memory, Dual
1024-Position Digital Potentiometer
AD5235
*RAB TOLERANCE
Figure 1.
In the scratchpad programming mode, a specific setting can
be programmed directly to the RDAC2 register, which sets the
resistance between Terminal W and Terminal A and Terminal W
and Terminal B. This setting can be stored into the EEMEM
and is restored automatically to the RDAC register during
system power-on.
The EEMEM content can be restored dynamically or through
external PR strobing, and a WP function protects EEMEM
contents. To simplify the programming, the independent or
simultaneous linear-step increment or decrement commands
can be used to move the RDAC wiper up or down, one step at
a time. For logarithmic ±6 dB changes in the wiper setting, the
left or right bit shift command can be used to double or halve the
RDAC wiper setting.
The AD5235 patterned resistance tolerance is stored in the
EEMEM. The actual end-to-end resistance can, therefore, be
known by the host processor in readback mode. The host can
execute the appropriate resistance step through a software
routine that simplifies open-loop applications as well as
precision calibration and tolerance matching applications.
The AD5235 is available in a thin, 16-lead TSSOP package.
The part is guaranteed to operate over the extended industrial
temperature range of −40°C to +85°C.
The terms nonvolatile memory and EEMEM are used interchangeably.
The terms digital potentiometer and RDAC are used interchangeably.
Rev. F
Information furnished by Analog Devices is believed to be accurate and reliable. However, no
responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other
rights of third parties that may result from its use. Specifications subject to change without notice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
Trademarks and registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
www.analog.com
Fax: 781.461.3113 ©2004–2012 Analog Devices, Inc. All rights reserved.
AD5235
Data Sheet
TABLE OF CONTENTS
Features .............................................................................................. 1
Programming the Variable Resistor ......................................... 22
Applications ....................................................................................... 1
Programming the Potentiometer Divider ............................... 22
General Description ......................................................................... 1
Programming Examples ............................................................ 23
Functional Block Diagram .............................................................. 1
EVAL-AD5235SDZ Evaluation Kit .......................................... 23
Revision History ............................................................................... 3
Applications Information .............................................................. 24
Specifications..................................................................................... 4
Bipolar Operation from Dual Supplies.................................... 24
Electrical Characteristics—25 kΩ, 250 kΩ Versions ............... 4
Gain Control Compensation .................................................... 24
Interface Timing and EEMEM Reliability Characteristics—
25 kΩ, 250 kΩ Versions ............................................................... 6
High Voltage Operation ............................................................ 24
Absolute Maximum Ratings ............................................................ 8
Bipolar Programmable Gain Amplifier ................................... 25
ESD Caution .................................................................................. 8
10-Bit Bipolar DAC .................................................................... 25
Pin Configuration and Function Descriptions ............................. 9
Programmable Voltage Source with Boosted Output ........... 25
Typical Performance Characteristics ........................................... 10
Programmable Current Source ................................................ 26
Test Circuits................................................................................. 14
Programmable Bidirectional Current Source ......................... 26
Theory of Operation ...................................................................... 16
Programmable Low-Pass Filter ................................................ 27
Scratchpad and EEMEM Programming.................................. 16
Programmable Oscillator .......................................................... 27
Basic Operation .......................................................................... 16
Optical Transmitter Calibration with ADN2841 ................... 28
EEMEM Protection .................................................................... 17
Resistance Scaling ...................................................................... 28
Digital Input and Output Configuration................................. 17
Serial Data Interface ................................................................... 17
Resistance Tolerance, Drift, and Temperature Coefficient
Mismatch Considerations ......................................................... 29
Daisy-Chain Operation ............................................................. 18
RDAC Circuit Simulation Model ............................................. 29
Terminal Voltage Operating Range .......................................... 18
Outline Dimensions ....................................................................... 30
Advanced Control Modes ......................................................... 20
Ordering Guide .......................................................................... 30
DAC.............................................................................................. 24
RDAC Structure .......................................................................... 21
Rev. F | Page 2 of 32
Data Sheet
AD5235
REVISION HISTORY
6/12—Rev. E to Rev. F
7/04—Rev. A to Rev. B
Changes to Table 1 Conditions ........................................................ 4
Removed Positive Supply Current RDY and/or SDO Floating
Parameters and Negative Supply Current RDY and/or SDO
Floating Parameters, Table 1 ............................................................ 5
Added Endnote 2 to Ordering Guide ...........................................30
Updated Formatting .......................................................... Universal
Edits to Features, General Description, and Block Diagram ...... 1
Changes to Specifications................................................................. 3
Replaced Timing Diagrams ............................................................. 6
Changes to Absolute Maximum Ratings........................................ 7
Changes to Pin Function Descriptions .......................................... 8
Changes to Typical Performance Characteristics ......................... 9
Additional Test Circuit (Figure 36)................................................. 9
Edits to Theory of Operation ........................................................ 14
Edits to Applications ....................................................................... 23
Updated Outline Dimensions........................................................ 27
4/11—Rev. D to Rev. E
Changes to Figure 12 ......................................................................11
4/11—Rev. C to Rev. D
Changes to EEMEM Performance .............................. Throughout
Changes to Features and General Descriptions Sections ............. 1
Changes to Specifications Section................................................... 4
Changes to Pin 5, Pin 13, Pin 14 Descriptions .............................. 9
Changes to Typical Performance Characteristics Section .........10
Changes to Table 7 ..........................................................................19
Changes to Table 9 ..........................................................................21
Changes to Rheostat Operation Section, Table 12, Table 13 .....22
Changes to Table 16, Table 19, and EVAL-AD5235SDZ
Evaluation Kit Section ....................................................................23
Changes to RDAC Circuit Simulation Model Section ...............29
Updated Outline Dimensions ........................................................30
Changes to Ordering Guide ...........................................................30
8/02—Rev. 0 to Rev. A
Change to Features and General Description ............................... 1
Change to Specifications .................................................................. 2
Change to Calculating Actual End-to-End Terminal
Resistance Section ........................................................................... 14
4/09—Rev. B to Rev. C
Changes to Figure 1........................................................................... 1
Changes to Specifications ................................................................. 3
Changes to SDO, Description Column, Table 4............................ 8
Changes to Figure 18 ......................................................................11
Changes to Theory of Operation Section ....................................14
Changes to Serial Data Interface Section .....................................15
Changes to Linear Increment and Decrement Instructions
Section, Logarithmic Taper Mode Adjustment Section, and
Figure 42 ...........................................................................................18
Changes to Rheostat Operations Section .....................................20
Changes to Bipolar Programmable Gain Amplifier Section,
Figure 49, Table 21, and 10-Bit Bipolar DAC Section ................23
Changes to Programmable Oscillator Section and Figure 56 ...25
Changes to Ordering Guide ...........................................................28
Rev. F | Page 3 of 32
AD5235
Data Sheet
SPECIFICATIONS
ELECTRICAL CHARACTERISTICS—25 kΩ, 250 kΩ VERSIONS
VDD = 2.7 V to 5.5 V, VSS = 0 V; VDD = 2.5 V, VSS = −2.5 V, VA = VDD, VB = VSS, −40°C < TA < +85°C, unless otherwise noted.
These specifications apply to versions with a date code 1209 or later.
Table 1.
Parameter
DC CHARACTERISTICS—RHEOSTAT
MODE (All RDACs)
Resistor Differential Nonlinearity 2
Resistor Integral Nonlinearity2
Nominal Resistor Tolerance
Resistance Temperature Coefficient
Wiper Resistance
Nominal Resistance Match
DC CHARACTERISTICS—
POTENTIOMETER DIVIDER MODE
(All RDACs)
Resolution
Differential Nonlinearity 3
Integral Nonlinearity3
Voltage Divider Temperature
Coefficient
Full-Scale Error
Zero-Scale Error
RESISTOR TERMINALS
Terminal Voltage Range 4
Capacitance Ax, Bx 5
Capacitance Wx5
Common-Mode Leakage Current5, 6
DIGITAL INPUTS AND OUTPUTS
Input Logic High
Input Logic Low
Input Logic High
Input Logic Low
Input Logic High
Symbol
Conditions
Min
R-DNL
R-INL
∆RAB/RAB
(∆RAB/RAB)/∆T × 106
RW
RWB
RWB
−1
−2
−8
N
DNL
INL
(∆VW/VW)/∆T × 106
Code = midscale
VWFSE
VWZSE
Code = full scale
Code = zero scale
CW
ICM
VIH
VIL
VIH
VIL
VIH
Input Logic Low
VIL
Output Logic High (SDO, RDY)
VOH
Output Logic Low
VOL
Input Current
Input Capacitance5
IIL
CIL
Max
Unit
+1
+2
+8
LSB
LSB
%
ppm/°C
60
Ω
Ω
%
10
+1
+1
Bits
LSB
LSB
ppm/°C
0
4
LSB
LSB
VDD
11
V
pF
80
pF
35
IW = 1 V/RWB, code = midscale
VDD = 5 V
VDD = 3 V
30
50
±0.1
RAB1/RAB2
VA, VB, VW
CA, CB
Typ 1
−1
−1
15
−6
0
VSS
f = 1 MHz, measured to GND,
code = midscale
f = 1 MHz, measured to GND,
code = midscale
VW = VDD/2
With respect to GND, VDD = 5 V
With respect to GND, VDD = 5 V
With respect to GND, VDD = 3 V
With respect to GND, VDD = 3 V
With respect to GND, VDD = +2.5 V,
VSS = −2.5 V
With respect to GND, VDD = +2.5 V,
VSS = −2.5 V
RPULL-UP = 2.2 kΩ to 5 V (see
Figure 38)
IOL = 1.6 mA, VLOGIC = 5 V (see
Figure 38)
VIN = 0 V or VDD
0.01
2.4
0.8
2.1
0.6
2.0
0.5
4.9
µA
V
V
V
V
V
V
V
5
Rev. F | Page 4 of 32
±1
0.4
V
±1
µA
pF
Data Sheet
Parameter
POWER SUPPLIES
Single-Supply Power Range
Dual-Supply Power Range
Positive Supply Current
Negative Supply Current
AD5235
Symbol
Conditions
Min
VDD
VDD/VSS
IDD
ISS
VSS = 0 V
2.7
±2.25
EEMEM Store Mode Current
IDD (store)
EEMEM Restore Mode Current 7
ISS (store)
IDD (restore)
Power Dissipation 8
Power Supply Sensitivity5
DYNAMIC CHARACTERISTICS5, 9
Bandwidth
Total Harmonic Distortion
VW Settling Time
ISS (restore)
PDISS
PSS
BW
THDW
tS
Resistor Noise Density
Crosstalk (CW1/CW2)
eN_WB
CT
Analog Crosstalk
CTA
VIH = VDD or VIL = GND
VDD = +2.5 V, VSS = −2.5 V
VIH = VDD or VIL = GND
VIH = VDD or VIL = GND,
VSS = GND, ISS ≈ 0
VDD = +2.5 V, VSS = −2.5 V
VIH = VDD or VIL = GND,
VSS = GND, ISS ≈ 0
VDD = +2.5 V, VSS = −2.5 V
VIH = VDD or VIL = GND
∆VDD = 5 V ± 10%
−3 dB, RAB = 25 kΩ/250 kΩ
VA = 1 V rms, VB = 0 V,
f = 1 kHz, code = midscale
RAB = 25 kΩ
RAB = 250 kΩ
VA = VDD, VB = 0 V, VW = 0.50% error
band, from zero scale to midscale
RAB = 25 kΩ
RAB = 250 kΩ
RAB = 25 kΩ/250 kΩ
VA1 = VDD, VB1 = VSS , measured VW2
with VW1 making full-scale change,
RAB = 25 kΩ/250 kΩ
VAB2 = 5 V p-p, f = 1 kHz, measured
VW1, Code 1 = midscale, Code 2 =
full scale, RAB = 25 kΩ/250 kΩ
−4
Typ 1
Max
Unit
2
5.5
±2.75
5
V
V
µA
−2
2
µA
mA
−2
320
mA
µA
−320
10
0.006
30
0.01
µA
µW
%/%
125/12
kHz
0.009
0.035
%
%
4
36
20/64
30/60
µs
µs
nV/√Hz
nV-s
−110/−100
dB
Typicals represent average readings at 25°C and VDD = 5 V.
