256-Position I2C Compatible Digital Potentiometer AD5245 FUNCTIONAL BLOCK DIAGRAM FEATURES 256-position End-to-end resistance 5 kΩ, 10 kΩ, 50 kΩ, 100 kΩ Compact SOT-23-8 (2.9 mm × 3 mm) package I2C® compatible interface Extra package address decode pin AD0 Full read/write of wiper register Power-on preset to midscale Single supply 2.7 V to 5.5 V Low temperature coefficient 45 ppm/°C Low power, IDD = 8 µA Wide operating temperature –40°C to +125°C Evaluation board available APPLICATIONS Mechanical potentiometer replacement in new designs Transducer adjustment of pressure, temperature, position, chemical, and optical sensors RF amplifier biasing Automotive electronics adjustment Gain control and offset adjustment VDD A SCL I2C INTERFACE SDA W AD0 WIPER REGISTER B GND Figure 1. PIN CONFIGURATION W 1 VDD 2 8 A AD5245 7 B 6 AD0 TOP VIEW SCL 4 (Not to Scale) 5 SDA GND 3 Figure 2. GENERAL OVERVIEW The AD5245 provides a compact 2.9 mm × 3 mm packaged solution for 256-position adjustment applications. These devices perform the same electronic adjustment function as mechanical potentiometers or variable resistors, with enhanced resolution, solid-state reliability, and superior low temperature coefficient performance. The wiper settings are controllable through an I2C compatible digital interface, which can also be used to read back the wiper register content. AD0 can be used to place up to two devices on the same bus. Command bits are available to reset the wiper position to midscale or to shut down the device into a state of zero power consumption. Operating from a 2.7 V to 5.5 V power supply and consuming less than 5 µA allows for usage in portable battery-operated applications. Note: The terms digital potentiometer, VR, and RDAC are used interchangeably. Purchase of licensed I2C components of Analog Devices or one of its sublicensed Associated Companies conveys a license for the purchaser under the Philips I2C Patent Rights to use these components in an I2C system, provided that the system conforms to the I2C Standard Specification as defined by Philips. Rev. 0 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 companies. One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.326.8703 © 2003 Analog Devices, Inc. All rights reserved. AD5245 TABLE OF CONTENTS Electrical Characteristics—5 kΩ Version ...................................... 3 Level Shifting for Bidirectional Interface ................................ 14 Electrical Characteristics—10 kΩ, 50 kΩ, 100 kΩ Versions ....... 4 ESD Protection ........................................................................... 14 Timing Characteristics—5 kΩ, 10 kΩ, 50 kΩ, 100 kΩ Versions 5 Terminal Voltage Operating Range.......................................... 14 Absolute Maximum Ratings............................................................ 5 Power-Up Sequence ................................................................... 15 Typical Performance Characteristics ............................................. 6 Layout and Power Supply Bypassing ....................................... 15 Test Circuits..................................................................................... 10 Pin Configuration and Function Descriptions........................... 15 I2C Interface..................................................................................... 11 Pin Configuration ...................................................................... 15 Operation......................................................................................... 12 Pin Function Descriptions ........................................................ 15 Programming the Variable Resistor ......................................... 12 Outline Dimensions ....................................................................... 16 Programming the Potentiometer Divider ............................... 13 Ordering Guide .......................................................................... 