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10 A, Rail-to-Rail I/O, Zero Input Crossover Distortion Amplifier ADA4505-2 FEATURES PSRR: 100 dB minimum CMRR: 105 dB typical Very low supply current: 10 A per amplifier maximum 1.8 V to 5 V single-supply or 0.9 to 2.5V dual-supply operation Rail-to-rail input and output 2.5 mV offset voltage maximum Very low input bias current: 0.5 pA typical PIN CONFIGURATION OUT A 1 -IN A 2 +IN A 3 V- 4 8 V+ OUT B +IN B 07416-004 ADA4505-2 TOP VIEW (Not to Scale) 7 6 5 -IN B Figure 1. 8-Lead MSOP (RM-8) APPLICATIONS Pressure and position sensors Remote security Medical monitors Battery-powered consumer equipment Hazard detectors GENERAL DESCRIPTION The ADA4505-2 is a dual micropower amplifier featuring railto-rail input and output swings while operating from a 1.8 V to 5 V single or from 0.9 V to 2.5 V dual power supply. Employing a new circuit technology, this low cost amplifier offers zero input crossover distortion (excellent PSRR and CMRR performance) and very low bias current, while operating with a supply current of less than 10 A per amplifier. This combination of features makes the ADA4505-2 amplifier an ideal choice for battery-powered applications because it minimizes errors due to power supply voltage variations over the lifetime of the battery, and maintains high CMRR even for a rail-to-rail op amp. Remote battery-powered sensors, handheld instrumentation and consumer equipment, hazard detectors (for example, smoke, fire, and gas), and patient monitors can benefit from the features of the ADA4505-2 amplifier. The ADA4505-2 is specified for both the industrial temperature range (-40C to +85C) and the extended industrial temperature range (-40C to +125C). The ADA4505-2 dual amplifiers are available in the standard 8-lead MSOP package. The ADA4505-2 is a member of a growing series of zero crossover op amps offered by Analog Devices, Inc., including the AD8506, which also operates from a 1.8 V to 5 V single or from 0.9 V to 2.5 V dual power supply. 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 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 (c)2008 Analog Devices, Inc. All rights reserved. ADA4505-2 TABLE OF CONTENTS Features .............................................................................................. 1 Applications ....................................................................................... 1 Pin Configuration ............................................................................. 1 General Description ......................................................................... 1 Revision History ............................................................................... 2 Specifications..................................................................................... 3 Electrical Characteristics--5 V Operation................................ 3 Electrical Characteristics--1.8 V Operation ............................ 4 Absolute Maximum Ratings............................................................ 5 Thermal Resistance ...................................................................... 5 ESD Caution...................................................................................5 Typical Performance Characteristics ..............................................6 Theory of Operation ...................................................................... 14 Applications Information .............................................................. 16 Pulse Oximeter Current Source ............................................... 