Resistor position nonlinearity error (R-INL) is the deviation from an ideal value measured between the maximum resistance and the minimum resistance wiper
positions. R-DNL measures the relative step change from ideal between successive tap positions. IWB = (VDD − 1)/RWB (see Figure 27).
3
INL and DNL are measured at VW with the RDAC configured as a potentiometer divider similar to a voltage output DAC. VA = VDD and VB = VSS. DNL specification limits of
±1 LSB maximum are guaranteed monotonic operating conditions (see Figure 28).
4
Resistor Terminal A, Resistor Terminal B, and Resistor Terminal W have no limitations on polarity with respect to each other. Dual-supply operation enables groundreferenced bipolar signal adjustment.
5
Guaranteed by design and not subject to production test.
6
Common-mode leakage current is a measure of the dc leakage from any Terminal A, Terminal B, or Terminal W to a common-mode bias level of VDD/2.
7
EEMEM restore mode current is not continuous. Current is consumed while EEMEM locations are read and transferred to the RDAC register.
8
PDISS is calculated from (IDD × VDD) + (ISS × VSS).
9
All dynamic characteristics use VDD = +2.5 V and VSS = −2.5 V.
1
2
Rev. F | Page 5 of 32
AD5235
Data Sheet
INTERFACE TIMING AND EEMEM RELIABILITY CHARACTERISTICS—25 kΩ, 250 kΩ VERSIONS
Guaranteed by design and not subject to production test. See the Timing Diagrams section for the location of measured values. All input
control voltages are specified with tR = tF = 2.5 ns (10% to 90% of 3 V) and timed from a voltage level of 1.5 V. Switching characteristics are
measured using both VDD = 2.7 V and VDD = 5 V.
Table 2.
Parameter
Clock Cycle Time (tCYC)
CS Setup Time
CLK Shutdown Time to CS Rise
Input Clock Pulse Width
Data Setup Time
Data Hold Time
CS to SDO-SPI Line Acquire
CS to SDO-SPI Line Release
CLK to SDO Propagation Delay 2
CLK to SDO Data Hold Time
CS High Pulse Width 3
CS High to CS High3
RDY Rise to CS Fall
CS Rise to RDY Fall Time
Store EEMEM Time 4, 5
Read EEMEM Time4
CS Rise to Clock Rise/Fall Setup
Preset Pulse Width (Asynchronous) 6
Preset Response Time to Wiper Setting6
Power-On EEMEM Restore Time6
FLASH/EE MEMORY RELIABILITY
Endurance 7
Symbol
t1
t2
t3
t4, t5
t6
t7
t8
t9
t10
t11
t12
t13
t14
t15
t16
t16
t17
tPRW
tPRESP
tEEMEM
Conditions
Clock level high or low
From positive CLK transition
From positive CLK transition
RP = 2.2 kΩ, CL < 20 pF
RP = 2.2 kΩ, CL < 20 pF
Min
20
10
1
10
5
5
Typ 1
40
50
50
0
10
4
0
0.15
15
7
Applies to Instructions 0x2, 0x3
Applies to Instructions 0x8, 0x9, 0x10
10
50
PR pulsed low to refresh wiper positions
30
30
TA = 25°C
1
100
Data Retention 8
Max
100
0.3
50
30
Unit
ns
ns
tCYC
ns
ns
ns
ns
ns
ns
ns
ns
tCYC
ns
ms
ms
µs
ns
ns
µs
µs
MCycles
kCycles
Years
Typicals represent average readings at 25°C and VDD = 5 V.
Propagation delay depends on the value of VDD, RPULL-UP, and CL.
3
Valid for commands that do not activate the RDY pin.
4
The RDY pin is low only for Instruction 2, Instruction 3, Instruction 8, Instruction 9, Instruction 10, and the PR hardware pulse: CMD_8 ~ 20 µs; CMD_9, CMD_10 ~ 7 µs;
CMD_2, CMD_3 ~ 15 ms; PR hardware pulse ~ 30 µs.
5
Store EEMEM time depends on the temperature and EEMEM writes cycles. Higher timing is expected at a lower temperature and higher write cycles.
6
Not shown in Figure 2 and Figure 3.
7
Endurance is qualified to 100,000 cycles per JEDEC Standard 22, Method A117 and measured at −40°C, +25°C, and +85°C.
8
Retention lifetime equivalent at junction temperature (TJ) = 85°C per JEDEC Standard 22, Method A117. Retention lifetime based on an activation energy of 1 eV
derates with junction temperature in the Flash/EE memory.
1
2
Rev. F | Page 6 of 32
Data Sheet
AD5235
Timing Diagrams
CPHA = 1
CS
t12
t13
t3
t1
t2
CLK
CPOL = 1
t5
B23
B0
t17
t4
t7
SDI
t6
HIGH
OR LOW
B23 (MSB)
t8
t11
t10
B23 (MSB)
B24*
SDO
HIGH
OR LOW
B0 (LSB)
t9
B0 (LSB)
t14
t15
t16
02816-002
RDY
*THE EXTRA BIT THAT IS NOT DEFINED IS NORMALLY THE LSB OF THE CHARACTER PREVIOUSLY TRANSMITTED.
THE CPOL = 1 MICROCONTROLLER COMMAND ALIGNS THE INCOMING DATA TO THE POSITIVE EDGE OF THE CLOCK.
Figure 2. CPHA = 1 Timing Diagram
CPHA = 0
CS
t12
t1
t2
B23
CLK
CPOL = 0
t3
t5
t13
t17
B0
t4
t7
t6
SDI
HIGH
OR LOW
HIGH
OR LOW
B23 (MSB IN)
B0 (LSB)
t10
t8
t11
t9
SDO
B23 (MSB OUT)
B0 (LSB)
*
t14
t15
t16
*THE EXTRA BIT THAT IS NOT DEFINED IS NORMALLY THE MSB OF THE CHARACTER JUST RECEIVED.
THE CPOL = 0 MICROCONTROLLER COMMAND ALIGNS THE INCOMING DATA TO THE POSITIVE EDGE OF THE CLOCK.
Figure 3. CPHA = 0 Timing Diagram
Rev. F | Page 7 of 32
02816-003
RDY
AD5235
Data Sheet
ABSOLUTE MAXIMUM RATINGS
TA = 25°C, unless otherwise noted.
Table 3.
Parameter
VDD to GND
VSS to GND
VDD to VSS
VA, VB, VW to GND
IA, IB, IW
Pulsed 1
Continuous
Digital Input and Output Voltage to GND
Operating Temperature Range 2
Maximum Junction Temperature (TJ max)
Storage Temperature Range
Lead Temperature, Soldering
Vapor Phase (60 sec)
Infrared (15 sec)
Thermal Resistance
Junction-to-Ambient θJA,TSSOP-16
Junction-to-Case θJC, TSSOP-16
Package Power Dissipation
Rating
–0.3 V to +7 V
+0.3 V to −7 V
7V
VSS − 0.3 V to VDD + 0.3 V
±20 mA
±2 mA
−0.3 V to VDD + 0.3 V
−40°C to +85°C
150°C
−65°C to +150°C
Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.
ESD CAUTION
215°C
220°C
150°C/W
28°C/W
(TJ max − TA)/θJA
Maximum terminal current is bounded by the maximum current handling of
the switches, maximum power dissipation of the package, and maximum
applied voltage across any two of the A, B, and W terminals at a given
resistance.
2
Includes programming of nonvolatile memory.
1
Rev. F | Page 8 of 32
Data Sheet
AD5235
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
16
RDY
2
15
CS
SDO 3
14
PR
AD5235
13
WP
TOP VIEW
(Not to Scale)
12
VDD
A1 6
11
A2
W1 7
10
W2
B1 8
9
B2
SDI
GND
4
VSS 5
02816-005
CLK 1
Figure 4. Pin Configuration
Table 4. Pin Function Descriptions
Pin No.
1
2
3
Mnemonic
CLK
SDI
SDO
4
5
GND
VSS
6
7
8
9
10
11
12
13
A1
W1
B1
B2
W2
A2
VDD
WP
14
PR
15
16
CS
RDY
Description
Serial Input Register Clock. Shifts in one bit at a time on positive clock edges.
Serial Data Input. Shifts in one bit at a time on positive clock CLK edges. MSB loads first.
Serial Data Output. Serves readback and daisy-chain functions. Command 9 and Command 10 activate the SDO
output for the readback function, delayed by 24 or 25 clock pulses, depending on the clock polarity before and
after the data-word (see Figure 2 and Figure 3). In other commands, the SDO shifts out the previously loaded SDI
bit pattern, delayed by 24 or 25 clock pulses depending on the clock polarity (see Figure 2 and Figure 3). This
previously shifted out SDI can be used for daisy-chaining multiple devices. Whenever SDO is used, a pull-up
resistor in the range of 1 kΩ to 10 kΩ is needed.
Ground Pin, Logic Ground Reference.
Negative Supply. Connect to 0 V for single-supply applications. If VSS is used in dual supply, it must be able to sink
2 mA for 15 ms when storing data to EEMEM.
Terminal A of RDAC1.
Wiper terminal of RDAC1. ADDR (RDAC1) = 0x0.
