AD8045

1 nV/√Hz, Low Power, Rail-to-Rail Output Amplifiers ADA4896-2/ADA4897-1 FEATURES Low wideband noise 1 nV/√Hz 2.8 pA/√Hz Low 1/f noise 2.4 nV/√Hz @ 10 Hz Low distortion: −115 dBc @ 100 kHz, VOUT = 2 V p-p Low power: 3 mA/amp Low input offset voltage: 0.5 mV maximum High speed 230 MHz, −3 dB bandwidth (G = +1) 120 V/μs slew rate 45 ns settling time to 0.1% Rail-to-rail output Wide supply range: 3 V to 10 V Disable feature (ADA4897-1) FUNCTIONAL BLOCK DIAGRAM NC 1 –IN 2 +IN 3 –VS 4 8 7 6 5 DISABLE +VS NC 09447-101 OUT Figure 1. 8-Lead SOIC (ADA4897-1) 8 7 VS = ±5V VOLTAGE NOISE (nV/√Hz) 6 5 4 3 2 1 0 APPLICATIONS Low noise preamplifier Ultrasound amplifiers PLL loop filters High performance ADC drivers DAC buffers 1 10 100 1k 10k 100k 1M 5M FREQUENCY (Hz) Figure 2. Voltage Noise vs. Frequency GENERAL DESCRIPTION The ADA4896-2/ADA4897-1 are unity gain stable, low noise, rail-to-rail output, high speed voltage feedback amplifiers that have a quiescent current of 3 mA. With the 1/f noise of 2.4 nV/√Hz at 10 Hz and a spurious-free dynamic range of −80 dBc at 2 MHz, the ADA4896-2/ADA4897-1 are an ideal solution in a variety of applications, including ultrasound, low noise preamplifiers, and drivers of high performance ADCs. The Analog Devices, Inc., proprietary next generation SiGe bipolar process and innovative architecture enable such high performance amplifiers. TheADA4896-2/ADA4897-1 have 230 MHz bandwidth, 120 V/μs slew rate, and settle to 0.1% in 45 ns. With a wide supply voltage range (3 V to 10 V), the ADA4896-2/ADA4897-1 are ideal candidates for systems that require high dynamic range, precision, and high speed. The ADA4896-2 is available in 8-lead LFCSP and 8-lead MSOP packages. The ADA4897-1 is available in 8-lead SOIC and 6-lead SOT-23 packages. Both the ADA4896-2 and ADA4897-1 work over the extended industrial temperature range of −40°C to +125°C. Table 1. Other Low Noise Amplifiers Part Number AD797 AD8021 AD8099 AD8045 ADA4899-1 ADA4898-1/ ADA4898-2 VN (nV/√Hz) @ 1 kHz 0.9 5 7 6 1.4 0.9 VN (nV/√Hz) @ 100 kHz 0.9 2.1 0.95 3 1 0.9 BW (MHz) 8 490 510 1000 600 65 Supply Voltage (V) 10 to 30 5 to 24 5 to 12 3.3 to 12 5 to 12 10 to 32 Table 2. Complementary ADCs Part Number AD7944 AD7985 AD7986 Bits 14 16 18 Speed (MSPS) 2.5 2.5 2 Power (mW) 15.5 15.5 15 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 ©2011 Analog Devices, Inc. All rights reserved. 09447-102 ADA4896-2/ADA4897-1 TABLE OF CONTENTS Features .............................................................................................. 1 Applications....................................................................................... 1 General Description ......................................................................... 1 Functional Block Diagram .............................................................. 1 Revision History ............................................................................... 2 Specifications..................................................................................... 3 ±5 V Supply ................................................................................... 3 +5 V Supply ................................................................................... 5 +3 V Supply ................................................................................... 7 Absolute Maximum Ratings............................................................ 9 Thermal Resistance ...................................................................... 9 Maximum Power Dissipation ..................................................... 9 ESD Caution.................................................................................. 9 Pin Configurations and Function Descriptions ......................... 10 Typical Performance Characteristics ........................................... 12 Theory of Operation ...................................................................... 18 Amplifier Description................................................................ 18 Input Protection ......................................................................... 18 Disable Operation ...................................................................... 18 DC Errors .................................................................................... 19 Noise Considerations................................................................. 19 Capacitance Drive ...................................................................... 20 Applications Information .............................................................. 