16 Four-Pole Low-Pass Butterworth Filter for Glucose Monitor ......................................................................... 17 Outline Dimensions ....................................................................... 18 Ordering Guide .......................................................................... 18 REVISION HISTORY 7/08--Revision 0: Initial Version Rev. 0 | Page 2 of 20 ADA4505-2 SPECIFICATIONS ELECTRICAL CHARACTERISTICS--5 V OPERATION VSY = 5 V, VCM = VSY/2, TA = 25C, unless otherwise specified. Table 1. Parameter INPUT CHARACTERISTICS Offset Voltage Input Bias Current Symbol VOS IB -40C TA +85C -40C TA +125C Input Offset Current IOS -40C TA +85C -40C TA +125C -40C TA +125C 0 V VCM 5 V -40C TA +85C -40C TA +125C 0.05 V VOUT 4.95 V -40C TA +125C -40C TA +125C 0.05 Conditions 0 V VCM 5 V -40C TA +125C Min Typ 0.5 0.5 Max 2.5 3 2 50 300 1 25 65 5 Unit mV mV pA pA pA pA pA pA V dB dB dB dB dB V/C G pF pF V V V V mV mV mV mV mA dB dB dB A A mV/s kHz Degrees V p-p nV/Hz fA/Hz Input Voltage Range Common-Mode Rejection Ratio CMRR Large Signal Voltage Gain Offset Voltage Drift Input Resistance Input Capacitance Differential Mode Input Capacitance Common Mode OUTPUT CHARACTERISTICS Output Voltage High AVO VOS/T RIN CIN(DM) CIN(CM) VOH 0 90 90 85 105 100 105 120 2 220 2.5 4.7 Output Voltage Low VOL Short-Circuit Limit POWER SUPPLY Power Supply Rejection Ratio ISC PSRR RL = 100 k to GND -40C TA +125C RL = 10 k to GND -40C TA +125C RL = 100 k to VSY -40C TA +125C RL = 10 k to VSY -40C TA +125C VOUT = VSY or GND VSY = 1.8 V to 5 V -40C TA +85C -40C TA +125C VOUT = VSY/2 -40C TA +125C RL = 100 k, CL = 20 pF, G = 1 RL = 1 M, CL = 20 pF, G = 1 RL = 1 M, CL = 20 pF, G = 1 f = 0.1 Hz to 10 Hz f = 1 kHz f = 1 kHz 4.98 4.98 4.9 4.9 4.99 4.95 2 10 40 5 5 25 25 100 100 95 110 Supply Current per Amplifier DYNAMIC PERFORMANCE Slew Rate Gain Bandwidth Product Phase Margin NOISE PERFORMANCE Voltage Noise Voltage Noise Density Current Noise Density ISY 7 10 15 SR GBP M en p-p en in 6 50 52 2.95 55 20 Rev. 0 | Page 3 of 20 ADA4505-2 ELECTRICAL CHARACTERISTICS--1.8 V OPERATION VSY = 1.8 V, VCM = VSY/2, TA = 25C, unless otherwise specified. Table 2. Parameter INPUT CHARACTERISTICS Offset Voltage Input Bias Current Symbol VOS IB -40C TA +85C -40C TA +125C Input Offset Current IOS -40C TA +85C -40C TA +125C -40C TA +125C 0 V VCM 1.8 V -40C TA +85C -40C TA +125C 0.05 V VOUT 1.75 V -40C TA +125C -40C TA +125C 0.05 Conditions 0 V VCM 1.8 V -40C TA +125C Min Typ 0.5 0.5 Max 2.5 3 2 50 300 1 25 50 1.8 Unit mV mV pA pA pA pA pA pA V dB dB dB dB dB V/C G pF pF V V V V mV mV mV mV mA dB dB dB A A mV/s kHz Degrees V p-p nV/Hz fA/Hz Input Voltage Range Common-Mode Rejection Ratio CMRR Large Signal Voltage Gain Offset Voltage Drift Input Resistance Input Capacitance Differential Mode Input Capacitance Common Mode OUTPUT CHARACTERISTICS Output Voltage High AVO VOS/T RIN CINDM CINCM VOH 0 85 85 80 95 95 100 115 2.5 220 2.5 4.7 Output Voltage Low VOL Short-Circuit Limit POWER SUPPLY Power Supply Rejection Ratio ISC PSRR RL = 100 k to GND -40C TA +125C RL = 10 k to GND -40C TA +125C RL = 100 k to VSY -40C TA +125C RL = 10 k to VSY -40C TA +125C VOUT = VSY or GND VSY = 1.8 V to 5 V -40C TA +85C -40C TA +125C VOUT = VSY/2 -40C TA +125C RL = 100 k, CL = 20 pF, G = 1 RL = 1 M, CL = 20 pF, G = 1 RL = 1 M, CL = 20 pF, G = 1 f = 0.1 Hz to 10 Hz f = 1 kHz f = 1 kHz 1.78 1.78 1.65 1.65 1.79 1.75 2 12 3.8 5 5 25 25 100 100 95 110 Supply Current per Amplifier DYNAMIC PERFORMANCE Slew Rate Gain Bandwidth Product Phase Margin NOISE PERFORMANCE Voltage Noise Voltage Noise Density Current Noise Density ISY 7 10 15 SR GBP M en p-p en in 6.5 50 52 2.95 55 20 