Terminal B of RDAC1.
Terminal B of RDAC2.
Wiper terminal of RDAC2. ADDR (RDAC2) = 0x1.
Terminal A of RDAC2.
Positive Power Supply.
Optional Write Protect. When active low, WP prevents any changes to the present contents, except PR strobe.
CMD_1 and COMD_8 refresh the RDAC register from EEMEM. Tie WP to VDD, if not used.
Optional Hardware Override Preset. Refreshes the scratchpad register with current contents of the EEMEM
register. Factory default loads midscale until EEMEM is loaded with a new value by the user. PR is activated
at the logic high transition. Tie PR to VDD, if not used.
Serial Register Chip Select Active Low. Serial register operation takes place when CS returns to logic high.
Ready. Active high open-drain output. Identifies completion of Instruction 2, Instruction 3, Instruction 8,
Instruction 9, Instruction 10, and PR.
Rev. F | Page 9 of 32
AD5235
Data Sheet
TYPICAL PERFORMANCE CHARACTERISTICS
0.20
0.20
+85°C
+25°C
–40°C
0.15
0.15
DNL ERROR (LSB)
0.05
0
–0.05
0.05
0
–0.05
–0.10
0
200
400
600
800
1000
DIGITAL CODE
–0.15
02816-006
–0.20
Figure 5. INL vs. Code, TA = −40°C, +25°C, +85°C Overlay, RAB = 25 kΩ
0
200
400
600
800
Figure 8. R-DNL vs. Code, TA = −40°C, +25°C, +85°C Overlay, RAB = 25 kΩ
200
0.16
0.14
POTENTIOMETER MODE TEMPCO (ppm/°C)
+85°C
+25°C
–40°C
0.12
0.10
0.08
0.06
0.04
0.02
0
–0.02
1000
DIGITAL CODE
02816-009
–0.10
–0.15
25kΩ
250kΩ
180
160
140
120
100
80
60
40
20
0
0
200
400
600
800
1000
DIGITAL CODE
0
02816-007
–0.04
256
512
768
Figure 9. (∆VW/VW)/∆T × 106 Potentiometer Mode Tempco
Figure 6. DNL vs. Code, TA = −40°C, +25°C, +85°C Overlay, RAB = 25 kΩ
200
0.20
+85°C
+25°C
–40°C
25kΩ
250kΩ
180
RHEOSTAT MODE TEMPCO (ppm/°C)
0.15
0.10
0.05
0
–0.05
–0.10
1023
CODE (Decimal)
02816-010
DNL ERROR (LSB)
0.10
–0.15
160
140
120
100
80
60
40
–0.20
0
200
400
600
DIGITAL CODE
800
1000
02816-008
20
Figure 7. R-INL vs. Code, TA = −40°C, +25°C, +85°C Overlay, RAB = 25 kΩ
Rev. F | Page 10 of 32
0
0
256
512
768
CODE (Decimal)
Figure 10. (∆RWB/RWB)/∆T × 106 Rheostat Mode Tempco
1023
02816-011
INL ERROR (LSB)
0.10
INL ERROR (LSB)
+85°C
+25°C
–40°C
Data Sheet
AD5235
60
2.7V
3.0V
3.3V
5.0V
5.5V
40
300
IDD (µA)
30
200
20
100
10
0
200
400
600
800
1000
CODE (Decimal)
0
02816-012
0
0
1
2
3
4
02816-015
WIPER ON RESISTANCE (Ω)
50
2.7V
3.0V
3.3V
5.0V
5.5V
400
5
VDIO (V)
Figure 11. Wiper On Resistance vs. Code
Figure 14. IDD vs. Digital Input Voltage
3
IDD
IDD
IDD
IDD
IDD
2
0.12
= 2.7V
= 3.3V
= 3.0V
= 5.0V
= 5.5V
0.10
250kΩ
0.08
THD + N (%)
IDD/ISS (µA)
1
0
–1
0.06
0.04
–20
25kΩ
0.02
0
20
40
60
80 85
TEMPERATURE (°C)
0
10
100
1k
10k
100k
FREQUENCY (Hz)
Figure 12. IDD vs. Temperature
02816-115
–3
–40
= 2.7V
= 3.3V
= 3.0V
= 5.0V
= 5.5V
02816-013
–2
ISS
ISS
ISS
ISS
ISS
Figure 15. THD + Noise vs. Frequency
50
10
FULL SCALE
MIDSCALE
ZERO SCALE
40
1
0.1
250kΩ
25kΩ
0.01
10
0
1
2
3
4
5
6
7
8
FREQUENCY (MHz)
9
10
Figure 13. IDD vs. Clock Frequency, RAB = 25 kΩ
0.001
0.0001
0.001
0.01
0.1
AMPLITUDE (V rms)
Figure 16. THD + Noise vs. Amplitude
Rev. F | Page 11 of 32
1
10
02816-116
THD + N (%)
20
02816-014
I DD (µA)
30
AD5235
Data Sheet
0
3
VDD = 5V ± 10% AC
VSS = 0V, VA = 4V, VB = 0V
MEASURED AT VW WITH CODE = 0x200
TA = 25°C
–10
0
–20
–3
PSRR (dB)
GAIN (dB)
RAB = 25kΩ
RAB = 250kΩ
–6
f–3dB = 12kHz
f–3dB = 125kHz
RAB = 250kΩ
–30
RAB = 25kΩ
–40
–50
–60
–9
VDD/VSS = ±2.5V
VA = 1V rms
D = MIDSCALE
100k
1M
FREQUENCY (Hz)
–80
10
100
1k
10k
100k
1M
FREQUENCY (Hz)
02816-019
10k
02816-016
–12
1k
–70
Figure 20. PSRR vs. Frequency
Figure 17. −3 dB Bandwidth vs. Resistance (See Figure 33)
0
CODE 0x200
–10
0x100
0x080
GAIN (dB)
–20
VDD
0x040
0x020
–30
0x010
VW (FULL SCALE)
0x008
–40
0x004
–50
VDD = 5V
VA = 5V
VB = 0V
TA = 25°C
1V/DIV
0x001
–60
10k
100k
1M
FREQUENCY (Hz)
10µs/DIV
02816-017
1k
Figure 21. Power-On Reset
Figure 18. Gain vs. Frequency vs. Code, RAB = 25 kΩ (See Figure 33)
2.5196
0
CODE 0x200
–10
2.512
0x080
AMPLITUDE (V)
GAIN (dB)
2.508
0x040
0x020
–30
0x010
–40
0x008
2.504
2.500
2.496
2.492
0x004
–50
VDD = VSS = 5V
CODE = 0x200 TO 0x1FF
2.516
0x100
–20
02816-020
0x002
2.488
0x002
2.484
0x001
–60
100k
1M
FREQUENCY (Hz)
0
20
40
60
80
100
120
TIME (µs)
Figure 22. Midscale Glitch Energy, RAB = 25 kΩ
Figure 19. Gain vs. Frequency vs. Code, RAB = 250 kΩ (See Figure 33)
Rev. F | Page 12 of 32
144
02816-021
10k
02816-018
2.4796
1k
Data Sheet
AD5235
2.60
2.3795
2.376
2.372
WIPER VOLTAGE (V)
2.55
AMPLITUDE (V)
2.368
2.364
2.360
2.356
2.352
2.50
2.45
2.348
2.40
2.3399
60
80
100
120
144
TIME (µs)
0
0.5
1.0
1.5
Figure 25. Digital Feedthrough
Figure 23. Midscale Glitch Energy, RAB = 250 kΩ
THEORECTICAL (IWB_MAX – mA)
100
CS (5V/DIV)
2.0
TIME (µs)
VDD = 5V
TA = 25°C
CLK (5V/DIV)
SDI (5V/DIV)
VA = VB = OPEN
TA = 25°C
10
1
RAB = 25kΩ
0.1
RAB = 250kΩ
0.01
IDD (2mA/DIV)
0
128
256
384
512
640
CODE (Decimal)
Figure 26. IWB_MAX vs. Code
Figure 24. IDD vs. Time when Storing Data to EEMEM
Rev. F | Page 13 of 32
768
896
1023
02816-025
40
02816-022
20
02816-023
0
02816-126
2.344
AD5235
Data Sheet
TEST CIRCUITS
Figure 27 to Figure 37 define the test conditions used in the Specifications section.
NC
A
DUT B
5V
IW
W
VIN
VMS
OFFSET BIAS
02816-026
NC = NO CONNECT
VOUT
OP279
OFFSET
GND
B
Figure 27. Resistor Position Nonlinearity Error (Rheostat Operation; R-INL, R-DNL)
02816-030
DUT
A
W
Figure 31. Inverting Gain
5V
W
B
W
OFFSET
GND
A
02816-027
VMS
A
W
VMS2
W
VIN
VW
DUT
2.5V
02816-028
RW = [VMS1 – VMS2]/IW
0.1V
ISW
CODE = 0x00
PSRR (dB) = 20 LOG
VMS
PSS (%/%) =
(
ΔVDD
W
+
)
B
0.1V
ISW
–
ΔVMS%
ΔVDD%
02816-029
W
ΔVMS
A = NC
Figure 30. Power Supply Sensitivity (PSS, PSRR)
VSS TO VDD
02816-033
V+ = VDD ±10%
B
–15V
RSW =
VA
~
VOUT
Figure 33. Gain vs. Frequency
DUT
V+
OP42
B
OFFSET
GND
Figure 29. Wiper Resistance
A
+15V
A
B
VDD
B
Figure 32. Noninverting Gain
IW = VDD/RNOMINAL
VMS1
DUT
OFFSET BIAS
Figure 28. Potentiometer Divider Nonlinearity Error (INL, DNL)
DUT
VOUT
02816-031
A
V+
OP279
VIN
V+ = VDD
1LSB = V+/2N
02816-032
DUT
Figure 34. Incremental On Resistance
Rev. F | Page 14 of 32
Data Sheet
AD5235
NC
A
VSS GND
B
ICM
W
TO OUTPUT
PIN
02816-034
VCM
NC
NC = NO CONNECT
VDD
NC
B1
A2
RDAC2
W2
W1
VSS
VOUT
B2
CTA = 20 LOG[VOUT/VIN]
NC = NO CONNECT
CL
50pF
IOH
Figure 37. Load Circuit for Measuring VOH and VOL
(The diode bridge test circuit is equivalent to the
application circuit with RPULL-UP of 2.2 kΩ.)
