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Dual Precision, Rail-to-Rail Output Operational Amplifier AD8698
FEATURES
Low offset voltage: 100 V max Low offset voltage drift: 2 V/C max Low input bias current: 700 pA max Low noise: 8 nV/Hz High common-mode rejection: 118 dB min Wide operating temperature: -40C to +85C No phase reversal
CONNECTION DIAGRAMS
8-Lead SOIC (R-8)
OUT A 1 -IN A 2 +IN A 3
8
8-Lead MSOP (RM-8)
V+ OUT B
04807-0-069
OUT A 1 -IN A 2 +IN A 3
8
V+ OUT B
04807-0-070
AD8698
7
AD8698
7
6 -IN B TOP VIEW V- 4 (Not to Scale) 5 +IN B
6 -IN B TOP VIEW V- 4 (Not to Scale) 5 +IN B
Figure 1.
APPLICATIONS
Photodiode amplifier Sensors and controls Multipole filters Integrator
GENERAL DESCRIPTION
The AD8698 is a high precision, rail-to-rail output, low noise, low input bias current operational amplifier. Offset voltage is a respectable 100 V max and drift over temperature is below 2 V/C, eliminating the need for manual offset trimming. The AD8698 is ideal for high impedance sensors, minimizing offset errors due to input bias and offset currents. The rail-to-rail output maximizes dynamic range in a variety of applications, such as photodiode amplifiers, DAC I/V amplifiers, filters, and ADC input amplifiers. The AD8698 dual amplifiers are offered in 8-lead MSOP and narrow 8-lead SOIC packages. The MSOP version is available in tape and reel only.
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: 781.326.8703 (c) 2004 Analog Devices, Inc. All rights reserved.
AD8698 TABLE OF CONTENTS
Specifications .................................................................................... 3 Absolute Maximum Ratings ........................................................... 5 Thermal Resistance ...................................................................... 5 ESD Caution.................................................................................. 5 Typical Performance Characteristics............................................. 6 Applications .................................................................................... 14 Input Overvoltage Protection................................................... 14 Driving Capacitive Loads .......................................................... 14 Instrumentation Amplifier ....................................................... 15 Composite Amplifier ................................................................. 15 Low Noise Applications ............................................................ 16 Driving ADCs ............................................................................. 16 Using the AD8698 in Active Filter Designs ........................... 16 Outline Dimensions....................................................................... 17 Ordering Guide .......................................................................... 17
REVISION HISTORY
4/04--Revision 0: Initial Version
Rev. 0 | Page 2 of 20
AD8698 SPECIFICATIONS
VS = 15 V, VCM = 0 V (@TA = 25oC, unless otherwise noted.) Table 1.
Parameter INPUT CHARACTERISTICS Offset Voltage Offset Voltage Drift Input Bias Current Input Offset Current Input Voltage Range Common-Mode Rejection Ratio Large Signal Voltage Gain Input Capacitance OUTPUT CHARACTERISTICS Output Voltage Swing VOS -40C < TA < +85C VOS/T IB IOS IVR CMRR AVO CDIFF CCM VOH VOH (Ref. to GND) POWER SUPPLY Power Supply Rejection Ratio Supply Current Supply Voltage DYNAMIC PERFORMANCE Slew Rate Gain Bandwidth Product Phase Margin NOISE PERFORMANCE Input Noise Voltage Input Voltage Noise Density Input Voltage Noise Density Current Noise Density VOL VOL PSRR ISY VS SR GBP OO en p-p en en in 0.1 Hz < f < 10 Hz f = 10 Hz f = 1 kHz f = 1 kHz 0.6 15 8 0.2 -40C < TA < +85C -40C < TA < +85C VCM = 13.5 V RL = 2 k, VO = 13.5 V -13.5V 118 900 -40C < TA < +85C -40C < TA < +85C 0.6 20 100 300 2 700 1500 700 1500 13.5 132 1450 6.5 4.6 14.93 14.8 -14.93 -14.82 114 132 2.8 -14.6 -14.5 V V V/C pA pA pA pA V dB V/mV pF pF V V V V dB mA mA V V/s MHz 60 Degrees V p-p nV/Hz nV/Hz pA/Hz Symbol Conditions Min Typ Max Unit
(Ref. to GND)
IL = 1 mA, -40C < TA < +85C IL = 5 mA, -40C < TA < +85C IL = 1 mA, -40C < TA < +85C IL = 5 mA, -40C < TA < +85C 2.5 V < VS < 15 V VO = 0 V -40C < TA < +85C -40C < TA < +85C RL = 2 k
14.85 14.6
2.5 0.4 1
3.2 3.8 15
Rev. 0 | Page 3 of 20
AD8698
VS = 2.5 V, VCM = 0 V (@TA = 25oC, unless otherwise noted.) Table 2.