16 I2C Compatible 2-Wire Serial Bus............................................ 13 ESD Caution................................................................................ 16 REVISION HISTORY Revision 0: Initial Version Rev. 0 | Page 2 of 16 AD5245 ELECTRICAL CHARACTERISTICS—5 kΩ VERSION (VDD = 5 V ± 10%, or 3 V ± 10%; VA = +VDD; VB = 0 V; –40°C < TA < +125°C; unless otherwise noted.) Table 1. Parameter Symbol Conditions DC CHARACTERISTICS—RHEOSTAT MODE Resistor Differential Nonlinearity2 R-DNL RWB, VA = no connect Resistor Integral Nonlinearity2 R-INL RWB, VA = no connect Nominal Resistor Tolerance3 ∆RAB TA = 25°C Resistance Temperature Coefficient ∆RAB/∆T VAB = VDD, Wiper = no connect Wiper Resistance RW DC CHARACTERISTICS—POTENTIOMETER DIVIDER MODE (Specifications apply to all VRs) Resolution N Differential Nonlinearity4 DNL Integral Nonlinearity4 INL Voltage Divider Temperature Coefficient ∆VW/∆T Code = 0x80 Full-Scale Error VWFSE Code = 0xFF Zero-Scale Error VWZSE Code = 0x00 RESISTOR TERMINALS Voltage Range5 VA,B,W Capacitance6 A, B CA,B f = 1 MHz, measured to GND, Code = 0x80 Capacitance6 W CW f = 1 MHz, measured to GND, Code = 0x80 Shutdown Supply Current7 IDD_SD VDD = 5.5 V Common-Mode Leakage ICM VA = VB = VDD/2 DIGITAL INPUTS AND OUTPUTS Input Logic High VIH Input Logic Low VIL Input Logic High VIH VDD = 3 V Input Logic Low VIL VDD = 3 V Input Current IIL VIN = 0 V or 5 V 6 Input Capacitance CIL POWER SUPPLIES Power Supply Range VDD RANGE Supply Current IDD VIH = 5 V or VIL = 0 V Power Dissipation8 PDISS VIH = 5 V or VIL = 0 V, VDD = 5 V Power Supply Sensitivity PSS ∆VDD = +5 V ± 10%, Code = Midscale DYNAMIC CHARACTERISTICS6, 9 Bandwidth –3dB BW_5K RAB = 5 kΩ, Code = 0x80 Total Harmonic Distortion THDW VA = 1 V rms, VB = 0 V, f = 1 kHz VW Settling Time tS VA= 5 V, VB = 0 V, ±1 LSB error band Resistor Noise Voltage Density eN_WB RWB = 2.5 kΩ, RS = 0 Rev. 0 | Page 3 of 16 Min Typ1 Max Unit –1.5 –4 –30 ±0.1 ±0.75 +1.5 +4 +30 LSB LSB % ppm/°C Ω 45 50 –1.5 –1.5 –6 0 ±0.1 ±0.6 15 –2.5 +2 GND 120 8 +1.5 +1.5 0 +6 VDD Bits LSB LSB ppm/°C LSB LSB 45 V pF 60 pF 0.01 1 1 2.4 0.8 2.1 0.6 ±1 5 2.7 3 ±0.02 5.5 8 0.2 ±0.05 µA nA V V V V µA pF V µA mW %/% 1.2 0.05 1 MHz % µs 6 nV/√Hz AD5245 ELECTRICAL CHARACTERISTICS—10 kΩ, 50 kΩ, 100 kΩ VERSIONS (VDD = 5 V ± 10%, or 3 V ± 10%; VA = VDD; VB = 0 V; –40°C < TA < +125°C; unless otherwise noted.) Table 2. Parameter DC CHARACTERISTICS—RHEOSTAT MODE Resistor Differential Nonlinearity2 Resistor Integral Nonlinearity2 Nominal Resistor Tolerance3 Resistance Temperature Coefficient Symbol Conditions RWB, VA = no connect RWB, VA = no connect TA = 25°C VAB = VDD, Wiper = no connect Wiper Resistance RW VDD = 5 V DC CHARACTERISTICS—POTENTIOMETER DIVIDER MODE (Specifications apply to all VRs) Resolution N Differential Nonlinearity4 DNL Integral Nonlinearity4 INL Voltage Divider Temperature Coefficient ∆VW/∆T Code = 0x80 Full-Scale Error VWFSE Code = 0xFF Zero-Scale Error VWZSE Code = 0x00 RESISTOR TERMINALS Voltage Range5 VA,B,W Capacitance6 A, B CA,B f = 1 MHz, measured to GND, Code = 0x80 Capacitance6 W CW f = 1 MHz, measured to GND, Code = 0x80 Shutdown Supply Current7 IDD_SD VDD = 5.5 V Common-Mode Leakage ICM VA = VB = VDD/2 DIGITAL INPUTS AND OUTPUTS Input Logic High VIH Input Logic Low VIL Input Logic High VIH VDD = 3 V Input Logic Low VIL VDD = 3 V Input Current IIL VIN = 0 V or 5 V Input Capacitance6 CIL POWER SUPPLIES Power Supply Range VDD RANGE Supply Current IDD VIH = 5 V or VIL = 0 V Power Dissipation8 PDISS VIH = 5 V or VIL = 0 V, VDD = 5 V Power Supply Sensitivity PSS ∆VDD = +5 V ± 10%, Code = Midscale DYNAMIC CHARACTERISTICS6, 9 Bandwidth –3dB BW RAB = 10 kΩ/50 kΩ/100 kΩ, Code = 0x80 Total Harmonic Distortion THDW VA =1 V rms, VB = 0 V, f = 1 kHz, RAB = 10 kΩ VW Settling Time (10 kΩ/50 kΩ/100 kΩ) tS VA = 5 V, VB = 0 V, ±1 LSB error band Resistor Noise Voltage Density eN_WB RWB = 5 kΩ, RS = 0 R-DNL R-INL ∆RAB ∆RAB/∆T Rev. 0 | Page 4 of 16 Min Typ1 Max Unit –1 –2 –30 ±0.1 ±0.25 +1 +2 +30 LSB LSB % ppm/°C 120 Ω 8 +1 +1 Bits LSB LSB ppm/°C LSB LSB 45 50 –1 –1 –3 0 ±0.1 ±0.3 15 –1 1 GND 0 3 45 VDD V pF 60 pF 0.01 1 1 2.4 0.8 2.1 0.6 ±1 5 2.7 µA nA V V V V µA pF 3 5.5 8 0.2 V µA mW ±0.02 ±0.05 %/% 600/100/40 kHz 0.05 % 2 µs 9 nV/√Hz AD5245 TIMING CHARACTERISTICS—5 kΩ, 10 kΩ, 50 kΩ, 100 kΩ VERSIONS (VDD = +5V ± 10%, or +3V ± 10%; VA = VDD; VB = 0 V; –40°C < TA < +125°C; unless otherwise noted.) Table 3. Parameter Symbol Conditions I2C INTERFACE TIMING CHARACTERISTICS6, 10 (Specifications Apply to All Parts) SCL Clock Frequency fSCL tBUF Bus Free Time between STOP and START t1 tHD;STA Hold Time (Repeated START) t2 After this period, the first clock pulse is generated. tLOW Low Period of SCL Clock t3 tHIGH High Period of SCL Clock t4 tSU;STA Setup Time for Repeated START Condition t5 tHD;DAT Data Hold Time t6 tSU;DAT Data Setup Time t7 tF Fall Time of Both SDA and SCL Signals t8 tR Rise Time of Both SDA and SCL Signals t9 tSU;STO Setup Time for STOP Condition t10 Min Typ1 Max Unit 400 kHz µs µs 1.3 0.6 1.3 0.6 0.6 50 0.9 100 300 300 0.6 µs µs µs µs ns ns ns µs NOTES 1 Typical specifications represent average readings at +25°C and VDD = 5 V. 2 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. Parts are guaranteed monotonic. 3 VAB = VDD, Wiper (VW) = no connect. 4 INL and DNL are measured at VW with the RDAC configured as a potentiometer divider similar to a voltage output D/A converter. VA = VDD and VB = 0 V. DNL specification limits of ±1 LSB maximum are guaranteed monotonic operating conditions. 5 Resistor terminals A, B, W have no limitations on polarity with respect to each other. 6 Guaranteed by design and not subject to production test. 7 Measured at the A terminal. The A terminal is open circuited in shutdown mode. 8 PDISS is calculated from (IDD × VDD). CMOS logic level inputs result in minimum power dissipation. 9 All dynamic characteristics use VDD = 5 V. 10 See timing diagrams for locations of measured values. ABSOLUTE MAXIMUM RATINGS1 (TA = +25°C, unless otherwise noted.) Table 4. Parameter VDD to GND VA, VB, VW to GND IMAX1 Digital Inputs and Output Voltage to GND Operating Temperature Range Maximum Junction Temperature (TJMAX) Storage Temperature Lead Temperature (Soldering, 10 sec) Thermal Resistance2 θJA: MSOP-10 Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only and 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. Value –0.3 V to +7 V VDD ±20 mA 0 V to +7 V –40°C to +125°C 150°C –65°C to +150°C 300°C 230°C/W NOTES 1 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 Package power dissipation = (TJMAX – TA)/θJA. Rev. 0 | Page 5 of 16 AD5245 TYPICAL PERFORMANCE CHARACTERISTICS 1.0 1.0 5V 0.8 POTENTIOMETER MODE DNL (LSB) RHEOSTAT MODE INL (LSB) –40°C +25°C +85°C +125°C 0.8 3V 0.6 0.4 0.2 0 –0.2 –0.4 –0.6 –0.8 –1.0 0.6 0.4 0.2 0 –0.2 –0.4 –0.6 –0.8 –1.0 0 32 64 96 128 160 192 224 256 0 32 64 CODE (Decimal) 160 192 224 256 Figure 6. DNL vs. Code, VDD = 5 V 1.0 1.0 0.8 0.8 5V 3V 0.6 POTENTIOMETER MODE INL (LSB) RHEOSTAT MODE DNL (LSB) 128 CODE (Decimal) Figure 3. R-INL vs. Code vs. Supply Voltages 0.4 0.2 0 –0.2 –0.4 –0.6 5V 3V 0.6 0.4 0.2 0 –0.2 –0.4 –0.6 –0.8 –0.8 –1.0 –1.0 0 32 64 96 128 160 192 224 0 256 32 64 96 128 160 192 224 256 CODE (Decimal) CODE (Decimal) Figure 7. INL vs. Code vs. Supply Voltages Figure 4. R-DNL vs. Code vs. Supply Voltages 1.0 1.0 _40°C +25°C +85°C +125°C 0.6 5V 0.8 POTENTIOMETER MODE DNL(LSB) 0.8 POTENTIOMETER MODE INL (LSB) 96 0.4 0.2 0 –0.2 –0.4 –0.6 3V 0.6 0.4 0.2 0 –0.2 –0.4 –0.6 –0.8 –0.8 –1.0 –1.0 0 32 64 96 128 160 192 224 256 0 CODE (Decimal) 32 64 96 128 160 192 CODE (Decimal) Figure 5. INL vs. Code, VDD = 5 V Figure 8. DNL vs. Code vs. Supply Voltages Rev. 0 | Page 6 of 16 224 256 AD5245 1.0 2.5 RHEOSTAT MODE INL (LSB) 0.6 2.0 ZSE, ZERO-SCALE ERROR (µA) –40 °C +25°C +85°C +125°C 0.8 0.4 0.2 0 –0.2 –0.4 –0.6 VDD = 5.5V 1.5 VDD = 2.7V 1.0 0.5 –0.8 0 –40 –1.0 0 32 64 96 128 160 192 224 256 0 80 120 Figure 12. Zero-Scale Error vs. Temperature Figure 9. R-INL vs. Code, VDD = 5 V 1.0 10 _40°C 0.8 +25°C +85°C +125°C 0.6 IDD SUPPLY CURRENT (µA) RHEOSTAT