02816-035
A1
RDAC1
VOH (MIN)
OR
VOL (MAX)
200µA
Figure 35. Common-Mode Leakage Current
VIN
IOL
02816-036
VDD
DUT
200µA
Figure 36. Analog Crosstalk
Rev. F | Page 15 of 32
AD5235
Data Sheet
THEORY OF OPERATION
The AD5235 digital potentiometer is designed to operate as a
true variable resistor. The resistor wiper position is determined
by the RDAC register contents. The RDAC register acts as a
scratchpad register, allowing unlimited changes of resistance
settings. The scratchpad register can be programmed with any
position setting using the standard SPI serial interface by loading
the 24-bit data-word. In the format of the data-word, the first four
bits are commands, the following four bits are addresses, and the
last 16 bits are data. When a specified value is set, this value can
be stored in a corresponding EEMEM register. During subsequent
power-ups, the wiper setting is automatically loaded to that value.
Storing data to the EEMEM register takes about 15 ms and
consumes approximately 2 mA. During this time, the shift
register is locked, preventing any changes from taking place.
The RDY pin pulses low to indicate the completion of this
EEMEM storage. There are also 13 addresses with two bytes
each of user-defined data that can be stored in the EEMEM
register from Address 2 to Address 14.
The scratchpad RDAC register directly controls the position of
the digital potentiometer wiper. For example, when the scratchpad
register is loaded with all 0s, the wiper is connected to Terminal B
of the variable resistor. The scratchpad register is a standard
logic register with no restriction on the number of changes
allowed, but the EEMEM registers have a program erase/write
cycle limitation.
BASIC OPERATION
The basic mode of setting the variable resistor wiper position
(programming the scratchpad register) is accomplished by
loading the serial data input register with Instruction 11 (0xB),
Address 0, and the desired wiper position data. When the proper
wiper position is determined, the user can load the serial data
input register with Instruction 2 (0x2), which stores the wiper
position data in the EEMEM register. After 15 ms, the wiper
position is permanently stored in nonvolatile memory.
Table 5 provides a programming example listing the sequence
of the serial data input (SDI) words with the serial data output
appearing at the SDO pin in hexadecimal format.
The following instructions facilitate the programming needs
of the user (see Table 7 for details):
0.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
SCRATCHPAD AND EEMEM PROGRAMMING
Do nothing.
Restore EEMEM content to RDAC.
Store RDAC setting to EEMEM.
Store RDAC setting or user data to EEMEM.
Decrement by 6 dB.
Decrement all by 6 dB.
Decrement by one step.
Decrement all by one step.
Reset EEMEM content to RDAC.
Read EEMEM content from SDO.
Read RDAC wiper setting from SDO.
Write data to RDAC.
Increment by 6 dB.
Increment all by 6 dB.
Increment by one step.
Increment all by one step.
Table 5. Write and Store RDAC Settings to EEMEM Registers
Table 14 to Table 20 provide programming examples that use
some of these commands.
SDI
0xB00100
SDO
0xXXXXXX
0x20XXXX
0xB00100
0xB10200
0x20XXXX
0x21XXXX
0xB10200
Action
Writes data 0x100 to the RDAC1 register,
Wiper W1 moves to 1/4 full-scale position.
Stores RDAC1 register content into the
EEMEM1 register.
Writes Data 0x200 to the RDAC2 register,
Wiper W2 moves to 1/2 full-scale position.
Stores RDAC2 register contents into the
EEMEM2 register.
At system power-on, the scratchpad register is automatically
refreshed with the value previously stored in the corresponding
EEMEM register. The factory-preset EEMEM value is midscale.
The scratchpad register can also be refreshed with the contents
of the EEMEM register in three different ways. First, executing
Instruction 1 (0x1) restores the corresponding EEMEM value.
Second, executing Instruction 8 (0x8) resets the EEMEM values
of both channels. Finally, pulsing the PR pin refreshes both
EEMEM settings. Operating the hardware control PR function
requires a complete pulse signal. When PR goes low, the internal
logic sets the wiper at midscale. The EEMEM value is not
loaded until PR returns high.
Rev. F | Page 16 of 32
Data Sheet
AD5235
EEMEM PROTECTION
VDD
WP
INPUT
300Ω
DIGITAL INPUT AND OUTPUT CONFIGURATION
All digital inputs are ESD protected, high input impedance that
can be driven directly from most digital sources. Active at logic
low, PR and WP must be tied to VDD, if they are not used. No
internal pull-up resistors are present on any digital input pins.
To avoid floating digital pins that might cause false triggering
in a noisy environment, add pull-up resistors. This is applicable
when the device is detached from the driving source when it is
programmed.
The SDO and RDY pins are open-drain digital outputs that only
need pull-up resistors if these functions are used. To optimize
the speed and power trade-off, use 2.2 kΩ pull-up resistors.
The equivalent serial data input and output logic is shown in
Figure 38. The open-drain output SDO is disabled whenever
chip-select (CS) is in logic high. ESD protection of the digital
inputs is shown in Figure 39 and Figure 40.
PR
VALID
COMMAND
COUNTER
WP
COMMAND
PROCESSOR
AND ADDRESS
DECODE
5V
RPULL-UP
CLK
SERIAL
REGISTER
(FOR DAISY
CHAIN ONLY)
SDO
CS
02816-037
GND
AD5235
SDI
Figure 38. Equivalent Digital Input and Output Logic
VDD
Figure 40. Equivalent WP Input Protection
SERIAL DATA INTERFACE
The AD5235 contains a 4-wire SPI-compatible digital interface
(SDI, SDO, CS, and CLK). The 24-bit serial data-word must be
loaded with MSB first. The format of the word is shown in Table 6.
The command bits (C0 to C3) control the operation of the digital
potentiometer according to the command shown in Table 7. A0
to A3 are the address bits. A0 is used to address RDAC1 or RDAC2.
Address 2 to Address 14 are accessible by users for extra EEMEM.
Address 15 is reserved for factory usage. Table 9 provides an
address map of the EEMEM locations. D0 to D9 are the values
for the RDAC registers. D0 to D15 are the values for the EEMEM
registers.
The AD5235 has an internal counter that counts a multiple of
24 bits (a frame) for proper operation. For example, AD5235
works with a 24-bit or 48-bit word, but it cannot work properly
with a 23-bit or 25-bit word. To prevent data from mislocking
(due to noise, for example), the counter resets, if the count is not a
multiple of four when CS goes high but remains in the register if it
is multiple of four. In addition, the AD5235 has a subtle feature
that, if CS is pulsed without CLK and SDI, the part repeats the
previous command (except during power-up). As a result, care
must be taken to ensure that no excessive noise exists in the CLK or
CS line that might alter the effective number-of-bits pattern.
The SPI interface can be used in two slave modes: CPHA = 1,
CPOL = 1 and CPHA = 0, CPOL = 0. CPHA and CPOL refer
to the control bits that dictate SPI timing in the following
MicroConverters® and microprocessors: ADuC812, ADuC824,
M68HC11, MC68HC16R1, and MC68HC916R1.
INPUTS
300Ω
GND
02816-038
LOGIC
PINS
GND
02816-039
The write protect (WP) pin disables any changes to the
scratchpad register contents, except for the EEMEM setting,
which can still be restored using Instruction 1, Instruction 8,
and the PR pulse. Therefore, WP can be used to provide a
hardware EEMEM protection feature.
Figure 39. Equivalent ESD Digital Input Protection
Rev. F | Page 17 of 32
AD5235
Data Sheet
DAISY-CHAIN OPERATION
The serial data output pin (SDO) serves two purposes. It can be
used to read the contents of the wiper setting and EEMEM values
using Instruction 10 and Instruction 9, respectively. The remaining
instructions (Instruction 0 to Instruction 8, Instruction 11 to
Instruction 15) are valid for daisy-chaining multiple devices in
simultaneous operations. Daisy-chaining minimizes the number
of port pins required from the controlling IC (see Figure 41). The
SDO pin contains an open-drain N-Ch FET that requires a pull-up
resistor, if this function is used. As shown in Figure 41, users need
to tie the SDO pin of one package to the SDI pin of the next package.
Users may need to increase the clock period because the pull-up
resistor and the capacitive loading at the SDO-to-SDI interface may
require additional time delay between subsequent devices.
When two AD5235s are daisy-chained, 48 bits of data are
required. The first 24 bits (formatted 4-bit command, 4-bit
address, and 16-bit data) go to U2, and the second 24 bits with
the same format go to U1. Keep CS low until all 48 bits are
clocked into their respective serial registers. CS is then pulled
high to complete the operation.
VDD
AD5235
SDI U1 SDO
CS
CLK
AD5235
CLK
CS
Figure 41. Daisy-Chain Configuration Using SDO
TERMINAL VOLTAGE OPERATING RANGE
The positive VDD and negative VSS power supplies of the AD5235
define the boundary conditions for proper 3-terminal digital
potentiometer operation. Supply signals present on Terminal A,
Terminal B, and Terminal W that exceed VDD or VSS are clamped by
the internal forward-biased diodes (see Figure 42).
VDD
Power-Up Sequence
Because there are diodes to limit the voltage compliance at
Terminal A, Terminal B, and Terminal W (see Figure 42), it
is important to power VDD and VSS first before applying any
voltage to Terminal A, Terminal B, and Terminal W. Otherwise,
the diode is forward-biased such that VDD and VSS are powered
unintentionally. For example, applying 5 V across Terminal A
and Terminal B prior to VDD causes the VDD terminal to exhibit
4.3 V. It is not destructive to the device, but it might affect the
rest of the user’s system. The ideal power-up sequence is GND,
VDD and VSS, digital inputs, and VA, VB, and VW. The order of
powering VA, VB, VW, and the digital inputs is not important as
long as they are powered after VDD and VSS.
Regardless of the power-up sequence and the ramp rates of the
power supplies, when VDD and VSS are powered, the power-on
preset activates, which restores the EEMEM values to the RDAC
registers.
SDI U2 SDO
02816-040
MOSI
MICROCONTROLLER
SCLK SS
RP
2.2kΩ
The GND pin of the AD5235 is primarily used as a digital
ground reference. To minimize the digital ground bounce,
the AD5235 ground terminal should be joined remotely to
the common ground (see Figure 43). The digital input control
signals to the AD5235 must be referenced to the device ground
pin (GND) and must satisfy the logic level defined in the
Specifications section. An internal level-shift circuit ensures
that the common-mode voltage range of the three terminals
extends from VSS to VDD, regardless of the digital input level.
Layout and Power Supply Bypassing
It is a good practice to employ compact, minimum lead-length
layout design. The leads to the input should be as direct as
possible with a minimum conductor length. Ground paths
should have low resistance and low inductance.
Similarly, it is good practice to bypass the power supplies with
quality capacitors for optimum stability. Bypass supply leads to
the device with 0.01 μF to 0.1 μF disk or chip ceramic capacitors.
Also, apply low ESR, 1 μF to 10 μF tantalum or electrolytic
capacitors at the supplies to minimize any transient disturbance
(see Figure 43).