Rev. 0 | Page 4 of 20 ADA4505-2 ABSOLUTE MAXIMUM RATINGS Table 3. Parameter Supply Voltage Input Voltage Input Current1 Differential Input Voltage2 Output Short-Circuit Duration to GND Storage Temperature Range Operating Temperature Range Junction Temperature Range Lead Temperature (Soldering, 60 sec) 1 THERMAL RESISTANCE Rating 5.5 V VSY 0.1 V 10 mA VSY Indefinite -65C to +150C -40C to +125C -65C to +150C 300C JA is specified for the worst-case conditions, that is, a device soldered in a circuit board for surface-mount packages. This was measured using a standard two-layer board. Table 4. Thermal Resistance Package Type 8-Lead MSOP (RM-8) JA 206 JC 44 Unit C/W ESD CAUTION Input pins have clamp diodes to the supply pins. Input current should be limited to 10 mA or less whenever the input signal exceeds the power supply rail by 0.5 V. 2 Differential input voltage is limited to 5 V or the supply voltage, whichever is less. 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. Rev. 0 | Page 5 of 20 ADA4505-2 TYPICAL PERFORMANCE CHARACTERISTICS TA = 25C, unless otherwise noted. 140 120 NUMBER OF AMPLIFIERS VSY = 1.8V VCM = VSY/2 140 120 NUMBER OF AMPLIFIERS VSY = 5V VCM = VSY/2 100 80 60 40 20 0 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0 0.5 VOS (mV) 100 80 60 40 20 0 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0 0.5 VOS (mV) 07416-007 1.0 1.5 2.0 2.5 3.0 1.0 1.5 2.0 2.5 3.0 Figure 2. Input Offset Voltage Distribution 14 12 NUMBER OF AMPLIFIERS Figure 5. Input Offset Voltage Distribution 14 12 NUMBER OF AMPLIFIERS VSY = 1.8V -40C TA 125C VSY = 5V -40C TA 125C 10 8 6 4 2 0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 TCVOS (V/C) 4.5 5.0 5.5 6.0 10 8 6 4 2 0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 TCVOS (V/C) 4.5 5.0 5.5 6.0 07416-009 Figure 3. Input Offset Voltage Drift Distribution 1500 VSY = 1.8V 1000 DEVICE 1 DEVICE 2 DEVICE 3 DEVICE 4 DEVICE 5 DEVICE 6 DEVICE 7 DEVICE 8 DEVICE 9 DEVICE 10 Figure 6. Input Offset Voltage Drift Distribution 1500 VSY = 5V 1000 DEVICE 1 500 DEVICE 2 DEVICE 3 DEVICE 4 0 DEVICE 5 DEVICE 6 DEVICE 7 DEVICE 8 DEVICE 9 DEVICE 10 -1000 500 VOS (V) 0 -500 -1000 07416-011 0 0.2 0.4 0.6 0.8 1.0 VCM (V) 1.2 1.4 1.6 1.8 0 1 2 VCM (V) 3 4 5 Figure 4. Input Offset Voltage vs. Common-Mode Voltage Figure 7. Input Offset Voltage vs. Common-Mode Voltage Rev. 0 | Page 6 of 20 07416-012 -1500 VOS (V) -500 -1500 07416-010 07416-008 ADA4505-2 TA = 25C, unless otherwise noted. 1000 VSY = 1.8V 1000 IB+ IB- VSY = 5V IB+ IB- 100 100 IB (pA) 10 IB (pA) 07416-013 10 1 1 0 25 50 75 TEMPERATURE (C) 100 125 0 25 50 75 TEMPERATURE (C) 100 125 Figure 8. Input Bias Current vs. Temperature 1000 125C 100 105C VSY = 1.8V IB+ AND IB- 1000 Figure 11. Input Bias Current vs. Temperature VSY = 5V IB+ AND IB- 125C 100 105C IB (pA) 10 IB (pA) 85C 10 85C 1 25C 07416-014 1 25C 07416-016 07416-018 0.1 0 0.2 0.4 0.6 0.8 1.0 VCM (V) 1.2 1.4 1.6 1.8 0.1 0 1 2 VCM (V) 3 4 5 Figure 9. Input Bias Current vs. Common-Mode Voltage OUTPUT VOLTAGE (VOH) TO SUPPLY RAIL (mV) 10k Figure 12. Input Bias Current vs. Common-Mode Voltage OUTPUT VOLTAGE (VOH) TO SUPPLY RAIL (mV) 10k VSY = 5V VSY = 1.8V 1k 1k 100 100 10 10 1 -40C +25C +85C +125C 0.01 0.1 1 LOAD CURRENT (mA) 10 100 07416-017 1 -40C +25C +85C +125C 0.01 0.1 1 LOAD CURRENT (mA) 10 100 0.1 0.1 0.01 0.001 0.01 0.001 Figure 10. Output Voltage (VOH) to Supply Rail vs. Load Current and Temperature Figure 13. Output Voltage (VOH) to Supply Rail vs. Load Current and Temperature Rev. 0 | Page 7 of 20 07416-015 0.1 0.1 ADA4505-2 TA = 25C, unless otherwise noted. OUTPUT VOLTAGE (VOL) TO SUPPLY RAIL (mV) 10k OUTPUT VOLTAGE (VOL) TO SUPPLY RAIL (mV) VSY = 1.8V 10k VSY = 5V 1k 1k 100 100 10 10 1 -40C +25C +85C +125C 07416-019 1 -40C +25C +85C +125C 0.01 0.1 1 LOAD CURRENT (mA) 10 100 07416-020 0.1 0.1 0.01 0.001 0.01 0.1 1 LOAD CURRENT (mA) 10 100 0.01 0.001 Figure 14. Output Voltage (VOL) to Supply Rail vs. Load Current and Temperature 1.800 Figure 17. Output Voltage (VOL) to Supply Rail vs. Load Current and Temperature 5.000 OUTPUT VOLTAGE (VOH) TO SUPPLY RAIL (V) OUTPUT VOLTAGE (VOH) TO SUPPLY RAIL (V) RL = 100k 1.795 4.995 RL = 100k 4.990 1.790 4.985 RL = 10k 1.785 RL = 10k 4.980 1.780 VSY = 1.8V 07416-021 4.975 VSY = 5V -25 -10 5 20 35 50 65 TEMPERATURE (C) 80 95 110 125 07416-022 1.775 -40 -25 -10 5 20 35 50 65 TEMPERATURE (C) 80 95 110 125 4.970 -40 Figure 15. Output Voltage (VOH) to Supply Rail vs. Temperature OUTPUT VOLTAGE (VOL) TO SUPPLY RAIL (mV) OUTPUT VOLTAGE (VOL) TO SUPPLY RAIL (mV) 25 VSY = 1.8V 20 Figure 18. Output Voltage (VOH) to Supply Rail vs. Temperature 25 VSY = 5V 20 15 RL = 10k 15 RL = 10k 10 10 5 RL = 100k 07416-023 5 RL = 100k 07416-024 0 -40 -25 -10 5 20 35 50 65 TEMPERATURE (C) 80 95 110 125 0 -40 -25 -10 5 20 35 50 65 TEMPERATURE (C) 80 95 110 125 Figure 16. Output Voltage (VOL) to Supply Rail vs. Temperature Figure 19. Output Voltage (VOL) to Supply Rail vs. Temperature Rev. 0 | Page 8 of 20 ADA4505-2 TA = 25C, unless otherwise noted. 100 80 60 40 GAIN (dB) VSY = 1.8V 225 180 135 90 PHASE (Degrees) 100 80 60 40 VSY = 5V 225 180 135 90 45 0 -45 -90 0 -20 -40 -60 -80 0 -45 -90 GAIN (dB) 20 45 20 0 -20 -40 -60 -80 -135 -180 07416-025 -135 -180 1k 10k FREQUENCY (Hz) 100k 07416-026 07416-030 -100 100 1k 10k FREQUENCY (Hz) 100k -225 1M -100 100 -225 1M Figure 20. Open-Loop Gain and Phase vs. Frequency 60 50 40 G = -100 VSY = 1.8V 60 50 40 Figure 23. Open-Loop Gain and Phase vs. Frequency VSY = 5V G = -100 CLOSED-LOOP GAIN (dB) CLOSED-LOOP GAIN (dB) 30 20 10 0 -10 -20 -30 -40 -50 07416-027 30 20 10 0 -10 -20 -30 -40 -50 G = -10 G = -10 G = -1 G = -1 1k 10k FREQUENCY (Hz) 100k 1M 1k 10k FREQUENCY (Hz) 100k 1M Figure 21. Closed-Loop Gain vs. Frequency.. 10k VSY = 1.8V 10k G = -10 1k G = -100 100 G = -1 ZOUT () Figure 24. Closed-Loop Gain vs. Frequency VSY = 5V G = -10 1k G = -100 100 G = -1 10 ZOUT () 10 1 1 07416-029 0.1 10 100 1k 10k FREQUENCY (Hz) 100k 1M 0.1 10 100 1k 10k FREQUENCY (Hz) 100k 1M Figure 22. Output Impedance vs. Frequency Figure 25. Output Impedance vs. Frequency Rev. 0 | Page 9 of 20 07416-028 -60 100 -60 100 PHASE (Degrees) ADA4505-2 TA = 25C, unless otherwise noted. 120 VSY = 1.8V 100 100 120 VSY = 5V 80 80 60 CMRR (dB) 07416-031 CMRR (dB) 60 40 40 20 20 1k 10k FREQUENCY (Hz) 100k 1M 1k 10k FREQUENCY (Hz) 100k 1M Figure 26. CMRR vs. Frequency 120 120 VSY = 1.8V 100 Figure 29. CMRR vs. Frequency VSY = 5V 100 80 PSRR (dB) 80 PSRR (dB) 60 60 40 40 20 PSRR+ PSRR- 07416-033 20 PSRR+ PSRR- 100 1k 10k FREQUENCY (Hz) 100k 1M 07416-034 07416-050 0 10 100 1k 10k FREQUENCY (Hz) 100k 1M 0 10 Figure 27. PSRR vs. Frequency 140 1.8V VSY 5V 130 Figure 30. PSRR vs. Frequency 1k VSY = 5V 120 110 en (nV/Hz) PSRR (dB) 100 VSY = 1.8V 100 90 -25 -10 5 20 35 50 65 TEMPERATURE (C) 80 95 110 125 07416-035 80 -40 10 1 10 FREQUENCY (Hz) 100 1000 Figure 28. PSRR vs. Temperature Figure 31. Voltage Noise Density vs. Frequency Rev. 0 | Page 10 of 20 07416-032 0 100 0 100 ADA4505-2 TA = 25C, unless otherwise noted. 