21 Typical Performance Values...................................................... 21 Low Noise Gain Selectable Amplifier...................................... 22 Medical Ultrasound Applications ............................................ 23 Layout Considerations............................................................... 24 Ground Plane.............................................................................. 24 Power Supply Bypassing ............................................................ 24 Outline Dimensions ....................................................................... 25 Ordering Guide .......................................................................... 27 REVISION HISTORY 7/11—Revision 0: Initial Version Rev. 0 | Page 2 of 28 ADA4896-2/ADA4897-1 SPECIFICATIONS ±5 V SUPPLY TA = 25°C, G = +1, RL = 1 kΩ to ground, unless otherwise noted. Table 3. Parameter DYNAMIC PERFORMANCE –3 dB Bandwidth Conditions G = +1, VOUT = 0.02 V p-p G = +1, VOUT = 2 V p-p G = +2, VOUT = 0.02 V p-p G = +2, VOUT = 2 V p-p, RL = 100 Ω G = +2, VOUT = 6 V step G = +2, VOUT = 2 V step G = +2, VOUT = 2 V step fC = 100 kHz, VOUT = 2 V p-p fC = 1 MHz, VOUT = 2 V p-p fC = 2 MHz, VOUT = 2 V p-p fC = 5 MHz, VOUT = 2 V p-p f = 10 Hz f = 100 kHz f = 10 Hz f = 100 kHz G = +101, RF = 1 kΩ, RG = 10 Ω −500 −17 −0.6 100 Min Typ 230 30 90 7 120 45 90 −115 −93 −80 −61 2.4 1 31 2.8 99 −28 0.2 −11 3 −0.02 110 10 M/10 k 3/11 −4.9 to +4.1 −120 81 4.96 −4.97 4.73 −4.84 80 135 39 3 to 10 3.0 0.25 −125 −121 +500 −4 +0.6 Max Unit MHz MHz MHz MHz V/μs ns ns dBc dBc dBc dBc nV/√Hz nV/√Hz pA/√Hz pA/√Hz nV p-p μV μV/°C μA nA/°C μA dB Ω pF V dB ns V V V V mA mA pF V mA mA dB dB Bandwidth for 0.1 dB Flatness Slew Rate Settling Time to 0.1% Settling Time to 0.01% NOISE/HARMONIC PERFORMANCE Harmonic Distortion (dBc) SFDR Input Voltage Noise Input Current Noise 0.1 Hz to 10 Hz Noise DC PERFORMANCE Input Offset Voltage Input Offset Voltage Drift Input Bias Current Input Bias Current Drift Input Bias Offset Current Open-Loop Gain INPUT CHARACTERISTICS Input Resistance Input Capacitance Input Common-Mode Voltage Range Common-Mode Rejection OUTPUT CHARACTERISTICS Output Overdrive Recovery Time +Output Voltage Swing −Output Voltage Swing +Output Voltage Swing −Output Voltage Swing Output Current Short-Circuit Current Capacitive Load Drive POWER SUPPLY Operating Range Quiescent Current per Amplifier Positive Power Supply Rejection Negative Power Supply Rejection VOUT = −4 V to +4 V Common mode/differential Common mode/differential VCM = −2 V to +2 V VIN = ±5 V, G = +2 R L = 1 kΩ RL = 1 kΩ RL = 100 Ω RL = 100 Ω 45 dBc SFDR Sinking/sourcing 30% overshoot, G = +2 −92 4.85 −4.85 4.5 −4.5 2.8 DISABLE = −5 V +VS = 4 V to 6 V, −VS = −5 V +VS = 5 V, −VS = −4 V to −6 V −96 −96 3.2 Rev. 0 | Page 3 of 28 ADA4896-2/ADA4897-1 Parameter DISABLE PIN (ADA4897-1) DISABLE Voltage Input Current Enabled Disabled Switching Speed Enabled Disabled Conditions Enabled Disabled DISABLE = +5 V DISABLE = −5 V Min Typ >+VS − 0.5 +VS − 0.5 +VS −0.5 +1, RF = 249 Ω and when G = +1, RF = 0 Ω. 2 VS = +5V VOUT = 20mV p-p 2 NORMALIZED CLOSED-LOOP GAIN (dB) 1 0 –1 NORMALIZED CLOSED-LOOP GAIN (dB) G = +1 VS = ±5V G = +1 20mV p-p 1 0 –1 –2 –3 –4 –5 100mV p-p 400mV p-p G = –1 OR G = +2 G = +10 –2 –3 –4 –5 –6 0.1 2V p-p 09447-010 1 10 FREQUENCY (MHz) 100 0.1 1 10 FREQUENCY (MHz) 100 Figure 8. Small Signal Frequency Response vs. Gain Figure 11. Frequency Response for Various VOUT 2 NORMALIZED CLOSED-LOOP GAIN (dB) NORMALIZED CLOSED LOOP GAIN (dB) G = +1 VOUT = 20mV p-p VS = +5V VS = +3V 0.8 1 0 –1 –2 –3 –4 –5 0.1 VS = +5V 0.7 VOUT = 2V p-p G = +2 0.6 RL = 1kΩ 0.5 0.4 0.3 0.2 0.1 0 –0.1 –0.2 RF = RG = 249Ω RF = RG = 100Ω VS = ±5V RF = RG = 49.9Ω 1 10 FREQUENCY (MHz) 100 09447-005 1 FREQUENCY (MHz) 10 50 Figure 9. Small Signal Frequency Response vs. Supply Voltage 2 VS = +5V G = +1 1 VOUT = 20mV p-p 0 –1 –2 +25°C –3 –4 –5 100k +125°C Figure 12. 0.1 dB Bandwidth at Selected RF Value 2 VS = +5V VOUT = 2V p-p –40°C G = –1 G = +1 NORMALIZED CLOSED-LOOP GAIN (dB) NORMALIZED CLOSED-LOOP GAIN (dB) 1 0 –1 –2 –3 –4 –5 G = +10 09447-038 1M 10M FREQUENCY (Hz) 100M 1G 1 10 FREQUENCY (MHz) 100 Figure 10. Small Signal Frequency Response vs. Temperature Figure 13. Large Signal Frequency Response vs. Gain Rev. 0 | Page 12 of 28 09447-006 –6 0.1 09447-061 –0.3 0.1 09447-008 ADA4896-2/ADA4897-1 4 NORMALIZED CLOSED-LOOP GAIN (dB) 3 2 1 0 –1 VS = +5V G = +2 RL = 100Ω VOUT = 20mV p-p –30 CL = 39pF –40 –50 VS = +5V VOUT = 2V p-p G = +10 RL = 100Ω, THIRD RL = 100Ω, SECOND DISTORTION (dBc) –60 –70 CL = 20pF RL = 1kΩ, THIRD –80 CL = 0pF –2 –3 0.1 –90 –100 0.1 RL = 1kΩ, SECOND 