Parameter INPUT CHARACTERISTICS Offset Voltage Offset Voltage Drift Input Bias Current Input Offset Current Input Voltage Range Common-Mode Rejection Ratio Large Signal Voltage Gain Input Capacitance OUTPUT CHARACTERISTICS Output Voltage Swing (Ref. to GND) (Ref. to GND) VOS -40C < TA < +85C VOS/T IB IOS IVR CMRR AVO CDIFF CCM VOH VOH VOL VOL POWER SUPPLY Power Supply Rejection Ratio Supply Current Supply Voltage DYNAMIC PERFORMANCE Slew Rate Gain Bandwidth Product Phase Margin NOISE PERFORMANCE Input Noise Voltage Input Voltage Noise Density Input Voltage Noise Density Current Noise Density -40C < TA < +85C -40C < TA < +85C VCM = 13.5 V RL = 2 k, VO = 13.5 V -1.5 105 600 -40C < TA < +85C -40C < TA < +85C 20 100 300 2 700 1500 700 1500 +1.5 120 1200 6.4 4.6 2.44 2.29 -2.43 -2.15 -2.2 -1.9 -1.6 114 132 2.3 dB mA mA V V/s MHz Degrees V p-p nV/Hz nV/Hz pA/Hz V V V/C pA pA pA pA V dB V/mV pF pF V V V V Symbol Conditions Min Typ Max Unit
IL = 1 mA, -40C < TA < +85C IL = 5 mA, -40C < TA < +85C IL = 1 mA, -40C < TA < +85C IL = 5 mA, TA = 25C IL= 5mA, -40C2.35 2.1
PSRR ISY Vs SR GBP Oo en p-p en en in
2.5 V < VS < 15 V VO = 0 V -40C < TA < +85C -40C < TA < +85C RL = 2 k
2.5 0.4 1 60
2.8 3.3 15
0.1 Hz < f < 10Hz f = 10 Hz f =1 kHz f = 1 kHz
0.6 15 8 0.2
Rev. 0 | Page 4 of 20
AD8698 ABSOLUTE MAXIMUM RATINGS
Table 3.
Parameter Supply Voltage Input Voltage Differential Input Voltage Output Short-Circuit Duration to Gnd Storage Temperature Range R, RM Packages Operating Temperature Range Junction Temperature Range R, RM Packages Lead Temperature Range (Soldering, 60 Sec) Rating 15 V VS VS Indefinite -65C to +150C -40C to +85C -65C to +150C +300C
THERMAL RESISTANCE
JA is specified for the worst-case conditions, i.e., JA is specified for devices soldered in circuit boards for surface-mount packages. Table 4. Thermal Resistance
Package Type MSOP-8 (RM) SOIC-8 (R) JA 210 158 JC 45 43 Unit C/W C/W
Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
ESD CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 1000 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.