80 VSY = 1.8V VIN = 10mV p-p 70 R = 100k L 60 80 VSY = 5V VIN = 10mV p-p 70 R = 100k L 60 OVERSHOOT (%) OVERSHOOT (%) 50 40 30 20 10 07416-036 50 40 30 20 10 0 10 OS+ OS- 100 CAPACITANCE (pF) 1000 07416-037 OS+ OS- 0 10 100 CAPACITANCE (pF) 1000 Figure 32. Small Signal Overshoot vs. Load Capacitance T LOAD = 100k || 100pF VSY = 1.8V Figure 35. Small Signal Overshoot vs. Load Capacitance T LOAD = 100k || 100pF VSY = 5V 3.959V p-p VOLTAGE (500mV/DIV) VOLTAGE (1V/DIV) 1.490V p-p 07416-038 TIME (200s/DIV) TIME (200s/DIV) Figure 33. Large Signal Transient Response T LOAD = 100k || 100pF VSY = 1.8V Figure 36. Large Signal Transient Response T LOAD = 100k || 100pF VSY = 5V VOLTAGE (2mV/DIV) VOLTAGE (2mV/DIV) 07416-040 TIME (200s/DIV) TIME (200s/DIV) Figure 34. Small Signal Transient Response Figure 37. Small Signal Transient Response Rev. 0 | Page 11 of 20 07416-041 07416-039 ADA4505-2 TA = 25C, unless otherwise noted. 20 20 VSY = 1.8V 16 16 12 12 ISY (A) 8 ISY (A) VSY = 5V 8 4 4 07416-054 0 0.5 1.0 1.5 2.0 2.5 3.0 VSY (V) 3.5 4.0 4.5 5.0 -25 -10 5 20 35 50 65 TEMPERATURE (C) 80 95 110 125 Figure 38. Supply Current vs. Supply Voltage VSY = 1.8V Figure 41. Total Supply Current vs. Temperature VSY = 5V 2.95V p-p 2.95V p-p INPUT NOISE VOLTAGE (0.5V/DIV) INPUT NOISE VOLTAGE (0.5V/DIV) 07416-052 TIME (s) TIME (s) Figure 39. 0.1 Hz to 10 Hz Noise VSY = 1.8V RL = 100k -20 G = -100 0 VIN = 0.5V p-p VIN = 1V p-p VIN = 1.7V p-p Figure 42. 0.1 Hz to 10 Hz Noise VSY = 5V RL = 100k -20 G = -100 0 VIN = 1V p-p VIN = 2V p-p VIN = 3V p-p VIN = 4V p-p VIN = 4.99V p-p CHANNEL SEPARATION (dB) CHANNEL SEPARATION (dB) -40 -60 -80 -100 -120 -140 100 100k 1k -40 -60 -80 -100 -120 -140 100 100k 1k 07416-053 07416-057 1k FREQUENCY (Hz) 10k 100k 1k FREQUENCY (Hz) 10k 100k Figure 40. Channel Separation vs. Frequency Figure 43. Channel Separation vs. Frequency Rev. 0 | Page 12 of 20 07416-058 07416-055 0 0 -40 ADA4505-2 TA = 25C, unless otherwise noted. 1.8 VSY = 1.8V VIN = 1.7V G=1 RL = 100k 6 VSY = 5V VIN = 4.9V G=1 RL = 100k 1.5 5 OUTPUT SWING (V) 0.9 OUTPUT SWING (V) 07416-059 1.2 4 3 0.6 2 0.3 1 100 1k FREQUENCY (Hz) 10k 100k 100 1k FREQUENCY (Hz) 10k 100k Figure 44. Output Swing vs. Frequency 10 VSY = 1.8V VIN = 100mV p-p RL = 100k 10 Figure 46. Output Swing vs. Frequency VSY = 5V VIN = 100mV p-p RL = 100k 1 1 THD + NOISE (%) THD + NOISE (%) G = -1 0.1 G = +1 G = -1 0.1 G = +1 0.01 0.01 07416-061 100 1k FREQUENCY (Hz) 10k 100k 100 1k FREQUENCY (Hz) 10k 100k Figure 45. THD + Noise vs. Frequency Figure 47. THD + Noise vs. Frequency Rev. 0 | Page 13 of 20 07416-062 0.001 10 0.001 10 07416-060 0 10 0 10 ADA4505-2 THEORY OF OPERATION The ADA4505-2 is a unity-gain stable CMOS rail-to-rail input/ output operational amplifier designed to optimize performance in current consumption, PSRR, CMRR, and zero crossover distortion, all imbedded in a small package. The typical offset voltage is 500 V, with a low peak-to-peak voltage noise of 2.95 V p-p from 0.1 Hz to 10 Hz and a voltage noise density of 55 nV/Hz at 1 kHz. The ADA4505-2 was designed to solve two key problems in low voltage battery-powered applications: battery voltage decrease over time and rail-to-rail input stage distortion. In battery-powered applications, the supply voltage available to the IC is the voltage of the battery. Unfortunately, the voltage of a battery decreases as it discharges itself through the load. This voltage drop over the lifetime of the battery causes an error in the output of the op amps. Some applications requiring precision measurements during the entire lifetime of the battery use voltage regulators to power up the op amps as a solution. If a design uses standard battery cells, the op amps experience a supply voltage change from roughly 3.2 V to 1.8 V during the lifetime of the battery. This means that for a PSRR of 70 dB minimum in a typical op amp, the input-referred offset error is approximately 