1 10 FREQUENCY (MHz) 100 09447-007 1 5 FREQUENCY (MHz) Figure 14. Small Signal Frequency Response vs. Capacitive Load Figure 17. Harmonic Distortion vs. Frequency, G = +10 –50 –60 –70 DISTORTION (dBc) –50 VS = +5V VOUT = 2V p-p G = +1 RL = 100Ω, SECOND DISTORTION (dBc) –60 –70 –80 –90 –100 –110 VS = ±5V G = +1 RL = 1kΩ –80 –90 –100 –110 –120 0.1 RL = 100Ω, THIRD RL = 1kΩ, THIRD 8V p-p THIRD 8V p-p SECOND 2V p-p SECOND 2V p-p THIRD 4V p-p SECOND 4V p-p THIRD RL = 1kΩ, SECOND 09447-021 1 FREQUENCY (MHz) 5 1 FREQUENCY (MHz) 5 Figure 15. Harmonic Distortion vs. Frequency, G = +1 –40 –50 –60 Figure 18. Harmonic Distortion vs. Frequency for Various Output Voltages –50 –60 –70 VS = +5V VOUT = 2V p-p G = +5 G = +2 VS = +5V, SECOND VS = +5V, THIRD RL = 100Ω, SECOND DISTORTION (dBc) DISTORTION (dBc) –80 –90 –100 –110 –120 –70 –80 VS = +3V, SECOND VS = ±5V, SECOND RL = 100Ω, THIRD –90 RL = 1kΩ , THIRD –100 VS = ±5V, THIRD VS = +3V, THIRD 1 5 09447-045 RL = 1kΩ, SECOND 1 5 09447-041 –110 0.1 –130 0.1 FREQUENCY (MHz) FREQUENCY (MHz) Figure 16. Harmonic Distortion vs. Frequency, G = +5 Figure 19. Harmonic Distortion vs. Frequency for Various Supplies Rev. 0 | Page 13 of 28 09447-026 –120 0.1 09447-067 ADA4896-2/ADA4897-1 90 80 70 –80 –100 –120 MAGNITUDE PHASE 18 VS = ±5V 100 UNITS 16 σ = 309.2µV/°C OPEN-LOOP GAIN (dB) NUMBER OF PARTS 60 50 40 30 20 10 0 –10 –20 10k 100k 1M OPEN-LOOP PHASE (Degrees) 14 12 10 8 6 4 2 09447-066 –140 –160 –180 –200 –220 –240 1G 10M 100M FREQUENCY (Hz) 09447-044 0 –600 –400 –200 0 200 400 600 800 1000 OFFSET DRIFT DISTRIBUTION (nV/°C) Figure 20. Open-Loop Gain and Phase vs. Frequency 8 7 VS = ±5V Figure 23. Input Offset Voltage Drift Distribution VS = +3V VS = +5V 10 OUTPUT VOLTAGE (mV) G = +1 VOUT = 20mV p-p TIME = 100ns/DIV VOLTAGE NOISE (nV/√Hz) 6 5 4 3 2 1 0 VS = ±5V 0 –10 09447-027 1 10 100 1k 10k 100k 1M 5M FREQUENCY (Hz) Figure 21. Voltage Noise vs. Frequency 100 Figure 24. Small Signal Transient Response for Various Supplies VS = ±5V CURRENT NOISE (pA/√Hz) CL = 39pF CL = 20pF CL = 0pF VS = ±5V G = +2 TME = 100ns/DIV 10 OUTPUT VOLTAGE (mV) 10 0 –10 1 10 100 1k 10k 100k 1M 5M FREQUENCY (Hz) Figure 22. Current Noise vs. Frequency 09447-060 1 Figure 25. Small Signal Transient Response for Various Capacitive Loads Rev. 0 | Page 14 of 28 09447-039 09447-050 ADA4896-2/ADA4897-1 VS = +3V VS = +5V 10 3 G = +2 TIME = 100ns/DIV 2× VIN VOUT TIME = 100ns/DIV VS = +5V G = +2 2 OUTPUT VOLTAGE (mV) VS = ±5V 1 VOLTAGE (V) 0 0 –1 –10 09447-040 –2 09447-051 –3 Figure 26. Small Signal Transient Response for Various Supplies, G = +2 AVERAGE OUTPUT OVERLOAD RECOVERY TIME (ns) Figure 29. Output Overdrive Recovery 250 1.5 G = +2 1.0 OUTPUT VOLTAGE (V) VS = ±5V VOUT = 2V p-p TIME = 100ns/DIV VS = +5V G = +2 200 G = +1 0.5 150 0 100 –0.5 50 –1.0 09447-009 –1.5 0 100 200 300 400 500 600 700 800 900 OVERLOAD DURATION (ns) Figure 27. Large Signal Transient Response for Various Gains Figure 30. Output Recovery Time vs. Overload Duration 4 3 VIN 105.0 TIME = 100ns/DIV VS = +5V G = +1 102.5 100.0 RISING EDGE VOUT = 3V p-p VS = +5V G = +2 INPUT AND OUTPUT VOLTAGE (V) SLEW RATE (V/µs) 2 1 0 –1 –2 –3 –4 VOUT 97.5 95.0 92.5 90.0 87.5 85.0 82.5 FALLING EDGE 09447-049 –25 –10 5 20 35 50 65 80 95 110 125 TEMPERATURE (°C) Figure 28. Input Overdrive Recovery Figure 31. Slew Rate vs. Temperature Rev. 0 | Page 15 of 28 09447-052 80.0 –40 09447-055 0 ADA4896-2/ADA4897-1 100000 0.3 VS = +5V G = +2 VOUT = 2V STEP RL = 1kΩ TIME = 10ns/DIV 10000 OUTPUT IMPEDANCE (Ω) VS = +5V G = +1 PIN = –30dBm PART DISABLED 0.2 1000 100 10 PART ENABLED 1 0.1 SETTLING (%) 0.1 0 –0.1 –0.2 09447-028 –0.3 1 10 FREQUENCY (MHz) 100 Figure 32. Settling Time to 0.1% –20 –30 COMMON-MODE REJECTION (dB) Figure 35. Output Impedance vs. Frequency –33.5 VS = +5V ∆VCM = 2V p-p INPUT OFFSET VOLTAGE (µV) –40 –50 –60 –70 –80 –90 –100 –110 –120 09447-029 –31.0 VS = +3V –28.5 VS = +5V VS = ±5V 10k 100k 1M 10M 100M –25 –10 5 20 35 50 65 80 95 110 125 FREQUENCY (Hz) TEMPERATURE (°C) Figure 33. CMRR vs. Frequency 0 –10 POWER SUPPLY REJECTION (dB) Figure 36. Input Offset Voltage vs. Temperature for Various Supplies –10.50 VS = ±5V –20 –30 –40 –50 –60 –70 –80 –90 –100 –110 –120 VS = +5V ∆VS = 2V p-p G = +1 INPUT BIAS CURRENT (µA) –10.75 VS = +5V –PSRR –11.00 +PSRR VS = +3V –11.25 09447-030 10k 100k 1M 10M 100M –25 –10 5 20 35 50 65 80 95 110 125 FREQUENCY (Hz) TEMPERATURE (°C) Figure 34. PSRR vs. Frequency Figure 37. Input Bias Current vs. Temperature for Various Supplies Rev. 0 | Page 16 of 28 09447-046 –130 1k –11.50 –40 09447-042 –130 1k –26.0 –40 09447-013 0.01 0.1 ADA4896-2/ADA4897-1 3.2 3.1 SUPPLY