Rev. 0 | Page 5 of 20
AD8698 TYPICAL PERFORMANCE CHARACTERISTICS
80 VS = 15V 70 60 50
GAIN (dB)
100 VS = 15V 80 60 40 20 0 -20 -40 10k
225 180 135 90 45 0 -45 -90 10M
PHASE MARGIN (Degrees)
04807-0-007 04807-0-009
04807-0-001
NUMBER OF AMPLIFIERS
40 30 20 10
04807-0-034
0 0 0.2 0.4 0.6 TCVOS (V/C) 0.8 1.0 1.2
100k FREQUENCY (Hz)
1M
Figure 2. Input Offset Voltage Drift Distribution
80 VS = 15V 70
CLOSED-LOOP GAIN (dB)
Figure 5. Open-Loop Gain and Phase vs. Frequency
50 VS = 15V 40 AV = 100 30 20 AV = 10 10 0 AV = 1 -10 -20
NUMBER OF AMPLIFIERS
60 50 40 30 20 10
04807-0-058
0 -100 -80
-60
-40
-20
0
20
40
60
80
100
1k
10k
100k FREQUENCY (Hz)
1M
10M
VOS (V)
Figure 3. Offset Voltage Distribution
70 VS = 15V 60
NUMBER OF AMPLIFIERS
Figure 6. Closed-Loop Gain vs. Frequency
60 VS = 15V
50 40 30 20 10 0 -400 -320 -240 -160 -80
OUTPUT IMPEDANCE ()
45
30 AV = 100 AV = 10
15 AV = 1 0 10
04807-0-060
0
80
160
240
320
400
100
1k
10k
100k
1M
IB (pA)
FREQUENCY (Hz)
Figure 4. Input Bias Distribution
Figure 7. Output Impedance vs. Frequency
Rev. 0 | Page 6 of 20
AD8698
VOLTAGE (mV)
VS = 15V VIN = 4V p-p CL = 1nF
0 VIN
VOLTAGE (1V/DIV)
-200 15
VOLTAGE (V)
VOUT
0
04807-0-037
VS = 15V VIN = 200mV p-p AV = -100
04807-0-041
TIME (100s/DIV)
TIME (10s/DIV)
Figure 8. Large Signal Transient Response
Figure 11. Positive Overvoltage Recovery
VOLTAGE (mV)
VS = 15V VIN = 200mV p-p CL = 1nF
VOLTAGE (100mV/DIV)
200 VIN
VS = 15V VIN = 200mV AV = -100
0 0
VOLTAGE (V)
-15 VOUT
04807-0-044
04807-0-040
TIME (100s/DIV)
TIME (400s/DIV)
Figure 9. Small Signal Transient Response
50 VS = 15V VIN = 200mV AV = 1
120
Figure 12. Negative Overvoltage Recovery
VS = 15V 100
OVERSHOOT (%)
80
CMRR (dB)
04807-0-013
30
60
20
40
10
20
0
500
1000
1500
2000
2500
3000
1k
10k
100k FREQUENCY (Hz)
1M
10M
CAPACITIVE LOAD (pF)
Figure 10. Overshoot vs. Load Capacitance
Figure 13. CMRR vs. Frequency
Rev. 0 | Page 7 of 20
04807-0-003
0
0
AD8698
100 VS = 15V 80
CURRENT NOISE DENSITY (nV/Hz)
100 VS = 15V
10
60 -PSRR 40 +PSRR
1
20
04807-0-005
100
1k
10k
100k
1M
1
10 FREQUENCY (Hz)
100
1k
FREQUENCY (Hz)
Figure 14. PSRR vs. Frequency
20 VS = 15V
Figure 17. Current Noise Density vs. Frequency
-ISC
VS = 15V
SHORT-CIRCUIT CURRENT (mA)
10
VOLTAGE (200nV/DIV)
0
-10
-20 +ISC -30
04807-0-035
-40
-20
0
20
40
60
80
100
TIME (1s/DIV)
TEMPERATURE (C)
Figure 15. Input Voltage Noise
100 VS = 15V
VOLTAGE NOISE DENSITY (nV/Hz)
Figure 18. Short-Circuit Current vs. Temperature
14.96 14.95 14.94 VS = 15V IL = 1mA
OUTPUT SWING (V)
14.93 14.92 14.91 14.90 14.89 14.88 -VOL VOH
10
04807-0-032
1
10 FREQUENCY (Hz)
100
1k
-40
-20
0
20
40
60
80
100
TEMPERATURE (C)
Figure 16. Voltage Noise Density vs. Frequency
Figure 19. Output Swing vs. Temperature
Rev. 0 | Page 8 of 20
04807-0-019
1 0.1
14.87 -60
04807-0-030
-40 -60
04807-0-033
0 10
0.1 0.1
AD8698
14.90 VS = 15V IL = 5mA 138 140 VS = 15V
14.85
OUTPUT VOLTAGE SWING (V)
14.80
14.75
PSRR (dB)
04807-0-020
VOH
136
134
14.70 -VOL 132
14.65
-40
-20