AD5235
A
VDD
C3
10µF
+
W
C1
0.1µF
C4
10µF
+
B
C2
0.1µF
VSS
VDD
VSS
02816-041
VSS
02816-042
GND
Figure 43. Power Supply Bypassing
Figure 42. Maximum Terminal Voltages Set by VDD and VSS
Rev. F | Page 18 of 32
Data Sheet
AD5235
In Table 6, command bits are C0 to C3, address bits are A0 to A3, Data Bit D0 to Data Bit D9 are applicable to RDAC, and D0 to D15 are
applicable to EEMEM.
Table 6. 24-Bit Serial Data-Word
RDAC
EEMEM
MSB
C3 C2
C3 C2
C1
C1
Command Byte 0
C0 0
0
0
C0 A3 A2 A1
A0
A0
X
D15
X
D14
X
D13
Data Byte 1
X
X
D12 D11
X
D10
D9
D9
D8
D8
D7
D7
D6
D6
Data Byte 0
D5 D4 D3
D5 D4 D3
D2
D2
D1
D1
LSB
D0
D0
Command instruction codes are defined in Table 7.
Table 7. Command Operation Truth Table 1, 2, 3
Command Byte 0
D9
X
B8
D8
X
Data Byte 0
B7
B0
D7 … D0
X
… X
…
X
X
X
…
X
X
…
X
X
X
…
X
A0
D15
…
D8
D7
…
D0
0
A0
X
…
X
X
X
…
X
X
X
X
X
…
X
X
X
…
X
0
0
0
A0
X
…
X
X
X
…
X
1
X
X
X
X
X
…
X
X
X
…
X
0
0
0
0
0
0
X
…
X
X
X
…
X
0
0
1
A3
A2
A1
A0
X
…
X
X
X
…
X
1
0
1
0
0
0
0
A0
X
…
X
X
X
…
X
11
1
0
1
1
0
0
0
A0
X
…
D9
D8
D7
…
D0
125
1
1
0
0
0
0
0
A0
X
…
X
X
X
…
X
135
1
1
0
1
X
X
X
X
X
…
X
X
X
…
X
145
1
1
1
0
0
0
0
A0
X
…
X
X
X
…
X
155
1
1
1
1
X
X
X
X
X
…
X
X
X
…
X
Command
Number
0
B23
C3
0
C2
0
C1
0
C0
0
A3
X
A2
X
1
0
0
0
1
0
2
0
0
1
0
34
0
0
1
45
0
1
55
0
65
Data Byte 1
A1
X
B16
A0
X
B15
X
X
…
…
0
0
A0
X
0
0
0
A0
1
A3
A2
A1
0
0
0
0
1
0
1
X
0
1
1
0
75
0
1
1
8
1
0
9
1
10
Operation
NOP. Do nothing. See Table 19
Restore EEMEM (A0) contents to RDAC (A0)
register. See Table 16.
Store wiper setting. Store RDAC (A0) setting to
EEMEM (A0). See Table 15.
Store contents of Serial Register Data Byte 0
and Serial Register Data Bytes 1 (total 16 bits)
to EEMEM (ADDR). See Table 18.
Decrement by 6 dB. Right-shift contents of
RDAC (A0) register, stop at all 0s.
Decrement all by 6 dB. Right-shift contents of
all RDAC registers, stop at all 0s.
Decrement contents of RDAC (A0) by 1,
stop at all 0s.
Decrement contents of all RDAC registers by 1,
stop at all 0s.
Reset. Refresh all RDACs with their corresponding
EEMEM previously stored values.
Read contents of EEMEM (ADDR) from
SDO output in the next frame. See Table 19.
Read RDAC wiper setting from SDO output
in the next frame. See Table 20.
Write contents of Serial Register Data Byte 0
and Serial Register Data Byte 1 (total 10 bits)
to RDAC (A0). See Table 14.
Increment by 6 dB: Left-shift contents of RDAC
(A0), stop at all 1s. See Table 17.
Increment all by 6 dB. Left-shift contents of
all RDAC registers, stop at all 1s.
Increment contents of RDAC (A0) by 1,
stop at all 1s. See Table 15.
Increment contents of all RDAC registers by 1,
stop at all 1s.
The SDO output shifts out the last 24 bits of data clocked into the serial register for daisy-chain operation. Exception: for any instruction following Instruction 9 or
Instruction 10, the selected internal register data is present in Data Byte 0 and Data Byte 1. The instructions following Instruction 9 and Instruction 10 must also be a
full 24-bit data-word to completely clock out the contents of the serial register.
2
The RDAC register is a volatile scratchpad register that is refreshed at power-on from the corresponding nonvolatile EEMEM register.
3
Execution of these operations takes place when the CS strobe returns to logic high.
4
Instruction 3 writes two data bytes (16 bits of data) to EEMEM. In the case of Address 0 and Address 1, only the last 10 bits are valid for wiper position setting.
5
The increment, decrement, and shift instructions ignore the contents of the shift register, Data Byte 0 and Data Byte 1.
1
Rev. F | Page 19 of 32
AD5235
Data Sheet
ADVANCED CONTROL MODES
the RDAC register is then set to Code 1. Similarly, if the data in
the RDAC register is greater than or equal to midscale and the data
is shifted left, then the data in the RDAC register is automatically
set to full scale. This makes the left-shift function as ideal a
logarithmic adjustment as possible.
The AD5235 digital potentiometer includes a set of user
programming features to address the wide number of
applications for these universal adjustment devices.
Key programming features include the following:
•
•
Table 8. Detail Left-Shift and Right-Shift Functions for 6 dB
Step Increment and Decrement
Linear Increment and Decrement Instructions
The increment and decrement instructions (Instruction 14,
Instruction 15, Instruction 6, and Instruction 7) are useful for
linear step adjustment applications. These commands simplify
microcontroller software coding by allowing the controller to
send just an increment or decrement command to the device.
The adjustment can be individual or in a ganged potentiometer
arrangement where both wiper positions are changed at the
same time.
For an increment command, executing Instruction 14
automatically moves the wiper to the next resistance segment
position. The master increment command, Instruction 15,
moves all resistor wipers up by one position.
Logarithmic Taper Mode Adjustment
Four programming instructions produce logarithmic taper
increment and decrement of the wiper position control by
an individual potentiometer or by a ganged potentiometer
arrangement where both wiper positions are changed at the
same time. The 6 dB increment is activated by Instruction 12
and Instruction 13, and the 6 dB decrement is activated by
Instruction 4 and Instruction 5. For example, starting with the
wiper connected to Terminal B, executing 11 increment
instructions (Command Instruction 12) moves the wiper in 6 dB
steps from 0% of the RAB (Terminal B) position to 100% of the RAB
position of the AD5235 10-bit potentiometer. When the wiper
position is near the maximum setting, the last 6 dB increment
instruction causes the wiper to go to the full-scale 1023 code
position. Further 6 dB per increment instructions do not
change the wiper position beyond its full scale (see Table 8).
Left-Shift (+6 dB/Step)
00 0000 0000
00 0000 0001
00 0000 0010
00 0000 0100
00 0000 1000
00 0001 0000
00 0010 0000
00 0100 0000
00 1000 0000
01 0000 0000
10 0000 0000
11 1111 1111
11 1111 1111
Right-Shift(–6 dB/Step)
11 1111 1111
01 1111 1111
00 1111 1111
00 0111 1111
00 0011 1111
00 0001 1111
00 0000 1111
00 0000 0111
00 0000 0011
00 0000 0001
00 0000 0000
00 0000 0000
00 0000 0000
Actual conformance to a logarithmic curve between the data
contents in the RDAC register and the wiper position for each
Right-Shift 4 command and Right-Shift 5 command execution
contains an error only for odd numbers of bits. Even numbers of
bits are ideal. Figure 44 shows plots of log error [20 × log10
(error/code)] for the AD5235. For example, Code 3 log error = 20 ×
log10 (0.5/3) = −15.56 dB, which is the worst case. The log error plot
is more significant at the lower codes (see Figure 44).
0
–20
The 6 dB step increments and 6 dB step decrements are achieved
by shifting the bit internally to the left or right, respectively. The
following information explains the nonideal ±6 dB step adjustment
under certain conditions. Table 8 illustrates the operation of the
shifting function on the RDAC register data bits. Each table row
represents a successive shift operation. Note that the left-shift
12 and 13 instructions were modified such that, if the data in
the RDAC register is equal to zero and the data is shifted left,
Rev. F | Page 20 of 32
–40
–60
–80
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
CODE (From 1 to 1023 by 2.0 × 103)
Figure 44. Log Error Conformance for Odd Numbers of Bits Only
(Even Numbers of Bits Are Ideal)
02816-043
•
The Right-Shift 4 instruction and Right-Shift 5 instruction are
ideal only if the LSB is 0 (ideal logarithmic = no error). If the
LSB is 1, the right-shift function generates a linear half-LSB
error, which translates to a number-of-bits dependent logarithmic
error, as shown in Figure 44. Figure 44 shows the error of the odd
numbers of bits for the AD5235.
Scratchpad programming to any desirable values
Nonvolatile memory storage of the scratchpad RDAC
register value in the EEMEM register
Increment and decrement instructions for the RDAC
wiper register
Left and right bit shift of the RDAC wiper register to
achieve ±6 dB level changes
26 extra bytes of user-addressable nonvolatile memory
GAIN (dB)
•
•
Data Sheet
AD5235
For example, if RAB_RATED = 250 kΩ and the data in the SDO
shows XXXX XXXX 1001 1100 0000 1111, RAB_ACTUAL can be
calculated as follows:
Using CS to Re-Execute a Previous Command
Another subtle feature of the AD5235 is that a subsequent CS
strobe, without clock and data, repeats a previous command.
MSB: 1 = positive
Next 7 LSB: 001 1100 = 28
8 LSB: 0000 1111 = 15 × 2−8 = 0.06
% tolerance = 28.06%
Therefore, RAB_ACTUAL = 320.15 kΩ
Using Additional Internal Nonvolatile EEMEM
The AD5235 contains additional user EEMEM registers for
storing any 16-bit data such as memory data for other components,
look-up tables, or system identification information. Table 9 provides an address map of the internal storage registers shown in the
functional block diagram (see Figure 1) as EEMEM1, EEMEM2,
and 26 bytes (13 addresses × 2 bytes each) of User EEMEM.
RDAC STRUCTURE
The patent-pending RDAC contains multiple strings of equal
resistor segments with an array of analog switches that acts as
the wiper connection. The number of positions is the resolution of
the device. The AD5235 has 1024 connection points, allowing it to
provide better than 0.1% setability resolution. Figure 45 shows
an equivalent structure of the connections among the three
terminals of the RDAC. The SWA and SWB are always on, while
the switches, SW(0) to SW(2N − 1), are on one at a time, depending
on the resistance position decoded from the data bits. Because
the switch is not ideal, there is a 50 Ω wiper resistance, RW.