CURRENT (mA) 5.5 VS = ±5V 5.0 4.5 DISABLE PIN TIME = 2µs/DIV VS = +5V G = +1 VIN = 1V +25°C –40°C 3.875 3.750 3.625 3.500 3.375 3.250 3.125 3.000 2.875 2.750 OUTPUT VOLTAGE (V) 09447-015 3.0 2.9 2.8 2.7 2.6 2.5 –40 VS = +3V 4.0 DISABLE PIN (V) VS = +5V 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 +125°C 2.625 09447-056 2.500 2.375 09447-043 –25 –10 5 20 35 50 65 80 95 110 125 –0.5 TEMPERATURE (°C) Figure 38. Supply Current vs. Temperature for Various Supplies Figure 41. Turn-Off Time vs. Temperature –40 –50 –60 CROSSTALK (dB) VS = +5V G = +2 VOUT = 2V p-p –30 –40 –50 –60 VS = +5V G = +2 RL = 100Ω VOUT = 2V p-p ISOLATION (dB) 09447-014 –70 –80 –90 –100 –110 –120 –130 0.01 0.1 1 FREQUENCY (MHz) 10 100 –70 –80 –90 –100 –110 –120 –130 –140 0.01 0.1 1 FREQUENCY (MHz) 10 100 Figure 39. Crosstalk OUT1 to OUT2 (ADA4896-2 Only) Figure 42. Forward Isolation vs. Frequency 5.5 5.0 4.5 4.0 DISABLE PIN (V) 3.875 DISABLE PIN 3.750 3.625 OUTPUT VOLTAGE (V) 3.500 +25°C –40°C +125°C 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0 –0.5 TIME = 200ns/DIV VS = +5V G = +1 VIN = 1V 3.375 3.250 3.125 3.000 2.875 2.750 2.625 2.500 2.375 Figure 40. Turn-On Time vs. Temperature Rev. 0 | Page 17 of 28 09447-054 ADA4896-2/ADA4897-1 THEORY OF OPERATION AMPLIFIER DESCRIPTION The ADA4896-2/ADA4897-1 are 1 nV/√Hz input noise amplifiers that consume 3 mA from supplies ranging from 3 V to 10 V. Utilizing the Analog Devices XFCB3 process, the bandwidth is in excess of 200 MHz and unity gain stable and the input structure results in an extremely low input of 1/f noise for a high speed amplifier. The rail-to-rail output stage is designed to drive a heavy feedback load required to achieve an overall low output referred noise. Unlike other low noise unity gain stable amplifiers, the large signal bandwidth has been enhanced beyond the typical fundamental limits to meet more demanding system requirements. The maximum offset of 500 μV and drift of 1 μV/°C make the ADA4896-2/ADA4897-1 an excellent amplifier choice even when the noise is not needed because there is minimal power penalty in achieving the low input noise or the high bandwidth. The ESD clamps start to conduct for input voltages that are more than 0.7 V above the positive supply and input voltages more than 0.7 V below the negative supply. It is recommended that the fault current be limited to less than 10 mA if an overvoltage condition is expected. DISABLE OPERATION Figure 44 shows the ADA4897-1 power-down circuitry. If the DISABLE pin is left unconnected, the base of the input PNP transistor is pulled high through the internal pull-up resistor to the positive supply and the part is turned on. Pulling the DISABLE pin to ≥2 V below the positive supply turns the part off, reducing the supply current to approximately 18 μA for a 5 V voltage supply. VCC IBIAS ESD DISABLE INPUT PROTECTION The ADA4896-2/ADA4897-1 are fully protected from ESD events, withstanding human body model ESD events of 2.5 kV and charge device model events of 1 kV with no measured performance degradation. The precision input is protected with an ESD network between the power supplies and diode clamps across the input device pair, as shown in Figure 43. VCC BIAS ESD VP ESD ESD ESD VN ESD TO AMPLIFIER BIAS VEE Figure 44. DISABLE Circuit VEE 09447-068 The DISABLE pin is protected with ESD clamps, as shown in Figure 44. Voltages beyond the power supplies cause these diodes to conduct. For protection of the DISABLE pin, the voltage to this pin should not exceed 0.7 V of the supply voltage, or the input current should be restricted to less than 10 mA with a series resistor. When the amplifier is disabled, its output goes to a high impedance state. The output impedance decreases as frequency increases; this effect can be observed in Figure 35. In disable mode, a forward isolation of 50 dB can be achieved at 10 MHz. Figure 42 shows the forward isolation vs. frequency data. TO THE REST OF THE AMPLIFIER Figure 43. Input Stage and Protection Diodes For differential voltages above approximately 0.7 V, the diode clamps start to conduct. Too much current can cause damage due to excessive heating. If large differential voltages must be sustained across the input terminals, it is recommended that the current through the input clamps be limited to below 10 mA. Series input resistors that are sized appropriately for the expected differential overvoltage provide the needed protection. Rev. 0 | Page 18 of 28 09447-037 ADA4896-2/ADA4897-1 DC ERRORS Figure 45 shows a typical connection diagram and the major dc error sources. RF – VIN + RG IB– – VIP + RS 09447-031 