440 V. If the same application uses the ADA4505-2 with a 100 dB minimum PSRR, the error is only 14 V. It is possible to calibrate out this error or to use an external voltage regulator to power the op amp, but these solutions can increase system cost and complexity. The ADA4505-2 solves the impasse with no additional cost or error-nullifying circuitry. The second problem with battery-powered applications is the distortion caused by the standard rail-to-rail input stage. Using a CMOS non-rail-to-rail input stage (that is, a single differential pair) limits the input voltage to approximately one VGS (gatesource voltage) away from one of the supply lines. Because VGS for normal operation is commonly over 1 V, a single differential pair input stage op amp greatly restricts the allowable input voltage range when using a low supply voltage. This limitation restricts the number of applications where the non-rail-to-rail input op amp was originally intended to be used. To solve this problem, a dual differential pair input stage is usually implemented (see Figure 48); however, this technique has its own drawbacks. One differential pair amplifies the input signal when the commonmode voltage is on the high end, whereas the other pair amplifies the input signal when the common-mode voltage is on the low end. This method also requires a control circuitry to operate the two differential pairs appropriately. Unfortunately, this topology leads to a very noticeable and undesirable problem: if the signal level moves through the range where one input stage turns off and the other one turns on, noticeable distortion occurs (see Figure 49). VDD VBIAS VIN+ IB Q3 Q1 Q2 Q4 VIN- IB VSS Figure 48. A Typical Dual Differential Pair Input Stage Op Amp (Dual PMOS Q1 and Q2 Transistors Form the Lower End of the Input Voltage Range Whereas Dual NMOS Q3 and Q4 Compose the Upper End) 300 250 200 150 100 50 VOS (V) VSY = 5V TA = 25C 0 -50 -100 -150 -200 -250 0 0.5 1.0 1.5 2.0 2.5 3.0 VCM (V) 3.5 4.0 4.5 5.0 07416-044 -300 Figure 49. Typical Input Offset Voltage vs. Common-Mode Voltage Response in a Dual Differential Pair Input Stage Op Amp (Powered by 5 V Supply; Results of Approximately 100 Units per Graph Are Displayed) This distortion forces the designer to come up with impractical ways to avoid the crossover distortion areas, therefore narrowing the common-mode dynamic range of the operational amplifier. The ADA4505-2 solves this crossover distortion problem by using an on-chip charge pump to power the input differential pair. The charge pump creates a supply voltage higher than the voltage of the battery, allowing the input stage to handle a wide range of input signal voltages without using a second differential pair. With this solution, the input voltage can vary from one supply extreme to the other with no distortion, thereby restoring the op amp full common-mode dynamic range. Rev. 0 | Page 14 of 20 07416-043 ADA4505-2 The charge pump has been carefully designed so that switching noise components at any frequency, both within and beyond the amplifier bandwidth, are much lower than the thermal noise floor. Therefore, the spurious-free dynamic range (SFDR) is limited only by the input signal and the thermal or flicker noise. There is no intermodulation between input signal and switching noise. Figure 50 displays a typical front-end section of an operational amplifier with an on-chip charge pump. VPP = POSITIVE PUMPED VOLTAGE = VDD + 1.8V VPP VB CASCODE STAGE AND RAIL-TO-RAIL OUTPUT STAGE VDD VOS (V) Figure 51, input offset voltage vs. input common-mode voltage response, shows the typical response of