0
20
40
60
80
100
-40
-20
0
20
40
60
80
100
TEMPERATURE (C)
TEMPERATURE (C)
Figure 20. Output Voltage Swing vs. Temperature
30 VS = 15V 20 100
Figure 23. PSRR vs. Temperature
VS = 15V
INPUT BIAS CURRENT (pA)
OFFSET VOLTAGE (V)
50
10
0
0
-10
-50
-20
04807-0-023
-40
-20
0
20
40
60
80
100
-40
-20
0
20
40
60
80
100
TEMPERATURE (C)
TEMPERATURE (C)
Figure 21. Offset Voltage vs. Temperature
155 VS = 15V 150 145 5 6
Figure 24. Input Bias Current vs. Temperature
VS = 15V
OUTPUT SWING (V)
VOL 4
CMRR (dB)
140 135 130 125 120 -60
3
2
1 VOH 0 5 10 LOAD CURRENT (mA) 15 20
04807-0-015
-40
-20
0
20
40
60
80
100
TEMPERATURE (C)
04807-0-027
0
Figure 22. CMRR vs. Temperature
Figure 25. Output Voltage Swing from Rails vs. Load Current
Rev. 0 | Page 9 of 20
04807-0-025
-30 -60
-100 -60
04807-0-029
14.60 -60
130 -60
AD8698
3.5 VS = 15V 80 100 VS = 2.5V 180 135 90 45 0 -45 -90 10M 225
60
40 20 0 -20
2.5
2.0
04807-0-017
-40
-20
0
20
40
60
80
100
100k FREQUENCY (Hz)
1M
TEMPERATURE (C)
Figure 26. Supply Current vs. Temperature
0 VS = 15V -20 60
Figure 29. Open-Loop Gain and Phase vs. Frequency
VS = 2.5V
CHANNEL SEPARATION (dB)
-40 -60 -80 -100 -120 -140 1k
OUTPUT IMPEDANCE ()
45
30 AV = 100 15 AV = 1 0 10 AV = 10
04807-0-010
10k
100k FREQUENCY (Hz)
1M
10M
100
1k
10k
100k
1M
FREQUENCY (Hz)
Figure 27. Channel Separation
70 VS = 2.5V 60
Figure 30. Output Impedance vs. Frequency
VS = 2.5V VIN = 2V p-p CL = 1nF
NUMBER OF AMPLIFIERS
40 30 20 10 0 -100 -80
VOLTAGE (500mV/DIV)
50
0
04807-0-059
-60
-40
-20
0
20
40
60
80
100
VOS (V)
TIME (100s/DIV)
Figure 28. Offset Voltage Distribution
Figure 31. Large Signal Transient Response
Rev. 0 | Page 10 of 20
04807-0-038
04807-0-008
04807-0-002
1.5 -60
-40 10k
PHASE MARGIN (Degrees)
SUPPLY CURRENT (mA)
3.0
GAIN (dB)
AD8698
VS = 2.5V VIN = 200mV p-p CL = 1nF
VOLTAGE (mV)
200 VIN
VS = 2.5V VIN = 200mV p-p AV = -100
VOLTAGE (100mV/DIV)
0 0
VOLTAGE (V)
-2.5 VOUT
04807-0-045
TIME (100s/DIV)
TIME (4s/DIV)
Figure 32. Small Signal Transient Response
50 VS = 2.5V VIN = 200mV AV = 1 120
Figure 35. Negative Overvoltage Recovery
VS = 2.5V 100
40
OVERSHOOT (%)
80
CMRR (dB)
30
60
20
40 10
20
04807-0-014
0
500
1000
1500
2000
2500
3000
1k
10k
100k FREQUENCY (Hz)
1M
10M
CAPACITIVE LOAD (pF)
Figure 33. Overshoot vs. Load Capacitance
100
Figure 36. CMRR vs. Frequency
VOLTAGE (mV)
VS = 2.5V 0 80 VIN
2.5
PSRR (dB)
-200
60 -PSRR 40 +PSRR
VOLTAGE (V)
VOUT
0
20 VS = 2.5V VIN = 200mV p-p AV = -100
04807-0-043
100
1k
10k
100k
1M
TIME (4s/DIV)
FREQUENCY (Hz)
Figure 34. Positive Overvoltage Recovery
Figure 37. PSRR vs. Frequency
Rev. 0 | Page 11 of 20
04807-0-006
0 10
04807-0-004
0
0
04807-0-042
AD8698
20 -ISC VS = 2.5V 20 30 VS = 2.5V
SHORT-CIRCUIT CURRENT (mA)
10
OFFSET VOLTAGE (V)
04807-0-031
10
0
0
-10
-10
-20
+ISC
-20
-40
-20
0
20
40
60
80
100
-40
-20
0
20
40
60
80
100
TEMPERATURE (C)
TEMPERATURE (C)
Figure 38. Short-Circuit Current vs. Temperature
2.46 2.45 2.44 2.43 VOH 2.42 2.41 -VOL 2.40 126 2.39
04807-0-021