MODE DNL (LSB) 40 TEMPERATURE (°C) CODE (Decimal) 0.4 0.2 0 –0.2 –0.4 –0.6 VDD = 5.5V 1 VDD = 2.7V –0.8 –1.0 0 32 64 96 128 160 192 224 0.1 –40 256 0 Figure 10. R-DNL vs. Code, VDD = 5 V 80 120 Figure 13. Supply Current vs. Temperature 2.5 70 60 2.0 IA SHUTDOWN CURRENT (nA) FSE, FULL-SCALE ERROR (LSB) 40 TEMPERATURE (°C) CODE (Decimal) 1.5 VDD = 2.7V 1.0 VDD = 5.5V 0.5 50 40 30 VDD = 5V 20 10 0 –40 0 40 80 0 –40 120 TEMPERATURE (°C) 0 40 80 TEMPERATURE (°C) Figure 11. Full-Scale Error vs. Temperature Figure 14. Shutdown Current vs. Temperature Rev. 0 | Page 7 of 16 120 AD5245 REF LEVEL 0.000dB 0 RHEOSTAT MODE TEMPCO (ppm/°C) 200 150 –6 0x80 –12 0x40 –18 0x20 MARKER 510 634.725Hz MAG (A/R) –9.049dB 0x10 –24 100 0x08 –30 0x04 –36 50 0x02 0x01 –42 –48 0 –54 –50 –60 0 32 64 96 128 160 192 224 1k START 1 000.000Hz 256 CODE (Decimal) Figure 15. Rheostat Mode Tempco ∆RWB/∆T vs. Code 10k 100k 1M STOP 1 000 000.000Hz Figure 18. Gain vs. Frequency vs. Code, RAB = 10 kΩ REF LEVEL 0.000dB 0 160 POTENTIOMETER MODE TEMPCO (ppm/°C) /DIV 6.000dB 140 /DIV 6.000dB 0x80 –6 120 –12 0x40 100 –18 0x20 80 –24 60 –30 0x10 0x08 0x04 –36 40 0x02 –42 20 MARKER 100 885.289Hz MAG (A/R) –9.014dB 0x01 –48 0 –54 –20 0 32 64 96 128 160 192 224 –60 256 1k START 1 000.000Hz CODE (Decimal) Figure 16. Potentiometer Mode Tempco ∆VWB/∆T vs. Code REF LEVEL 0.000dB 0 /DIV 6.000dB 10k Figure 19. Gain vs. Frequency vs. Code, RAB = 50 kΩ REF LEVEL 0.000dB 0 MARKER 1 000 000.000Hz MAG (A/R) –8.918dB /DIV 6.000dB 0x80 –6 0x80 –6 0x40 –12 0x40 –12 0x20 –18 0x20 –18 0x10 –24 0x10 –30 0x08 –36 0x04 –42 0x02 –24 0x08 –30 0x04 0x02 0x01 –36 100k 1M STOP 1 000 000.000Hz –42 0x01 –48 –48 MARKER 54 089.173Hz MAG (A/R) –9.052dB –54 –54 –60 –60 1k START 1 000.000Hz 10k 1k START 1 000.000Hz 100k 1M STOP 1 000 000.000Hz 10k 100k 1M STOP 1 000 000.000Hz Figure 20. Gain vs. Frequency vs. Code, RAB = 100 kΩ Figure 17. Gain vs. Frequency vs. Code, RAB = 5 kΩ Rev. 0 | Page 8 of 16 AD5245 REF LEVEL –5.000dB /DIV 0.500dB –5.5 5kΩ – 1.026 MHz 10kΩ – 511 MHz 50kΩ – 101 MHz 100kΩ – 54 MHz –6.0 –6.5 –7.0 1 –7.5 VW –8.0 –8.5 R = 50kΩ SCL R = 5kΩ 2 –9.0 R = 10kΩ R = 100kΩ –9.5 Ch 1 200mV BW Ch 2 5.00 V BW M 100ns A CH2 3.00 V –10.0 Figure 24. Digital Feedthrough –10.5 10k 100k 1M START 1 000.000Hz 10M STOP 1 000 000.000Hz Figure 21. –3 dB Bandwidth @ Code = 0x80 60 CODE = 0x80, VA= VDD, VB = 0V VA = 5V VB = 0V PSRR (dB) 40 1 VW PSRR @ VDD = 3V DC ± 10% p-p AC SCL 20 2 Ch 1 PSRR @ VDD = 5V DC ± 10% p-p AC 0 100 1k 10k 100k 100mV BW Ch 2 5.00 V BW M 200ns A CH1 152mV Figure 25. Midscale Glitch, Code 0x80–0x7F 1M FREQUENCY (Hz) Figure 22. PSRR vs. Frequency 900 VDD = 5V 800 VA = 5V VB = 0V 700 IDD (µA) 600 1 500 400 SCL 300 CODE = 0xFF 2 200 Ch 1 100 0 10k VW CODE = 0x55 100k 1M FREQUENCY (Hz) 5.00V BW Ch 2 5.00 V BW M 200ns A CH1 3.00 V Figure 26. Large Signal Settling Time, Code 0xFF–0x00 10M Figure 23. IDD vs. Frequency Rev. 0 | Page 9 of 16 AD5245 TEST CIRCUITS Figure 27 to Figure 35 illustrate the test circuits that define the test conditions used in the product specification tables. OP279 V+ = VDD 1LSB = V+/2N DUT A 5V VIN W W V+ B OFFSET GND VMS VOUT A DUT B OFFSET BIAS Figure 27. Test Circuit for Potentiometer Divider Nonlinearity Error (INL, DNL) Figure 32. Test Circuit for Noninverting Gain NO CONNECT A DUT A IW W VMS –15V RSW = DUT W I W = VDD /R NOMINAL VW 0.1V ISW CODE = 0x00 W B 0.1V ISW B VMS1 VOUT Figure 33. Test Circuit for Gain vs. Frequency DUT VMS2 AD8610 B 2.5V Figure 28. Test Circuit for Resistor Position Nonlinearity Error (Rheostat Operation; R-INL, R-DNL) A DUT OFFSET GND B +15V W VIN RW = [VMS1 – VMS2]/I W VSS TO VDD Figure 29. Test Circuit for Wiper Resistance Figure 34. Test Circuit for Incremental ON Resistance VA V+ = VDD 10% VDD PSRR (dB) = 20 LOG A V+ W PSS (%/%) = B ∆V MS% NC ∆V (∆V MS ) DD ∆V DD% VMS VDD DUT A VSS GND B NC Figure 30. Test Circuit for Power Supply Sensitivity (PSS, PSSR) A DUT OFFSET GND B W OP279 ICM VCM NC = NO CONNECT Figure 35. Test Circuit for Common-Mode Leakage current 5V