Wiper resistance is a function of supply voltage and temperature.
The lower the supply voltage or the higher the temperature, the
higher the resulting wiper resistance. Users should be aware of
the wiper resistance dynamics, if accurate prediction of the
output resistance is needed.
Table 9. EEMEM Address Map
Address
0000
0001
0010
0011
…
1110
1111
EEMEM Content for …
RDAC11
RDAC2
USER12
USER2
…
USER13
RAB1 tolerance3
RDAC data stored in EEMEM locations is transferred to the corresponding
RDAC register at power-on, or when Instruction 1, Instruction 8, and PR are
executed.
2
USERx are internal nonvolatile EEMEM registers available to store and
retrieve constants and other 16-bit information using Instruction 3 and
Instruction 9, respectively.
3
Read only.
1
SWA
A
Calculating Actual End-to-End Terminal Resistance
SW(2N–1)
The resistance tolerance is stored in the EEMEM register during
factory testing. The actual end-to-end resistance can, therefore,
be calculated, which is valuable for calibration, tolerance matching,
and precision applications. Note that this value is read only and
the RAB2 matches with RAB1, typically 0.1%.
RDAC
WIPER
REGISTER
AND
DECODER
The resistance tolerance in percentage is contained in the last
16 bits of data in EEMEM Register 15. The format is the sign
magnitude binary format with the MSB designate for sign
(0 = negative and 1 = positive), the next 7 MSB designate the
integer number, and the 8 LSB designate the decimal number
(see Table 11).
RS
W
SW(2N–2)
RS
SW(1)
RS
SW(0)
RS = RAB/2N
DIGITAL
CIRCUITRY
OMITTED FOR
CLARITY
SWB
B
02816-044
EEMEM No.
1
2
3
4
…
15
16
Figure 45. Equivalent RDAC Structure
Table 10. Nominal Individual Segment Resistor Values
Device Resolution
1024-Step
25 kΩ
24.4
250 kΩ
244
Table 11. Calculating End-to-End Terminal Resistance
Bit
Sign
Mag
D15
D14
D13
Sign
26
25
D12
D11
D10
D9
24
23
22
21
7 Bits for Integer Number
D8
20
.
Decimal
Point
Rev. F | Page 21 of 32
D7
D6
D5
D4
D3
D2
D1
D0
2−1
2−2
2−3
2−4
2−5
2−6
8 Bits for Decimal Number
2−7
2−8
AD5235
Data Sheet
PROGRAMMING THE VARIABLE RESISTOR
Table 12. RWB (D) at Selected Codes for RAB = 25 kΩ
Rheostat Operation
D (Dec)
1023
512
1
0
The nominal resistance of the RDAC between Terminal A
and Terminal B, RAB, is available with 25 kΩ and 250 kΩ with
1024 positions (10-bit resolution). The final digits of the part
number determine the nominal resistance value, for example,
25 kΩ = 24.4 Ω; 250 kΩ = 244 Ω.
The 10-bit data-word in the RDAC latch is decoded to select one
of the 1024 possible settings. The following description provides
the calculation of resistance, RWB, at different codes of a 25 kΩ
part. The first connection of the wiper starts at Terminal B for
Data 0x000. RWB(0) is 30 Ω because of the wiper resistance, and
it is independent of the nominal resistance. The second connection
is the first tap point where RWB(1) becomes 24.4 Ω + 30 Ω = 54.4 Ω
for Data 0x001. The third connection is the next tap point
representing RWB(2) = 48.8 Ω + 30 Ω = 78.8 Ω for Data 0x002,
and so on. Each LSB data value increase moves the wiper up the
resistor ladder until the last tap point is reached at RWB(1023) =
25006 Ω. See Figure 45 for a simplified diagram of the equivalent
RDAC circuit. When RWB is used, Terminal A can be left
floating or tied to the wiper.
Like the mechanical potentiometer that the RDAC replaces, the
AD5235 part is symmetrical. The resistance between Wiper W
and Terminal A also produces a digitally controlled complementary
resistance, RWA. Figure 46 shows the symmetrical
programmability of the various terminal connections. When RWA
is used, Terminal B can be left floating or tied to the wiper.
Setting the resistance value for RWA starts at a maximum value
of resistance and decreases as the data loaded in the latch is
increased in value.
The general transfer equation for this operation is
RWA (D) =
RWB
1024 − D
× R AB + RW
1024
(2)
For example, the output resistance values in Table 13 are set for
the given RDAC latch codes (applies to RAB = 25 kΩ digital
potentiometers).
75
Table 13. RWA(D) at Selected Codes for RAB = 25 kΩ
50
D (Dec)
1023
512
1
0
25
0
256
512
CODE (Decimal)
768
1023
02816-045
RWA(D), RWB(D) (% RWF)
RWA
Output State
Full scale
Midscale
1 LSB
Zero scale (wiper contact resistor)
Note that, in the zero-scale condition, a finite wiper resistance
of 50 Ω is present. Care should be taken to limit the current
flow between W and B in this state to no more than 20 mA to
avoid degradation or possible destruction of the internal switches.
100
0
RWB(D) (Ω)
25,006
12,530
54.4
30
Figure 46. RWA(D) and RWB(D) vs. Decimal Code
The general equation that determines the programmed output
resistance between Terminal Bx and Terminal Wx is
RWB (D) =
D
× R AB + RW
1024
(1)
where:
D is the decimal equivalent of the data contained in the RDAC
register.
RAB is the nominal resistance between Terminal A and Terminal B.
RW is the wiper resistance.
For example, the output resistance values in Table 12 are set for
the given RDAC latch codes (applies to RAB = 25 kΩ digital
potentiometers).
RWA(D) (Ω)
54.4
12,530
25,006
25,030
Output State
Full scale
Midscale
1 LSB
Zero scale (wiper contact resistance)
The typical distribution of RAB from channel to channel is
±0.2% within the same package. Device-to-device matching is
process lot dependent upon the worst case of ±30% variation.
However, the change in RAB with temperature has a 35 ppm/°C
temperature coefficient.
PROGRAMMING THE POTENTIOMETER DIVIDER
Voltage Output Operation
The digital potentiometer can be configured to generate an output
voltage at the wiper terminal that is proportional to the input
voltages applied to Terminal A and Terminal B. For example,
connecting Terminal A to 5 V and Terminal B to ground
produces an output voltage at the wiper that can be any value
from 0 V to 5 V. Each LSB of voltage is equal to the voltage
applied across Terminal A to Terminal B divided by the 2N
position resolution of the potentiometer divider.
Rev. F | Page 22 of 32
Data Sheet
AD5235
Because the AD5235 can also be supplied by dual supplies, the
general equation defining the output voltage at VW with respect
to ground for any given input voltages applied to Terminal A
and Terminal B is
VW (D) =
D
× V AB + V B
1024
Table 16. Restoring the EEMEM Values to RDAC Registers
SDI
0x10XXXX
SDO
0xXXXXXX
Action
Restores the EEMEM1 value to the
RDAC1 register.
(3)
Equation 3 assumes that VW is buffered so that the effect of wiper
resistance is minimized. Operation of the digital potentiometer in
divider mode results in more accurate operation over temperature.
Here, the output voltage is dependent on the ratio of the internal
resistors and not the absolute value; therefore, the drift improves
to 15 ppm/°C. There is no voltage polarity restriction between
Terminal A, Terminal B, and Terminal W as long as the
terminal voltage (VTERM) stays within VSS < VTERM < VDD.
Table 17. Using Left-Shift by One to Increment 6 dB Steps
SDI
0xC0XXXX
SDO
0xXXXXXX
0xC1XXXX
0xC0XXXX
Action
Moves Wiper 1 to double the
present data contained in the
RDAC1 register.
Moves Wiper 2 to double the
present data contained in the
RDAC2 register.
PROGRAMMING EXAMPLES
Table 18. Storing Additional User Data in EEMEM
The following programming examples illustrate a typical sequence
of events for various features of the AD5235. See Table 7 for the
instructions and data-word format. The instruction numbers,
addresses, and data appearing at the SDI and SDO pins are in
hexadecimal format.
SDI
0x32AAAA
SDO
0xXXXXXX
0x335555
0x32AAAA
Table 14. Scratchpad Programming
SDI
0xB00100
SDO
0xXXXXXX
0xB10200
0xB00100
Action
Writes Data 0x100 into RDAC1 register,
Wiper W1 moves to 1/4 full-scale
position.
Loads Data 0x200 into RDAC2 register,
Wiper W2 moves to 1/2 full-scale
position.
Table 19. Reading Back Data from Memory Locations
SDI
0x92XXXX
SDO
0xXXXXXX
0x00XXXX
0x92AAAA
Table 15. Incrementing RDAC Followed by Storing the
Wiper Setting to EEMEM
SDI
0xB00100
SDO
0xXXXXXX
0xE0XXXX
0xB00100
0xE0XXXX
0xE0XXXX
0x20XXXX
0xXXXXXX
Action
Writes Data 0x100 into RDAC1
register, Wiper W1 moves to 1/4 fullscale position.
Increments RDAC1 register by one to
0x101.
Increments RDAC1 register by one to
0x102. Continue until desired wiper
position is reached.
Stores RDAC2 register data into
EEMEM1. Optionally, tie WP to GND to
protect EEMEM values.
The EEMEM values for the RDACs can be restored by poweron, by strobing the PR pin, or by the two commands shown in
Table 16.
Action
Stores Data 0xAAAA in the extra
EEMEM location USER1. (Allowable to
address in 13 locations with a
maximum of 16 bits of data.)
Stores Data 0x5555 in the extra
EEMEM location USER2. (Allowable to
address in 13 locations with a
maximum of 16 bits of data.)
Action
Prepares data read from USER1
EEMEM location.
NOP Instruction 0 sends a 24-bit word
out of SDO, where the last 16 bits
contain the contents in USER1 EEMEM
location.
Table 20. Reading Back Wiper Settings
SDI
0xB00200
0xC0XXXX
SDO
0xXXXXXX
0xB00200
0xA0XXXX
0xC0XXXX
0xXXXXXX
0xA003FF
Action
Writes RDAC1 to midscale.
Doubles RDAC1 from midscale to full
scale.
Prepares reading wiper setting from
RDAC1 register.
Reads back full-scale value from SDO.
EVAL-AD5235SDZ EVALUATION KIT
Analog Devices, Inc., offers a user-friendly EVAL-AD5235SDZ
evaluation kit that can be controlled by a PC in conjunction
with the SDP platform. The driving program is self-contained;
no programming languages or skills are needed.