09447-034 NOISE CONSIDERATIONS Figure 46 illustrates the primary noise contributors for the typical gain configurations. The total rms output noise is the root-mean-square of all the contributions. RF vn _ RF = 4kT × RF ven ien vn _ RS = 4kT × RS RS iep + vout_en – + VOS – + VOUT – vn _ RG = 4kT × RG RG IB+ Figure 45. Typical Connection Diagram and DC Error Sources The ideal transfer function (all error sources set to 0 and infinite dc gain) can be written as ⎛R VOUT = ⎜1 + F ⎜R G ⎝ ⎞ ⎛R ⎟ × VIP − ⎜ F ⎟ ⎜R ⎠ ⎝G ⎞ ⎟ × VIN ⎟ ⎠ Figure 46. Noise Sources in Typical Connection The output noise spectral density can be calculated by (1) vout _ en = ⎛R 4kTRF + ⎜1 + F ⎜R G ⎝ 2 2 ⎞ ⎛R ⎞ ⎟ 4kTRs + iep RS 2 + ven + ⎜ F ⎟ 4kTRG + ien 2 RF 2 ⎟ ⎜R ⎟ ⎠ ⎝ G⎠ 2 This reduces to the familiar forms for inverting and noninverting op amp gain expressions, as follows: (Noninverting gain, VIN = 0 V) ⎛R VOUT = ⎜ 1 + F ⎜R G ⎝ ⎞ ⎟ ×VIP ⎟ ⎠ [ ] 2 (6) where: k is Boltzmann’s Constant. T is the absolute temperature, degrees Kelvin. ien is the amplifier input current noise spectral density, pA/√Hz. ven is the amplifier input voltage spectral density, nV/√Hz. RS is the source resistance, as shown in Figure 46. RF and RG are the feedback network resistances, as shown in Figure 46. Source resistance noise, amplifier voltage noise (ven), and the voltage noise from the amplifier current noise (iep × RS) are all subject to the noise gain term (1 + RF/RG). Note that with a 1 nV/√Hz input voltage noise and 2.8 pA/√Hz input current, the noise contributions of the amplifier are relatively small for source resistances between approximately 50 Ω and 700 Ω. Figure 47 shows the total RTI noise due to the amplifier vs. the source resistance. In addition, the value of the feedback resistors used impacts the noise. It is recommended that the value of the feedback resistors be maintained between 250 Ω and 1 kΩ to keep the total noise low. 500 (2) (Inverting gain, VIP = 0 V) ⎛ − RF VOUT = ⎜ ⎜R ⎝G ⎞ ⎟ ×VIN ⎟ ⎠ (3) The total output voltage error is the sum of errors due to the amplifier offset voltage and input currents. The output error due to the offset voltage can be estimated as VOUTERROR = VCM VP − VPNOM VOUT ⎞ ⎛ R F ⎞ ⎛ ⎟ + + ⎜ VOFFSET NOM + ⎟ × ⎜1 + CMRR PSRR A ⎠ ⎜ RG ⎟ ⎝ ⎝ ⎠ (4) where: VOFFSETNOM is the offset voltage at the specified supply voltage, which is measured with the input and output at midsupply. VCM is the common-mode voltage. VP is the power supply voltage. VPNOM is the specified power supply voltage. CMRR is the common-mode rejection ratio. PSRR is the power supply rejection ratio. A is the dc open-loop gain. The output error due to the input currents can be estimated as ⎛R VOUTERROR = (RF || RG ) × ⎜ 1 + F ⎜R G ⎝ ⎞ ⎛R ⎟ I B − − RS × ⎜ 1 + F ⎟ ⎜R G ⎠ ⎝ ⎞ ⎟ × I B+ ⎟ ⎠ NOISE (nV/√Hz) 50 AMPLIFIER AND RESISTOR NOISE 5 (5) TOTAL AMPLIFIER NOISE SOURCE RESISTANCE NOISE Note that setting RS equal to RF||RG compensates for the voltage error due to the input bias current. 500 5k 50k SOURCE RESISTANCE (Ω ) Figure 47. RTI Noise vs. Source Resistance Rev. 0 | Page 19 of 28 09447-057 0.5 50 ADA4896-2/ADA4897-1 CAPACITANCE DRIVE Capacitance at the output of an amplifier creates a delay within the feedback path that, if within the bandwidth of the loop, can create excessive ringing and oscillation. The ADA4897-1/ADA4896-2 show the most peaking at a gain of +2, as demonstrated in Figure 8. Putting a small snub resistor (RSNUB) in series with the amplifier output and the capacitive load mitigates the problem. Figure 48 shows the effect of using a snub resistor (RSNUB) on reducing the peaking for the worst-case frequency response (gain of +2). Using RSNUB = 100 Ω eliminates the peaking entirely, with the trade-off that the closed-loop gain is reduced by 0.8 dB due to attenuation at the output. RSNUB can be adjusted from 0 Ω to 100 Ω to maintain an acceptable level of peaking and closedloop gain, as shown in Figure 48. 