VIN W VOUT OFFSET BIAS Figure 31. Test Circuit for Inverting Gain Rev. 0 | Page 10 of 16 AD5245 I2C INTERFACE Table 5. Write Mode S 0 1 0 1 1 0 AD0 W A X RS SD Slave Address Byte X X X X X A D7 D6 D5 D4 D3 D2 D1 D0 Instruction Byte A P Data Byte Table 6. Read Mode S 0 1 0 1 1 0 Slave Address Byte AD0 R A D7 D6 D5 D4 D3 Data Byte D2 D1 D0 A P S = Start Condition R = Read P = Stop Condition RS = Reset wiper to Midscale 80H A = Acknowledge X = Don’t Care SD = Shutdown connects wiper to B terminal and open circuits A terminal. It does not change contents of wiper register. W = Write D7, D6, D5, D4, D3, D2, D1, D0 = Data Bits t8 t2 t9 SCL t6 t2 t3 t7 t4 t5 t10 t9 t8 SDA t1 P S S P Figure 36. I2C Interface Detailed Timing Diagram 1 9 1 9 1 9 SCL SDA START BY MASTER 0 1 0 1 1 0 AD0 X R/W RS ACK BY AD5245 FRAME 1 SLAVE ADDRESS BYTE SD X X X X X D7 D6 D5 ACK BY AD5245 FRAME 2 INSTRUCTION BYTE D4 D3 D2 D1 FRAME 3 DATA BYTE Figure 37. Writing to the RDAC Register 1 9 1 9 SCL SDA START BY MASTER 0 1 0 1 1 0 AD0 FRAME 1 SLAVE ADDRESS BYTE D7 R/W ACK BY AD5245 D6 D5 D4 D3 D2 D1 FRAME 2 RDAC REGISTER Figure 38. Reading Data from a Previously Selected RDAC Register in Write Mode Rev. 0 | Page 11 of 16 D0 NO ACK BY MASTER STOP BY MASTER D0 ACK BY AD5245 STOP BY MASTER AD5245 OPERATION The AD5245 is a 256-position digitally controlled variable resistor (VR) device. The general equation determining the digitally programmed output resistance between W and B is An internal power-on preset places the wiper at midscale during power-on, which simplifies the fault condition recovery at power-up. PROGRAMMING THE VARIABLE RESISTOR Rheostat Operation The nominal resistance of the RDAC between terminals A and B is available in 5 kΩ, 10 kΩ, 50 kΩ, and 100 kΩ. The final two or three digits of the part number determine the nominal resistance value, e.g., 10 kΩ = 10; 50 kΩ = 50. The nominal resistance (RAB) of the VR has 256 contact points accessed by the wiper terminal, plus the B terminal contact. The 8-bit data in the RDAC latch is decoded to select one of the 256 possible settings. Assume a 10 kΩ part is used, the wiper’s first connection starts at the B terminal for data 0x00. Since there is a 60 Ω wiper contact resistance, such connection yields a minimum of 60 Ω resistance between terminals W and B. The second connection is the first tap point, which corresponds to 99 Ω (RWB = RAB/256 + RW = 39 Ω + 60 Ω) for data 0x01. The third connection is the next tap point, representing 177 Ω (2 × 39 Ω + 60 Ω) for data 0x02, and so on. Each LSB data value increase moves the wiper up the resistor ladder until the last tap point is reached at 9961 Ω (RAB – 1 LSB + RW). Figure 39 shows a simplified diagram of the equivalent RDAC circuit where the last resistor string will not be accessed; therefore, there is 1 LSB less of the nominal resistance at full scale in addition to the wiper resistance. A SD BIT RS D7 D6 D5 D4 D3 D2 D1 D0 RWB (D ) = RDAC LATCH RS AND DECODER (1) In summary, if RAB = 10 kΩ and the A terminal is open circuited, the following output resistance RWB will be set for the indicated RDAC latch codes. Table 7. Codes and Corresponding RWB Resistance D (Dec.) 255 128 1 0 RWB (Ω) 9,961 5,060 99 60 Output State Full Scale (RAB – 1 LSB + RW) Midscale 1 LSB Zero Scale (Wiper Contact Resistance) Note that in the zero-scale condition a finite wiper resistance of 60 Ω is present. Care should be taken to limit the current flow between W and B in this state to a maximum pulse current of no more than 20 mA. Otherwise, degradation or possible destruction of the internal switch contact can occur. Similar to the mechanical potentiometer, the resistance of the RDAC between the wiper W and terminal A also produces a digitally controlled complementary resistance RWA. When these terminals are used, the B terminal can be opened. Setting the resistance value for RWA starts at a maximum value of resistance and decreases as the data loaded in the latch increases in value. The general equation for this