Figure 41. Offset Voltage vs. Temperature
134
VS = 2.5V IL = 1mA 132
VS = 2.5V
OUTPUT VOLTAGE (V)
CMRR (dB)
130
128
-40
-20
0
20
40
60
80
100
-40
-20
0
20
40
60
80
100
TEMPERATURE (C)
TEMPERATURE (C)
Figure 39. Output Swing vs. Temperature
2.5 VS = 2.5V IL = 5mA -20
Figure 42. CMRR vs. Temperature
VS = 2.5V
VOH 2.1 -VOL 1.9
INPUT OFFSET CURRENT (pA)
OUTPUT VOLTAGE SWING (V)
2.3
-30
-40
-50
-60
1.7
-70
04807-0-022
-40
-20
0
20
40
60
80
100
-40
-20
0
20
40
60
80
100
TEMPERATURE (C)
TEMPERATURE (C)
Figure 40. Output Voltage Swing vs. Temperature
Figure 43. Input Bias Current vs. Temperature
Rev. 0 | Page 12 of 20
04807-0-026
1.5 -60
-80 -60
04807-0-028
2.38 -60
124 -60
04807-0-024
-30 -60
-30 -60
AD8698
2500 VS = 2.5V 2000 2.5 3.0
SUPPLY CURRENT (mA)
OUTPUT SWING (mV)
2.0
1500 VOL 1000 VOH 500
1.5
1.0
0.5
04807-0-016
0
5
10 LOAD CURRENT (mA)
15
20
0
5
10
15
20
25
30
35
SUPPLY VOLTAGE (V)
Figure 44. Output Voltage Swing from Rails vs. Load Current
3.0 VS = 2.5V 2.5 -20 0
Figure 47. Supply Current vs. Supply Voltage
VS = 2.5V
CHANNEL SEPARATION (dB)
SUPPLY CURRENT (mA)
-40 -60 -80 -100 -120 -140 1k
2.0
1.5
1.0
0.5
04807-0-018
-40
-20
0
20
40
60
80
100
10k
100k FREQUENCY (Hz)
1M
10M
TEMPERATURE (C)
Figure 45. Supply Current vs. Temperature
Figure 48. Channel Separation
VS = 5V VIN = 11.4V p-p
VOLTAGE (2V/DIV)
TIME (400s/DIV)
Figure 46. No Phase Reversal
Rev. 0 | Page 13 of 20
04807-0-039
04807-0-011
0 -60
04807-0-012
0
0
AD8698 APPLICATIONS
INPUT OVERVOLTAGE PROTECTION
The AD8698 has internal protective circuitry which allows voltages at either input to exceed the supply voltage. However, if voltages applied at either input exceed the supply voltage by more than 2 V, it is recommended to use a resistor in series with the inputs to limit the input current and prevent damaging the device. The value of the resistor can be calculated from the following formula:
VOLTAGE (100mV/DIV)
VS = 15V CL = 68nF RS = 30 CS = 5nF AV = 1
VIN - VS 5 mA RS + 500
TIME (10s/DIV)
DRIVING CAPACITIVE LOADS
The AD8698 is stable even when driving heavy capacitive loads in any configuration. Although the AD8698 will safely drive capacitive loads well over 10 nF, it is recommended to use external compensation should the amplifier be subjected to driving a load exceeding 50 nF. This is particularly important in positive unity gain configurations, the worst case for stability. Figure 49 shows the output of the AD8698 with a 68 nF load in response to a 400 mV signal at its positive input; the overshoot is less than 25% without any external compensation. Using a simple "snubber" network reduces the overshoot to less than 10% as shown in Figure 50.
VS = 15V CL = 68nF AV = 1
Figure 50. Compensated Capacitive Load Drive with Snubber
The snubber network consists of a simple RC network whose values are determined empirically.
V- V+ RS
+ -
400mV
CS
Figure 51. Snubber Network
Table 5 provides a few starting values for optimum
compensation.