3 NORMALIZED CLOSED-LOOP GAIN (dB) VS = 5V VOUT = 200mV p-p 2 G = +2 1 0 –1 –2 –3 –4 RSNUB = 0Ω RSNUB = 100Ω RSNUB = 50Ω R2 249Ω R1 249Ω ADA4896-2 RSNUB VOUT RL 1kΩ CL 39pF 09447-058 VIN –5 100k 1M 10M FREQUENCY (Hz) 100M Figure 48. Using a Snub Resistor to Reduce Peaking Due to Output Capacitive Load Rev. 0 | Page 20 of 28 ADA4896-2/ADA4897-1 APPLICATIONS INFORMATION TYPICAL PERFORMANCE VALUES To reduce design time and eliminate uncertainty, Table 10 provides a convenient reference for typical gains, component values, and performance parameters. The supply voltage used is 5 V. The bandwidth is obtained with a small signal output of 200 mV p-p, and the slew rate is obtained with a 2 V output step. Note that as the gain increases, the small-signal bandwidth decreases, as is expected from the gain bandwidth product relationship. In addition, the phase margin improves with higher gains, and the amplifier becomes more stable. As a result, the peaking in the frequency response is reduced (see Figure 49). NORMALIZED CLOSED-LOOP GAIN (dB) 2 VS = +5V VOUT = 200mV p-p 1 RF = 249Ω RL = 1kΩ 0 –1 G = +10 –2 G = +20 –3 –4 –5 –6 100k G = +5 G = +2 G = +1 1M 10M FREQUENCY (Hz) 100M 500M Figure 49. Small Signal Frequency Response at Various Gains Table 10. Recommended Values and Typical Performance Gain +1 +2 +5 +10 +20 RF (Ω) 0 249 249 249 249 RG (Ω) N/A 249 61.9 27.4 13.0 −3 dB BW (MHz) 92 54 30 17 9 Slew Rate (tR/tF) (V/μs) 78/158 101/140 119/137 87/88 37/37 Peaking (dB) 0.8 1.2 0 0 0 Output Voltage Noise Only (nV/√Hz) 1 2 5 10 20 Total Output Noise Including Resistors (nV/√Hz) 1.0 3.6 6.8 12.0 21.1 Rev. 0 | Page 21 of 28 09447-020 ADA4896-2/ADA4897-1 LOW NOISE GAIN SELECTABLE AMPLIFIER RF2 450Ω RF1 150Ω +5V RG1 150Ω 2 8 S1B 1 V01 D1 S1A V1 V2 S2B D2 5 S2A 4 09447-100 USING S3B IS OPTIONAL S3B D3 +5V 6 8 7 V02 RL ADA4896-2 ADA4896-2 VIN 3 4 –5V –5V Figure 50. Using the ADA4896-2 and the ADG633 to Construct a Low Noise Gain Selectable Amplifier to Drive a Low Resistive Load A gain selectable amplifier makes processing a wide range of input signals possible. The traditional gain selectable amplifier involves switches in the feedback loops connecting to the inverting input. In this case the switch resistance degrades the noise performance of the amplifier, as well as adding significant capacitance on the inverting input node. The noise and capacitance issue can be especially bothersome when working with low noise amplifiers. Also, the switch resistances contribute to nonlinear gain error, which is undesirable. Figure 50 presents an innovative switching technique used in the gain selectable amplifier such that the 1 nV/Hz noise performance of the ADA4896-2 is preserved, while the nonlinear gain error is much reduced. With this technique, one can also choose switches with minimal capacitance, which optimizes the bandwidth of the circuit. In this circuit, the switches are implemented with the ADG633 and they are configured such that either S1A and S2A are on, or S1B and S2B are on. In this example, when the S1A and S2A switches are on, the first stage amplifier gain is +4. When the S1B and S2B switches are on, the first stage amplifier gain is +2. The first set of switches of the ADG633 is put in the output side of the feedback loop and the second set of switches is used to sample at a point (V1 and V2) where switch resistances and nonlinear resistances do not matter. This way, the gain error can be reduced while preserving the noise performance of the ADA4896-2/ADA4897-1. 0.05 0 It should be noted that the input bias current of the output buffer can cause problems with the impedance of the S2A and S2B sampling switches. Both sampling switches are not only nonlinear with voltage but with temperature as well. If this is an issue, place the unused switch of the ADG633 in the feedback path of the output buffer, as shown in Figure 50, to balance the bias currents. The following derivation shows that sampling at V1 yields the desired signal gain without gain error. RS denotes the switch resistance. V2 can be derived with the same method. ⎛ R + RS1 ⎞ ⎟ V01 = VIN × ⎜1 + F1 ⎜ RG1 ⎟ ⎠ ⎝ ⎛ RF1 + RG1 ⎞ ⎟ V1 = V01 × ⎜ ⎜R +R +R ⎟ G1 S1 ⎠ ⎝ F1 (1) (2) Substituting (1) into (2), the following derivation is obtained ⎛ R⎞ V1 = VIN × ⎜1 + F1 ⎟ ⎜ R⎟ G1 ⎠ ⎝ (3) Figure 51 compares the gain errors when the output signal is sampled at V01 vs. V02 for a range of dc inputs. Note that sampling at V02 reduces the gain error significantly, as predicted in Equation 3. Figure 52 shows the normalized frequency response of the circuit at V02. 6 NORMALIZED CLOSED-LOOP GAIN (dB) 14.0 13.5 –0.05 VS = ±5V 3 VIN = 100mV p-p RL = 1kΩ 0 –3 –6 –9 –12 –15 –18 –21 –24 –27 1M 10M FREQUENCY (Hz) 100M 500M 09447-064 –0.10 –0.15 V02 GAIN ERROR AT V01 (%) GAIN ERROR AT V02 (%) 13.0 G = +4 G = +2 12.5 –0.20 –0.25 –0.30 V01 12.0 11.5 –0.35 0 0.5 1.0 INPUT VOLTAGE (V) 1.5 2.0 Figure 51. Gain Errors at V01 vs. V02 Rev. 0 | Page 22 of 28 09447-063 –0.40 11.0 –30 100k Figure 52. Frequency Response of V02/VIN ADA4896-2/ADA4897-1 MEDICAL ULTRASOUND APPLICATIONS Tx BEAMFORMER BEAMFORMER CENTRAL CONTROL AD9279 HV MUX/ DEMUX T/R SWITCHES LNA VGA AAF ADC Rx BEAMFORMER (B AND F MODES) TRANSDUCER ARRAY CW (ANALOG) BEAMFORMER ADA4896-2/ ADA4897-1 SPECTRAL DOPPLER PROCESSING MODE IMAGE