Rev. F | Page 23 of 32
AD5235
Data Sheet
APPLICATIONS INFORMATION
BIPOLAR OPERATION FROM DUAL SUPPLIES
The AD5235 can be operated from ±2.5 V dual supplies, which
enable control of ground referenced ac signals or bipolar operation.
AC signals as high as VDD and VSS can be applied directly across
Terminal A to Terminal B with the output taken from Terminal W.
See Figure 47 for a typical circuit connection.
+2.5V
SS
VDD
CS
CLK
SDI
SCLK
MOSI
W
MICROCONTROLLER
GND
±2.5V p-p
±1.25V p-p
AD5235
B
GND
Similarly, W and A terminal capacitances are connected to the
output (not shown); their effect at this node is less significant
and the compensation can be avoided in most cases.
HIGH VOLTAGE OPERATION
A
VSS
D = MIDSCALE
–2.5V
02816-046
VDD
Alternatively, it avoids the ringing or oscillation at the worst
case. For critical applications, find C2 empirically to suit the
oscillation. In general, C2 in the range of a few picofarads to
no more than a few tenths of picofarads is usually adequate
for the compensation.
Figure 47. Bipolar Operation from Dual Supplies
The digital potentiometer can be placed directly in the feedback or
input path of an op amp for gain control, provided that the voltage
across Terminal A to Terminal B, Terminal W to Terminal A or
Terminal W to Terminal B does not exceed |5 V|. When high
voltage gain is needed, set a fixed gain in the op amp and let the
digital potentiometer control the adjustable input. Figure 49
shows a simple implementation.
C
GAIN CONTROL COMPENSATION
R
A digital potentiometer is commonly used in gain control such
as the noninverting gain amplifier shown in Figure 48.
B
A
Figure 49. 15 V Voltage Span Control
W
Similarly, a compensation capacitor, C, may be needed to dampen
the potential ringing when the digital potentiometer changes
steps. This effect is prominent when stray capacitance at the
inverted node is augmented by a large feedback resistor. Typically,
a picofarad Capacitor C is adequate to combat the problem.
VO
02816-047
VI
0V TO 15V
02816-048
AD5235
V–
W
Figure 48. Typical Noninverting Gain Amplifier
When the RDAC B terminal parasitic capacitance is connected
to the op amp noninverting node, it introduces a zero for the 1/βO
term with 20 dB/dec, whereas a typical op amp gain bandwidth
product (GBP) has −20 dB/dec characteristics. A large R2 and
finite C1 can cause the frequency of this zero to fall well below
the crossover frequency. Therefore, the rate of closure becomes
40 dB/dec, and the system has a 0° phase margin at the crossover
frequency. If an input is a rectangular pulse or step function, the
output can ring or oscillate. Similarly, it is also likely to ring when
switching between two gain values; this is equivalent to a stop
change at the input.
DAC
For DAC operation (see Figure 50), it is common to buffer the
output of the digital potentiometer unless the load is much larger
than RWB. The buffer serves the purpose of impedance
conversion and can drive heavier loads.
Depending on the op amp GBP, reducing the feedback resistor
might extend the frequency of the zero far enough to overcome
the problem. A better approach is to include a compensation
capacitor, C2, to cancel the effect caused by C1. Optimum
compensation occurs when R1 × C1 = R2 × C2. This is not
an option because of the variation of R2. As a result, one can
use the previous relationship and scale C2 as if R2 were at its
maximum value. Doing this might overcompensate and
compromise the performance when R2 is set at low values.
Rev. F | Page 24 of 32
5V
1 U1
VIN
AD5235
VOUT
GND
2
5V
3
A
B
AD1582
W
V+
AD8601
VO
V–
A1
Figure 50. Unipolar 10-Bit DAC
02816-049
B
U1
VO
A1
A
R2
250kΩ
C1
11pF
V+
5V
C2
2.2pF
R1
47kΩ
2R
15V
Data Sheet
AD5235
BIPOLAR PROGRAMMABLE GAIN AMPLIFIER
For applications requiring bipolar gain, Figure 51 shows one
implementation. Digital Potentiometer U1 sets the adjustment
range; the wiper voltage (VW2) can, therefore, be programmed
between VI and −KVI at a given U2 setting. Configure OP2177
(A2) as a noninverting amplifier that yields a transfer function of
VIN VOUT
R2
W1
VDD
C
OP2177
U1
V–
A1
5
–2.5V
–2.5VREF
V+
AD8552
V–
U1 = U2 = AD5235
–2.5V
Figure 52. 10-Bit Bipolar DAC
VSS
PROGRAMMABLE VOLTAGE SOURCE WITH
BOOSTED OUTPUT
2D2 1 V
I
1024
For applications that require high current adjustment, such as a
laser diode driver or tunable laser, a boosted voltage source can
be considered (see Figure 53).
VI
R2 = 9 × R1
−10
−5
0
5
9.92
10-BIT BIPOLAR DAC
2N7002
AD5235
A
Table 21. Result of Bipolar Gain Amplifier
R1 = R2
−2
−1
0
1
1.984
VO
(5)
Table 21 shows the result of adjusting D2, with OP2177 (A2)
configured as a unity gain, a gain of 2, and a gain of 10. The
result is a bipolar amplifier with linearly programmable gain
and 1024-step resolution.
R1 = ∞, R2 = 0
−1
−0.5
0
0.5
0.992
+2.5V
A1
In the simpler (and much more usual) case where K = 1, VO is
simplified to
D2
0
256
512
768
1023
A2
W1
U1
U1 = MIDSCALE
R1
Figure 51. Bipolar Programmable Gain Amplifier
R2
VO 1
R1
B1
VSS
–KV I
V+
AD5235
A2
A1
B
SIGNAL CC
U2
W
RBIAS
IL
V+
AD8601
LD
V–
02816-052
B1
B2
ADR421
02816-050
A1
TRIM
GND
VO
V–
B2
A2
VI
+2.5VREF
V+
A2
6
VO
V–
02816-051
2 U3
OP2177
V+
AD8552
W2
VI
VDD
U2
(6)
+2.5V
(4)
where K is the ratio of RWB1/RWA1 set by U1.
W1
2D2
VO
1 VREF
1024
U2
VO R2 D2
1
(1 K ) K
V I R1 1024
AD5235
Without consideration of the wiper resistance, the output of this
circuit is approximately
Figure 53. Programmable Booster Voltage Source
In this circuit, the inverting input of the op amp forces VO to be
equal to the wiper voltage set by the digital potentiometer. The
load current is then delivered by the supply via the N-Ch FET N1
(see Figure 53). N1 power handling must be adequate to dissipate
(VI − VO) × IL power. This circuit can source a 100 mA maximum
with a 5 V supply.
For precision applications, a voltage reference, such as ADR421,
ADR03, or ADR370, can be applied at Terminal A of the digital
potentiometer.
If the circuit in Figure 51 is changed with the input taken from a
precision reference, U1 is set to midscale, and AD8552 (A2) is
configured as a buffer, a 10-bit bipolar DAC can be realized (as
shown in Figure 52). Compared to the conventional DAC, this
circuit offers comparable resolution but not the precision because
of the wiper resistance effects. Degradation of the nonlinearity
and temperature coefficient is prominent near the low values
of the adjustment range. Alternatively, this circuit offers a unique
nonvolatile memory feature that, in some cases, outweighs any
shortfalls in precision.
Rev. F | Page 25 of 32
AD5235
Data Sheet
PROGRAMMABLE CURRENT SOURCE
A programmable current source can be implemented with the
circuit shown in Figure 54.
+5V
U1
VS
3
SLEEP OUTPUT
GND
REF191
6
0V TO (2.048V + VL)
R2A + R2B
R1
IL =
× VW
R2B
B
C1
1µF
W
RS
102Ω
A
4
+5V
V+
(8)
R2
15kΩ
R1
150k Ω
–
AD5235
For applications that require bidirectional current control or
higher voltage compliance, a Howland current pump can be a
solution (see Figure 55). If the resistors are matched, the load
current is
OP1177
U2
–5V
+15V
VL
RL
100Ω
IL
–
02861-053
V–
+
–2.048V TO VL
+2.5V
Figure 54. Programmable Current Source
VREF × D
RS × 1024
V+
OP2177
+ V– A2
+15V
A
The REF191 is a unique low supply headroom and high current
handling precision reference that can deliver 20 mA at 2.048 V.
The load current is simply the voltage across Terminal W to
Terminal B of the digital potentiometer divided by RS.
IL =
C1
10pF
(7)
The circuit is simple but be aware that there are two issues.
First, dual-supply op amps are ideal because the ground potential
of REF191 can swing from −2.048 V at zero scale to VL at full
scale of the potentiometer setting. Although the circuit works
under single supply, the programmable resolution of the system
is reduced by half. Second, the voltage compliance at VL is
limited to 2.5 V, or equivalently, a 125 Ω load. When higher
voltage compliance is needed, consider digital potentiometers,
such as, AD5260, AD5280, and AD7376. Figure 55 shows an
alternate circuit for high voltage compliance.
To achieve higher current, such as when driving a high power
LED, replace U1 with an LDO, reduce RS, and add a resistor in
series with the A terminal of the digital potentiometer. This
limits the current of the potentiometer and increases the current
adjustment resolution.
+
AD5235
W
B
–2.5V
V+
OP2177
– V–
R2B
50Ω
–15V
R1
150k Ω
A1
–15V
R2A
14.95k Ω
VL
RL
500Ω
IL
02816-054
2
PROGRAMMABLE BIDIRECTIONAL CURRENT
SOURCE
Figure 55. Programmable Bidirectional Current Source
R2B, in theory, can be made as small as necessary to achieve the
current needed within the A2 output current driving capability.
In this circuit, OP2177 delivers ±5 mA in either direction, and
the voltage compliance approaches 15 V. Without the additions
of C1 and C2, the output impedance (looking into VL) can be
ZO =
R1' R2B (R1 + R2A)
R1 R2' − R1' (R2A + R2B)
(9)
ZO can be infinite, if Resistors R1' and R2' match precisely with
R1 and R2A + R2B, respectively, which is desirable. On the other
hand, if the resistors do not match, ZO can be negative and cause
oscillation. As a result, C1, in the range of a few picofarad, is
needed to prevent oscillation from the negative impedance.
Rev. F | Page 26 of 32
Data Sheet
AD5235
PROGRAMMABLE LOW-PASS FILTER
In analog-to-digital conversions (ADCs), it is common to
include an antialiasing filter to band limit the sampling signal.
Therefore, the dual-channel AD5235 can be used to construct a
second-order Sallen-Key low-pass filter, as shown in Figure 56.