operation is RWA (D ) = W × R AB + R W where D is the decimal equivalent of the binary code loaded in the 8-bit RDAC register, RAB is the end-to-end resistance, and RW is the wiper resistance contributed by the on resistance of the internal switch. RS RS D 256 256 − D × R AB + RW 256 (2) For RAB = 10 kΩ and the B terminal open circuited, the following output resistance RWA will be set for the indicated RDAC latch codes. Table 8. Codes and Corresponding RWA Resistance B Figure 39. AD5245 Equivalent RDAC Circuit D (Dec.) 255 128 1 0 RWA (Ω) 99 5,060 9,961 10,060 Output State Full Scale Midscale 1 LSB Zero Scale Typical device to device matching is process lot dependent and may vary by up to ±30%. Since the resistance element is processed in thin film technology, the change in RAB with temperature has a very low 45 ppm/°C temperature coefficient. Rev. 0 | Page 12 of 16 AD5245 selected device waits for data to be written to or read from its serial register. If the R/W bit is high, the master will read from the slave device. On the other hand, if the R/W bit is low, the master will write to the slave device. PROGRAMMING THE POTENTIOMETER DIVIDER Voltage Output Operation The digital potentiometer easily generates a voltage divider at wiper-to-B and wiper-to-A proportional to the input voltage at A-to-B. Unlike the polarity of VDD to GND, which must be positive, voltage across A-B, W-A, and W-B can be at either polarity. 2. If ignoring the effect of the wiper resistance for approximation, connecting the A terminal to 5 V and the B terminal to ground produces an output voltage at the wiper-to-B starting at 0 V up to 1 LSB less than 5 V. Each LSB of voltage is equal to the voltage applied across terminal AB divided by the 256 positions of the potentiometer divider. The general equation defining the output voltage at VW with respect to ground for any valid input voltage applied to terminals A and B is VW (D ) = D 256 VA + 256 − D VB 256 The second MSB, RS, is the midscale reset. A logic high on this bit moves the wiper to the center tap where RWA = RWB. This feature effectively writes over the contents of the register, and thus, when taken out of reset mode, the RDAC will remain at midscale. The third MSB, SD, is a shutdown bit. A logic high causes an open circuit at terminal A while shorting the wiper to terminal B. This operation yields almost 0 Ω in rheostat mode or 0 V in potentiometer mode. It is important to note that the shutdown operation does not disturb the contents of the register. When brought out of shutdown, the previous setting will be applied to the RDAC. Also, during shutdown, new settings can be programmed. When the part is returned from shutdown, the corresponding VR setting will be applied to the RDAC. (3) For a more accurate calculation, which includes the effect of wiper resistance, VW, can be found as VW (D ) = RWB (D ) 256 VA + RWA (D ) 256 VB (4) Operation of the digital potentiometer in the divider mode results in a more accurate operation over temperature. Unlike the rheostat mode, the output voltage is dependent mainly on the ratio of the internal resistors RWA and RWB and not the absolute values. Therefore, the temperature drift reduces to 15 ppm/°C. The remainder of the bits in the instruction byte are don’t cares (see Table 5). 3. After acknowledging the instruction byte, the last byte in write mode is the data byte. Data is transmitted over the serial bus in sequences of nine clock pulses (eight data bits followed by an acknowledge bit). The transitions on the SDA line must occur during the low period of SCL and remain stable during the high period of SCL (see Table 5). 4. In the read mode, the data byte follows immediately after the acknowledgment of the slave address byte. Data is transmitted over the serial bus in sequences of nine clock pulses (a slight difference with the write mode, where there are eight data bits followed by an acknowledge bit). Similarly, the transitions on the SDA line must occur during the low period of SCL and remain stable during the high period of SCL (see Figure 38). 