AND MOTION PROCESSING (B MODE) COLOR DOPPLER (PW) PROCESSING (F MODE) DISPLAY Figure 53. Simplified Ultrasound System Block Diagram Overview of the Ultrasound System Medical ultrasound systems are among the most sophisticated signal processing systems in widespread use today. By transmitting acoustic energy into the body and receiving and processing the returning reflections, ultrasound systems can generate images of internal organs and structures, map blood flow and tissue motion, and provide highly accurate blood velocity information. Figure 53 shows a simplified block diagram of an ultrasound system. The ultrasound system consists of two main operations, the time gain control (TGC) operation and the continuous wave (CW) Doppler operation. The AD9279 integrates the essential components of these two operations into a single IC. It contains eight channels of a variable gain amplifier (VGA) with a low noise preamplifier (LNA), an antialiasing filter (AAF), an analog-to-digital converter (ADC), and an I/Q demodulator with programmable phase rotation. For detailed information about how to use the AD9279 in the ultrasound system, refer to the AD9279 data sheet. The rail-to-rail output feature and the high output current drive of the ADA4896-2/ADA4897-1 make them a suitable candidate for the I-to-V converter, summer, and as a ADC driver. Figure 54 shows an interconnection block diagram of all eight channels of the AD9279. Two stages of ADA4896-2 amplifiers are used. The first stage does an I-to-V conversion and filters the high frequency content that results from the demodulation process. The second stage of ADA4896-2 amplifiers is used to sum the output currents of multiple AD9279, to provide gain, and to drive the AD7982, an 18-bit SAR ADC. The output-referred noise of the CW signal path depends on the LNA gain and the selection of the first stage summing amplifier and the value of RFILT. To determine the output referred noise, it is important to know the active low-pass filter (LPF) values RA, RFILT, and CFILT, as shown as Figure 54. Typical filter values for all eight channels of a single AD9279 are 100 Ω for RA, 500 Ω for RFILT, and 2.0 nF for CFILT; these values implement a 100 kHz single-pole LPF. The gain of the I-to-V converter can be increased by increasing the filter resistor, RFILT. To keep the corner frequency the same, decrease the filter capacitor, CFILT, by the same factor. The factor limiting the magnitude of the gain is the output swing and drive capability of the op amp selected for the I-to-V converter, in this example, the ADA4896-2/ADA4897-1. Because any amplifier has limited drive capability, there is a finite number of channels that can be summed. ADA4896-2/ADA4897-1 in the Ultrasound System The ADA4896-2/ADA4897-1 are used in the CW Doppler path in the ultrasound application after the I/Q demodulators of the AD9279. Doppler signals can be typically between 100 Hz to 100 kHz.The low noise floor, high dynamic range of the ADA4896-2/ADA4897-1 makes them an excellent choice for processing weak Doppler signals. Rev. 0 | Page 23 of 28 09447-033 AUDIO OUTPUT ADA4896-2/ADA4897-1 RFILT CFILT 50Ω 1.5V 1.5V ADA4896-2/ ADA4897-1 2.5V 2.5V ADA4896-2/ ADA4897-1 50Ω 4nF Φ CHANNEL A LNA CWI+ AD7982 I 18-BIT ADC Φ CWI– RA CFILT RFILT RFILT CFILT 50Ω CWQ+ RA AD7982 Q Φ CHANNEL H LNA CWQ– 1.5V 1.5V ADA4896-2/ ADA4897-1 2.5V 2.5V ADA4896-2/ ADA4897-1 50Ω 4nF 18-BIT ADC Φ RA CFILT RFILT 4 LO GENERATION AD9279 09447-032 4LO+ Figure 54. Using the ADA4896-2/ADA4897-1 as Filters, I-to-V Converters, Current Summers, and ADC Drivers After the I/Q Outputs of the AD9279 LAYOUT CONSIDERATIONS To ensure optimal performance, careful and deliberate attention must be paid to the board layout, signal routing, power supply bypassing, and grounding. helps to ensure that the power supply pins are provided a low ac impedance across a wide band of frequencies. This is important for minimizing the coupling of noise into the amplifier. This can be especially important when the amplifier PSR is starting to roll off—the bypass capacitors can help lessen the degradation in PSR performance. Starting directly at the ADA4896-2/ADA4897-1 power supply pins, the smallest value capacitor should be placed on the same side of the board as the amplifier, and as close as possible to the amplifier power supply pin. The ground end of the capacitor should be connected directly to the ground plane. Keeping the capacitor’s distance short but equal from the load is important and can improve distortion performance. This process should be repeated for the next largest value capacitor. It