C1
At the resonant frequency, fO, the overall phase shift is zero, and
the positive feedback causes the circuit to oscillate. With R = R',
C = C', and R2 = R2A /(R2B + RDIODE), the oscillation frequency is
ωO =
+2.5V
A
VI
B
W
R
B
V+
W
AD8601
A
R
RWA (D) =
02816-055
–2.5V
ADJUSTED
CONCURRENTLY
The design equations are
VO
=
VI
ωf 2
ωf
S2 +
S + ωf 2
Q
ωO =
1
R1 R2 C1 C2
(11)
1
1
+
R1 C1 R2 C2
(12)
(10)
First, users should select convenient values for the capacitors.
To achieve maximally flat bandwidth, where Q = 0.707, let C1
be twice the size of C2 and let R1 equal R2. As a result, the user
can adjust R1 and R2 concurrently to the same setting to
achieve the desirable bandwidth.
PROGRAMMABLE OSCILLATOR
2
VO = I D R2B + VD
3
In Figure 56 and Figure 57, the frequency tuning requires that
both RDACs be adjusted concurrently to the same settings.
Because the two channels might be adjusted one at a time, an
intermediate state occurs that might not be acceptable for some
applications. Of course, the increment/decrement instructions
(Instruction 5, Instruction 7, Instruction 13, and Instruction 15)
can all be used. Different devices can also be used in daisy-chain
mode so that parts can be programmed to the same settings
simultaneously.
B
R
25kΩ
A
A
W
+2.5V
2.2nF
W
+
B
U1
V+
VO
OP1177
– V–
–2.5V
R = R' = AD5235
R2B = AD5231
D1 = D2 = 1N4148
R2B
10kΩ
B
W
R1
1kΩ
A
R2A
2.1kΩ
D1
D2
AMPLITUDE
ADJUSTMENT
02816-056
C
2.2nF
R'
25kΩ
C'
(15)
VO, ID, and VD are interdependent variables. With proper
selection of R2B, an equilibrium is reached such that VO
converges. R2B can be in series with a discrete resistor to
increase the amplitude, but the total resistance cannot be too
large to saturate the output.
In a classic Wien bridge oscillator, the Wien network (R||C, R'C')
provides positive feedback, whereas R1 and R2 provide negative
feedback (see Figure 57).
VP
(14)
When the frequency is set, the oscillation amplitude can be
turned by R2B because
Figure 56. Sallen-Key Low-Pass Filter
FREQUENCY
ADJUSTMENT
1024 − D
× R AB + RW
1024
At resonance, setting R2/R1 = 2 balances the bridge. In practice,
R2/R1 should be set slightly larger than 2 to ensure that the
oscillation can start. On the other hand, the alternate turn-on
of the diodes, D1 and D2, ensures that R2/R1 is smaller than 2,
momentarily stabilizing the oscillation.
VO
V–
U1
C2
Q=
(13)
where R is equal to RWA such that :
R2
R1
1
1
or fO =
RC
2πRC
Figure 57. Programmable Oscillator with Amplitude Control
Rev. F | Page 27 of 32
AD5235
Data Sheet
OPTICAL TRANSMITTER CALIBRATION WITH
ADN2841
RESISTANCE SCALING
The AD5235, together with the multirate 2.7 Gbps laser diode
driver, ADN2841, forms an optical supervisory system in which
the dual digital potentiometers can be used to set the laser average
optical power and extinction ratio (see Figure 58). The AD5235
is particularly suited for the optical parameter settings because
of its high resolution and superior temperature coefficient
characteristics.
The AD5235 offers 25 kΩ or 250 kΩ nominal resistance. When
users need lower resistance but must maintain the number of
adjustment steps, they can parallel multiple devices. For example,
Figure 59 shows a simple scheme of paralleling two channels of
RDACs. To adjust half the resistance linearly per step, program
both RDACs concurrently with the same settings.
A2
A1
W1
B2
W2
02816-058
B1
VCC
VCC
Figure 59. Reduce Resistance by Half with Linear Adjustment Characteristics
IMPD
In voltage divider mode, by paralleling a discrete resistor, as
shown in Figure 60, a proportionately lower voltage appears at
Terminal A to Terminal B. This translates into a finer degree of
precision because the step size at Terminal W is smaller. The
voltage can be found as
ADN2841
PSET IMODP
B1
RDAC1
IBIAS
A2
W2
EEMEM
CLKN
B2
RDAC2
VW (D) =
ERSET
CLKN
CLKP
DATAP
DATAN
(RAB // R2)
D
×
× VDD
R3 + RAB // R2 1024
(16)
VDD
R3
02816-057
SDI
DATAN
CONTROL
DATAP
CLK
A1
W1
EEMEM
CLKP
CS
R2
Figure 58. Optical Supervisory System
A
R1
W
B
0
Figure 60. Lowering the Nominal Resistance
Figure 59 and Figure 60 show that the digital potentiometers
change steps linearly. Alternatively, pseudo log taper adjustment
is usually preferred in applications such as audio control. Figure 61
shows another type of resistance scaling. In this configuration,
the smaller the R2 with respect to RAB, the more the pseudo log
taper characteristic of the circuit behaves.
A1
B1
W1
R
02816-060
The ADN2841 is a 2.7 Gbps laser diode driver that uses a
unique control algorithm to manage the average power and
extinction ratio of the laser after its initial factory calibration.
The ADN2841 stabilizes the data transmission of the laser by
continuously monitoring its optical power and correcting the
variations caused by temperature and the degradation of the
laser over time. In the ADN2841, the IMPD monitors the laser
diode current. Through its dual-loop power and extinction
ratio control calibrated by the dual RDACs of the AD5235, the
internal driver controls the bias current, IBIAS, and consequently
the average power. It also regulates the modulation current,
IMODP, by changing the modulation current linearly with slope
efficiency. Therefore, any changes in the laser threshold current
or slope efficiency are compensated for. As a result, the optical
supervisory system minimizes the laser characterization efforts
and, therefore, enables designers to apply comparable lasers
from multiple sources.
02816-059
AD5235
Figure 61. Resistor Scaling with Pseudo Log Adjustment Characteristics
The equation is approximated as
REQUIVALENT =
D × R AB + 51, 200
D × R AB + 51, 200 + 1024 × R
(17)
Users should also be aware of the need for tolerance matching
as well as for temperature coefficient matching of the components.
Rev. F | Page 28 of 32
Data Sheet
AD5235
RDAC CIRCUIT SIMULATION MODEL
In rheostat mode operation, such as gain control, the tolerance
mismatch between the digital potentiometer and the discrete
resistor can cause repeatability issues among various systems
(see Figure 62). Because of the inherent matching of the silicon
process, it is practical to apply the dual-channel device in this
type of application. As such, R1 can be replaced by one of the
channels of the digital potentiometer and programmed to a
specific value. R2 can be used for the adjustable gain. Although it
adds cost, this approach minimizes the tolerance and temperature
coefficient mismatch between R1 and R2. This approach also
tracks the resistance drift over time. As a result, these less than
ideal parameters become less sensitive to system variations.
The internal parasitic capacitances and the external capacitive
loads dominate the ac characteristics of the RDACs. Configured
as a potentiometer divider, the −3 dB bandwidth of the AD5235
(25 kΩ resistor) measures 125 kHz at half scale. Figure 17 provides
the large signal bode plot characteristics of the two available
resistor versions, 25 kΩ and 250 kΩ. A parasitic simulation model
is shown in Figure 64.
A
80pF
W
The following code provides a macro model net list for the
25 kΩ RDAC:
C1
–
AD8601
+
VO
02816-061
U1
* REPLACED WITH ANOTHER
CHANNEL OF RDAC
Figure 62. Linear Gain Control with Tracking Resistance Tolerance,
Drift, and Temperature Coefficient
Note that the circuit in Figure 63 can track tolerance, temperature
coefficient, and drift in this particular application. The characteristic
of the transfer function is, however, a pseudo log rather than a
linear gain function.
A
R
.PARAM D = 1024, RDAC = 25E3
*
.SUBCKT DPOT (A, W, B)
*
CA
A
0
11E-12
RWA A
W
{(1-D/1024)* RDAC + 30}
CW
W
0
80E-12
RWB W
B
{D/1024 * RDAC + 30}
CB
B
0
11E-12
*
.ENDS DPOT
B
C1
W
Vi
+
U1
VO
02816-062
–
AD8601
CB
11pF
Figure 64. RDAC Circuit Simulation Model (RDAC = 25 kΩ)
W
Vi
B
CA
11pF
B R2 A
R1*
RDAC
25kΩ
02816-063
RESISTANCE TOLERANCE, DRIFT, AND
TEMPERATURE COEFFICIENT MISMATCH
CONSIDERATIONS
Figure 63. Nonlinear Gain Control with Tracking Resistance Tolerance and Drift
Rev. F | Page 29 of 32
AD5235
Data Sheet
OUTLINE DIMENSIONS
5.10
5.00
4.90
16
9
4.50
4.40
4.30
6.40
BSC
1
8
PIN 1
1.20
MAX
0.15
0.05
0.20
0.09
0.65
BSC
0.30
0.19
COPLANARITY
0.10
SEATING
PLANE
0.75
0.60
0.45
8°
0°
COMPLIANT TO JEDEC STANDARDS MO-153-AB
Figure 65. 16-Lead Thin Shrink Small Outline Package [TSSOP]
(RU-16)
Dimensions shown in millimeters
ORDERING GUIDE
Model 1, 2
AD5235BRUZ25
AD5235BRUZ25-RL7
AD5235BRUZ250
AD5235BRUZ250-R7
EVAL-AD5235SDZ
RAB (kΩ)
25
25
250
250
Temperature
Range
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
−40°C to +85°C
Package
Description
16-Lead TSSOP
16-Lead TSSOP
16-Lead TSSOP
16-Lead TSSOP
Evaluation Board
Package
Option
RU-16
RU-16
RU-16
RU-16
Ordering
Quantity
96
1,000
96
1,000
1
Branding 3
5235B25
5235B25
5235B250
5235B250
Z = RoHS Compliant Part.
The evaluation board is shipped with the 25 kΩ RAB resistor option; however, the board is compatible with all available resistor value options.
3
Line 1 contains the ADI logo followed by the date code, YYWW. Line 2 contains the model number followed by the end-to-end resistance value (note: D = 250 kΩ).
—OR—
Line 1 contains the model number. Line 2 contains the ADI logo followed by the end-to-end resistance value. Line 3 contains the date code, YYWW.
1
2
Rev. F | Page 30 of 32
Data Sheet
AD5235
NOTES
Rev. F | Page 31 of 32
AD5235
Data Sheet
NOTES
©2004–2012 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D02816-0-6/12(F)
Rev. F | Page 32 of 32