5. When all data bits have been read or written, a STOP condition is established by the master. A STOP condition is defined as a low-to-high transition on the SDA line while SCL is high. In write mode, the master will pull the SDA line high during the tenth clock pulse to establish a STOP condition (see Figure 37). In read mode, the master will issue a No Acknowledge for the ninth clock pulse (i.e., the SDA line remains high). The master will then bring the SDA line low before the tenth clock pulse which goes high to establish a STOP condition (see Figure 38). I2C COMPATIBLE 2-WIRE SERIAL BUS The first byte of the AD5245 is a slave address byte (see Table 5 and Table 6). It has a 7-bit slave address and a R/W bit. The six MSBs of the slave address are 010110, and the following bit is determined by the state of the AD0 pin of the device. AD0 allows the user to place up to two of the I2C compatible devices on one bus. The 2-wire I2C serial bus protocol operates as follows: 1. The master initiates data transfer by establishing a START condition, which is when a high-to-low transition on the SDA line occurs while SCL is high (see Figure 37). The following byte is the slave address byte, which consists of the 7-bit slave address followed by an R/W bit (this bit determines whether data will be read from or written to the slave device). A write operation contains an extra instruction byte that a read operation does not contain. Such an instruction byte in write mode follows the slave address byte. The first bit (MSB) of the instruction byte is a don’t care. The slave whose address corresponds to the transmitted address responds by pulling the SDA line low during the ninth clock pulse (this is termed the acknowledge bit). At this stage, all other devices on the bus remain idle while the Rev. 0 | Page 13 of 16 AD5245 VDD1 = 3.3V A repeated write function gives the user flexibility to update the RDAC output a number of times after addressing and instructing the part only once. During the write cycle, each data byte will update the RDAC output. For example, after the RDAC has acknowledged its slave address and instruction bytes, the RDAC output will update after these two bytes. If another byte is written to the RDAC while it is still addressed to a specific slave device with the same instruction, this byte will update the output of the selected slave device. If different instructions are needed, the write mode has to start again with a new slave address, instruction, and data byte. Similarly, a repeated read function of the RDAC is also allowed. VDD2 = 5V RP RP RP RP G S SDA1 D M1 SCL1 SDA2 G S D M2 3.3V SCL2 5V AD5245 E2PROM Figure 41. Level Shifting for Operation at Different Potentials Readback RDAC Value The AD5245 allows the user to read back the RDAC values in the read mode. Refer to Table 5 and Table 6 for the programming format. Multiple Devices on One Bus ESD PROTECTION All digital inputs are protected with a series input resistor and parallel Zener ESD structures shown in Figure 42 and Figure 43. This applies to the digital input pins SDA, SCL, and AD0. 340Ω Figure 40 shows two AD5245 devices on the same serial bus. Each has a different slave address since the states of their AD0 pins are different. This allows each RDAC within each device to be written to or read from independently. The master device output bus line drivers are open-drain pull-downs in a fully I2C compatible interface. LOGIC VSS Figure 42. ESD Protection of Digital Pins A,B,W +5V RP VSS RP Figure 43. ESD Protection of Resistor Terminals SDA MASTER SCL +5V SDA SCL SDA SCL AD0 AD0 AD5245 AD5245 TERMINAL VOLTAGE OPERATING RANGE The AD5245 VDD and GND power supply defines the boundary conditions for proper 3-terminal digital potentiometer operation. Supply signals present on terminals A, B, and W that exceed VDD or GND will be clamped by the internal forward biased diodes (see Figure 44). Figure 40. Multiple AD5245 Devices on One I2C Bus