two devices from Figure 7. Figure 51 has been expanded so that it is easier to compare with Figure 49, typical input offset voltage vs. common-mode voltage response in a dual differential pair input stage op amp. 300 250 200 150 100 50 0 -50 -100 VSY = 5V TA = 25C +IN Q1 Q2 -IN OUT -150 -200 -250 VSS 07416-045 0 0.5 1.0 1.5 2.0 2.5 3.0 VCM (V) 3.5 4.0 4.5 5.0 Figure 50. Typical Front-End Section of an Op Amp with Embedded Charge Pump Figure 51. Input Offset Voltage vs. Input Common-Mode Voltage Response (Powered by a 5 V Supply; Results of Two Units Are Displayed) This solution improves the CMRR performance tremendously. For instance, if the input varies from rail-to-rail on a 2.5 V supply rail, using a part with a CMRR of 70 dB minimum, an input-referred error of 790 V is introduced. Another part with a CMRR of 52 dB minimum generates a 6.3 mV error. The ADA4505-2 CMRR of 90 dB minimum causes only a 79 V error. As with the PSRR error, there are complex ways to minimize this error, but the ADA4505-2 solves this problem without incurring unnecessary circuitry complexity or increased cost. Rev. 0 | Page 15 of 20 07416-046 -300 ADA4505-2 APPLICATIONS INFORMATION PULSE OXIMETER CURRENT SOURCE A pulse oximeter is a noninvasive medical device used for measuring continuously the percentage of hemoglobin (Hb) saturated with oxygen and the pulse rate of a patient. Hemoglobin that is carrying oxygen (oxyhemoglobin) absorbs light in the infrared (IR) region of the spectrum; hemoglobin that is not carrying oxygen (deoxyhemoglobin) absorbs visible red (R) light. In pulse oximetry, a clip containing two LEDs (sometimes more, depending on the complexity of the measurement algorithm) and the light sensor (photodiode) is placed on the finger or earlobe of the patient. One LED emits red light (600 nm to 700 nm) and the other emits light in the near IR (800 nm to 900 nm) region. The clip is connected by a cable to a processor unit. The LEDs are rapidly and sequentially excited by two current sources (one for each LED), whose dc levels depend on the LED being driven, based on manufacturer requirements, and the detector is synchronized to capture the light from each LED as it is transmitted through the tissue. An example design of a dc current source driving the red and infrared LEDs is shown in Figure 52. These dc current sources allow 62.5 mA and 101 mA to flow through the red and infrared LEDs, respectively. First, to prolong battery life, the LEDs are driven only when needed. One-third of the ADG733 SPDT analog switch is used to disconnect/connect the 1.25 V voltage reference from/to each current circuit. When driving the LEDs, the ADR1581 1.25 V voltage reference is buffered by 1/2 of the ADA4505-2; the presence of this voltage on the noninverting input forces the output of the op amp (due to the negative feedback) to maintain a level that makes its inverting input-totrack the noninverting pin. Therefore, the 1.25 V appears in parallel with the 20 R1 or 12.4 R5 current source resistor, creating the flow of the 62.5 mA or 101 mA current through the red or infrared LED as the output of the op amp turns on the Q1 or Q2 N-MOSFET IRLMS2002. The maximum total quiescent currents for the 1/2 ADA4505-2, ADR1581, and ADG733 are 15 A, 70 A, and 1 A, respectively, making a total of 86 A current consumption (430 W power consumption) per circuit, which is good for a system powered by a battery. If the accuracy and temperature drift of the total design need to be improved, then a more accurate and low temperature coefficient drift voltage reference and current source resistor should be utilized. C3 and C4 