is recommended that a 0.1 μF ceramic 0508 case be used. The 0508 case size offers low series inductance and excellent high frequency performance. A 10 μF electrolytic capacitor should be placed in parallel with the 0.1 μF capacitor. Depending on the circuit parameters, some enhancement to performance can be realized by adding additional capacitors. Each circuit is different and should be individually analyzed for optimal performance. GROUND PLANE It is important to avoid ground in the areas under and around the input and output of the ADA4896-2/ADA4897-1. Stray capacitance created between the ground plane and the input and output pads of a device are detrimental to high speed amplifier performance. Stray capacitance at the inverting input, along with the amplifier input capacitance, lowers the phase margin and can cause instability. Stray capacitance at the output creates a pole in the feedback loop. This can reduce phase margin and can cause the circuit to become unstable. POWER SUPPLY BYPASSING Power supply bypassing is a critical aspect in the performance of the ADA4896-2/ADA4897-1. A parallel connection of capacitors from each of the power supply pins to ground works best. Smaller value capacitors offer better high frequency response, whereas larger value electrolytics offer better low frequency performance. Paralleling different values and sizes of capacitors RESET 4LO– Rev. 0 | Page 24 of 28 ADA4896-2/ADA4897-1 OUTLINE DIMENSIONS 5.00 (0.1968) 4.80 (0.1890) 4.00 (0.1574) 3.80 (0.1497) 8 1 5 4 6.20 (0.2441) 5.80 (0.2284) 1.27 (0.0500) BSC 0.25 (0.0098) 0.10 (0.0040) COPLANARITY 0.10 SEATING PLANE 1.75 (0.0688) 1.35 (0.0532) 0.50 (0.0196) 0.25 (0.0099) 8° 0° 0.25 (0.0098) 0.17 (0.0067) 1.27 (0.0500) 0.40 (0.0157) 45° 0.51 (0.0201) 0.31 (0.0122) COMPLIANT TO JEDEC STANDARDS MS-012-AA CONTROLLING DIMENSIONS ARE IN MILLIMETERS; INCH DIMENSIONS (IN PARENTHESES) ARE ROUNDED-OFF MILLIMETER EQUIVALENTS FOR REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN. Figure 55. 8-Lead Standard Small Outline Package [SOIC_N] Narrow Body (R-8) Dimensions shown in millimeters and (inches) 3.20 3.00 2.80 3.20 3.00 2.80 PIN 1 IDENTIFIER 8 5 1 5.15 4.90 4.65 4 0.65 BSC 0.95 0.85 0.75 0.15 0.05 COPLANARITY 0.10 0.40 0.25 15° MAX 1.10 MAX 0.23 0.09 0.80 0.55 0.40 10-07-2009-B 6° 0° COMPLIANT TO JEDEC STANDARDS MO-187-AA Figure 56. 8-Lead Mini Small Outline Package [MSOP] (RM-8) Dimensions shown in millimeters Rev. 0 | Page 25 of 28 012407-A ADA4896-2/ADA4897-1 3.00 2.90 2.80 1.70 1.60 1.50 PIN 1 INDICATOR 6 5 4 3.00 2.80 2.60 1 2 3 0.95 BSC 1.90 BSC 1.30 1.15 0.90 1.45 MAX 0.95 MIN 0.20 MAX 0.08 MIN 10° 4° 0° 0.55 0.45 0.35 12-16-2008-A 0.15 MAX 0.05 MIN 0.50 MAX 0.30 MIN SEATING PLANE 0.60 BSC COMPLIANT TO JEDEC STANDARDS MO-178-AB Figure 57. 6-Lead Small Outline Transistor Package [SOT-23] (RJ-6) Dimensions shown in millimeters 3.10 3.00 SQ 2.90 5 2.44 2.34 2.24 0.50 BSC 8 PIN 1 INDEX AREA 0.50 0.40 0.30 TOP VIEW EXPOSED PAD 1.70 1.60 1.50 4 BOTTOM VIEW 1 PIN 1 INDICATOR (R 0.15) 0.80 0.75 0.70 SEATING PLANE 0.30 0.25 0.20 0.05 MAX 0.02 NOM COPLANARITY 0.08 0.203 REF FOR PROPER CONNECTION OF THE EXPOSED PAD, REFER TO THE PIN CONFIGURATION AND FUNCTION DESCRIPTIONS SECTION OF THIS DATA SHEET. 01-24-2011-B COMPLIANT TO JEDEC STANDARDS MO-229-WEED Figure 58. 8-Lead Lead Frame Chip Scale Package [LFCSP_WD] 3 mm × 3 mm Body, Very Very Thin, Dual Lead (CP-8-11) Dimensions shown in millimeters Rev. 0 | Page 26 of 28 ADA4896-2/ADA4897-1 ORDERING GUIDE Model 1 ADA4896-2ARMZ ADA4896-2ARMZ-R7 ADA4896-2ARMZ-RL ADA4896-2ACPZ-R2 ADA4896-2ACPZ-R7 ADA4896-2ACPZ-RL ADA4896-2ACP-EBZ ADA4896-2ARM-EBZ ADA4897-1ARZ ADA4897-1ARZ-R7 ADA4897-1ARZ-RL ADA4897-1ARJZ-R2 ADA4897-1ARJZ-R7 ADA4897-1ARJZ-RL ADA4897-1AR-EBZ ADA4897-1ARJ-EBZ 1 Temperature Range −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 −40°C to +125°C −40°C to +125°C −40°C to +125°C −40°C to +125°C Package Description 8-Lead MSOP 8-Lead MSOP 8-Lead MSOP 8-Lead LFCSP_WD 8-Lead LFCSP_WD 8-Lead LFCSP_WD Evaluation Board for the 8-Lead LFCSP Evaluation Board for the 8-Lead MSOP 8-Lead SOIC_N 8-Lead SOIC_N 8-Lead SOIC_N 6-Lead SOT-23 6-Lead SOT-23 6-Lead SOT-23 Evaluation Board for the 8-Lead SOIC_N Evaluation Board for the 6-Lead SOT-23 Package Option RM-8 RM-8 RM-8 CP-8-11 CP-8-11 CP-8-11 Ordering Quantity 50 1,000 3,000 250 1,500 5,000 Branding H2P H2P H2P H2P H2P H2P R-8 R-8 R-8 RJ-6 RJ-6 RJ-6 98 1,000 2,500 250 3,000 10,000 H2K H2K H2K Z = RoHS Compliant Part. Rev. 0 | Page 27 of 28 ADA4896-2/ADA4897-1 NOTES ©2011 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D09447-0-7/11(0) Rev. 0 | Page 28 of 28