VDD LEVEL SHIFTING FOR BIDIRECTIONAL INTERFACE While most legacy systems may be operated at one voltage, a new component may be optimized at another. When two systems operate the same signal at two different voltages, proper level shifting is needed. For instance, one can use a 3.3 V E2PROM to interface with a 5 V digital potentiometer. A level shifting scheme is needed to enable a bidirectional communication so that the setting of the digital potentiometer can be stored to and retrieved from the E2PROM. Figure 41 shows one of the implementations. M1 and M2 can be any N-channel signal FETs, or if VDD falls below 2.5 V, low threshold FETs such as the FDV301N. Rev. 0 | Page 14 of 16 A W B VSS Figure 44. Maximum Terminal Voltages Set by VDD and VSS AD5245 POWER-UP SEQUENCE Since the ESD protection diodes limit the voltage compliance at terminals A, B, and W (see Figure 44), it is important to power VDD/GND before applying any voltage to terminals A, B, and W; otherwise, the diode will be forward biased such that VDD will be powered unintentionally and may affect the rest of the user’s circuit. The ideal power-up sequence is in the following order: GND, VDD, digital inputs, and then VA/B/W. The relative order of powering VA, VB, VW, and the digital inputs is not important as long as they are powered after VDD/GND. device should be bypassed with disc or chip ceramic capacitors of 0.01 µF to 0.1 µF. Low ESR 1 µF to 10 µF tantalum or electrolytic capacitors should also be applied at the supplies to minimize any transient disturbance and low frequency ripple (see Figure 45). Note that the digital ground should also be joined remotely to the analog ground at one point to minimize the ground bounce. VDD VDD C3 + C1 10µF 0.1µF AD5245 LAYOUT AND POWER SUPPLY BYPASSING It is a good practice to employ compact, minimum lead length layout design. The leads to the inputs should be as direct as possible with a minimum conductor length. Ground paths should have low resistance and low inductance. GND Figure 45. Power Supply Bypassing Similarly, it is also a good practice to bypass the power supplies with quality capacitors for optimum stability. Supply leads to the PIN CONFIGURATION AND FUNCTION DESCRIPTIONS PIN CONFIGURATION PIN FUNCTION DESCRIPTIONS Table 9. W 1 VDD 2 8 A AD5245 7 B 6 AD0 TOP VIEW SCL 4 (Not to Scale) 5 SDA GND 3 Figure 46. Pin 1 2 3 4 5 6 Name W VDD GND SCL SDA AD0 7 8 B A Rev. 0 | Page 15 of 16 Description W Terminal. Positive Power Supply. Digital Ground. Serial Clock Input. Positive edge triggered. Serial Data Input/Output. Programmable address bit 0 for multiple package decoding. B Terminal. A Terminal. AD5245 OUTLINE DIMENSIONS 2.90 BSC 8 7 6 5 1 2 3 4 1.60 BSC 2.80 BSC PIN 1 0.65 BSC 1.95 BSC 1.30 1.15 0.90 1.45 MAX 0.15 MAX 0.38 0.22 0.22 0.08 8° 4° 0° SEATING PLANE 0.60 0.45 0.30 COMPLIANT TO JEDEC STANDARDS MO-178BA Figure 47. 8-Lead Small Outline Transistor Package [SOT-23] (RJ-8) Dimensions shown in millimeters ORDERING GUIDE Model AD5245BRJ5-R2 AD5245BRJ5-RL7 AD5245BRJ10-R2 AD5245BRJ10-RL7 AD5245BRJ50-R2 AD5245BRJ50-RL7 AD5245BRJ100-R2 AD5245BRJ100-RL7 AD5245EVAL RAB (Ω) 5k 5k 10k 10k 50k 50k 100k 100k See Note 1 Temperature –40°C to +125°C –40°C to +125°C –40°C to +125°C –40°C to +125°C –40°C to +125°C –40°C to +125°C –40°C to +125°C –40°C to +125°C Package Description SOT-23-8 SOT-23-8 SOT-23-8 SOT-23-8 SOT-23-8 SOT-23-8 SOT-23-8 SOT-23-8 Evaluation Board Package Option RJ-8 RJ-8 RJ-8 RJ-8 RJ-8 RJ-8 RJ-8 RJ-8 1 The evaluation board is shipped with the 10 kΩ RAB resistor option; however, the board is compatible with all available resistor value options. The AD5245 contains 2532 transistors. Die size: 30.7 mil × 76.8 mil = 2,358 sq. mil. ESD CAUTION ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although this product features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality. © 2003 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective companies. C03436–0–5/03(0) Rev. 0 | Page 16 of 16 Branding D0G D0G D0H D0H D0J D0J D0K D0K