are used to improve stabilization of U1; R3 and R7 are used to provide some current limit into the U1 inverting pin; and R2 and R6 are used to slow down the rise time of the N-MOSFET when it turns on. These elements may not be needed, or some bench adjustments may be required. +5V CONNECT TO RED LED +5V C1 0.1F 62.5mA R2 V 22 OUT1 Q1 IRLMS2002 R3 1k R1 20 0.1% 1/8 W MIN 8 C2 0.1F U1 1/2 U2 ADG733 +5V R4 53.6k VREF = 1.25V U3 ADR1581 ADA4505-2 5 16 VDD 14 D1 S1A 12 S1B 13 7 V+ V- 4 6 15 D2 S2A 2 S2B 1 C3 22pF 4 D3 S3A 5 S3B 3 RED CURRENT SOURCE 8 GND VSS 9 A2 10 A1 11 A0 6 EN 7 CONNECT TO INFRARED LED U1 1/2 +5V 101mA R6 22 VOUT2 Q2 IRLMS2002 R7 1k ADA4505-2 8 1 V+ V- 4 3 2 I_BIT2 I_BIT1 I_BIT0 I_ENA C4 22pF Figure 52. Pulse Oximeter Red and Infrared Current Sources Using the ADA4505-2 as a Buffer to the Voltage Reference Device Rev. 0 | Page 16 of 20 07416-047 R5 INFRARED CURRENT 12.4 SOURCE 0.1% 1/4 W MIN ADA4505-2 FOUR-POLE LOW-PASS BUTTERWORTH FILTER FOR GLUCOSE MONITOR There are several methods of glucose monitoring: spectroscopic absorption of infrared light in the 2 m to 2.5 m range, reflectance spectrophotometry, and the amperometric type using electrochemical strips with glucose oxidase enzymes. The amperometric type generally uses three electrodes: a reference electrode, a control electrode, and a working electrode. Although this is a very old technique and widely used, signal-to-noise ratio and repeatability can be improved using the ADA4505-2 with its low peak-to-peak voltage noise of 2.95 V p-p from 0.1 Hz to 10 Hz and voltage noise density of 55 nV/Hz at 1 kHz. Another consideration is operation from a 3.3 V battery. Glucose signal currents are usually less than 3 A full scale, so the I-to-V C1 1000pF R1 5M +3.3V WORKING CONTROL 3 8 converter requires low input bias current. The ADA4505-2 is an excellent choice because it provides 0.5 pA typical and 2 pA maximum of input bias current at ambient temperature. A low-pass filter with a cutoff frequency of 80 Hz to100 Hz is desirable in a glucose meter device to remove extraneous noise; this can be a simple two- or four-pole Butterworth. Low power op amps with bandwidths of 50 kHz to 500 kHz should be adequate. The ADA4505-2 with its 50 kHz GBP and 10 A typical of current consumption meets these requirements. A circuit design of a four-pole Butterworth filter (preceded by a one-pole low-pass filter) is shown in Figure 53. With a 3.3 V battery, the total power consumption of this design is 198 W typical at ambient temperature. +3.3V V+ 1 R2 22.6k R3 22.6k C3 0.047F ADA4505-2 5 8 U1 1/2 +3.3V R5 22.6k C5 0.047F 2 REFERENCE V- 2 4 ADA4505-2 C2 0.1F U1 1/2 V+ 7 R4 22.6k ADA4505-2 3 8 U2 1/2 V- 6 4 V+ 1 V- 4 VOUT C4 0.1F DUPLICATE OF CIRCUIT ABOVE 07416-048 Figure 53. A Four-Pole Butterworth Filter That Can Be Used in a Glucose Meter Rev. 0 | Page 17 of 20 ADA4505-2 OUTLINE DIMENSIONS 3.20 3.00 2.80 3.20 3.00 2.80 PIN 1 8 5 1 5.15 4.90 4.65 4 0.65 BSC 0.95 0.85 0.75 0.15 0.00 0.38 0.22 SEATING PLANE 1.10 MAX 8 0 0.80 0.60 0.40 0.23 0.08 COPLANARITY 0.10 COMPLIANT TO JEDEC STANDARDS MO-187-AA Figure 54. 8-Lead Mini Small Outline Package [MSOP] (RM-8) Dimensions shown in millimeters ORDERING GUIDE Model ADA4505-2ARMZ-R21 ADA4505-2ARMZ-RL1 1 Temperature Range -40C to +125C -40C to +125C Package Description 8-Lead MSOP 8-Lead MSOP Package Option RM-8 RM-8 Branding A21 A21 Z = RoHS Compliant Part. Rev. 0 | Page 18 of 20 ADA4505-2 NOTES Rev. 0 | Page 19 of 20 ADA4505-2 NOTES (c)2008 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D07416-0-7/08(0) Rev. 0 | Page 20 of 20 |
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