Table 5. Compensation Values
CL (nF) 47 68 100 RS () 20 30 50 CS (nF) 7 5 3
VOLTAGE (100mV/DIV)
The use of the snubber network does not recover the loss of bandwidth incurred by the load capacitance. The AD8698 maintains a unity gain bandwidth of 1 MHz with load capacitances of up to 1 nF.
TIME (10s/DIV)
Figure 49. Heavy Capacitive Load Drive without Compensation
Rev. 0 | Page 14 of 20
04807-0-057.
04807-0-063
CL
04807-0-061
AD8698
10M
V1
V+ V-
R1 1k
R2 10k
UNITY GAIN BANDWIDTH (MHz)
1M
1/2 AD8698
R3 9k
100k
R4 2k R5 10k
V+ V-
OP184
10k
R3 9k V+ R1 9.8k R7 400
04807-0-064
V-
04807-0-062
1k
V2
1
10 LOAD CAPACITANCE (nF)
100
1/2 AD8698
Figure 53. Three Op Amp In-Amp
Figure 52. Unity Gain Bandwidth vs. Load Capacitance
COMPOSITE AMPLIFIER
The dc accuracy of the AD8698 and the ac performance of the OP184 are combined in the circuit shown in Figure 54. The composite amplifier provides a higher bandwidth, a lower offset voltage, and a higher loop, thereby reducing the gain error substantially. The circuit shown exhibits a total output rms noise of less than 500 V, corresponding to less than 3 mV of peak-to-peak noise over approximately a 3 MHz bandwidth. Cf is used to minimize peaking. The circuit has an inverting gain of 10. In applications with higher closed-loop gains, Cf is necessary to maintain a sufficient phase margin and ensure stability. This results in a narrower closed-loop bandwidth.
R2 10k R1 1k VIN V- V- V+
04807-0-065
Figure 52 shows the unity gain bandwidth as a function of load capacitance.
INSTRUMENTATION AMPLIFIER
Instrumentation amplifiers are used in applications requiring precision, accuracy, and high CMRR. One popular application is signal conditioning in process control, test automation, and measurement instrumentation, where the amplifier is used to amplify small signals. The triple op amp implementation uses the AD8698 at the front end with the OP184 for optimum accuracy. The circuit in Figure 53 enjoys a high overall gain, excellent dc performance, high CMRR, as well as the benefit of an output that swings to the supplies. The CMRR of the in-amp will be limited by the choice of resistor tolerance. R5 is an optional potentiometer that can be used to calibrate the circuit for maximum gain. R7 can be trimmed for optimum CMRR. The output voltage is given by:
2R3 R 2 VO =VIN 1 + R 4 R1
Cf 20pF
V+
OP184
1/2 AD8698
Figure 54. Composite Amplifier Circuit
Rev. 0 | Page 15 of 20
AD8698
LOW NOISE APPLICATIONS
In some applications, it is critical to minimize the noise, and although the AD8698 has a low noise of typically 8 nV/Hz at 1 kHz, paralleling the two amplifiers within the same package reduces the total noise referred to the input to approximately 5.5 nV/Hz. This simple technique is depicted in Figure 55.
VIN V+ V- R1 1k R2 10k R3 100
If a higher gain is desired, the corner frequency should be chosen accordingly. For example, if the amplifier is configured with a gain of 10, the corner frequency of the filter should not be more than 10 kHz. An example of an active filter is the Sallen Key. This topology gives the user the flexibility of implementing a low-pass or a high-pass filter by simply interchanging the resistors and the capacitors. In the high-pass filter of Figure 56, the damping factor Q is set to 1/2 for a maximally flat response (Butterworth).
VOUT
The gain is unity and the bandwidth is 10 kHz with the values shown.
C1 1nF R1 11k
V- V+ R3 1k R4 10k R5 100
04807-0-066
VIN C2 1nF
V+ R2 22k V-
04807-0-067
Figure 55. Paralleling Amplifiers
DRIVING ADCs
The AD8698 can drive extremely heavy capacitive loads without any compensation. Sometimes capacitors are placed at the output of the amplifier to absorb transient currents while the op amp is interfaced with the ADC. Most op amps need a small resistor with the output to isolate the load capacitance. This results in a loss of bandwidth and slows the amplifier down substantially. However, the AD8698 maintains a unity gain bandwidth of 1 MHz with loads of up to 1 nF, as shown in Figure 52.
Figure 56. Two Pole High-Pass Filter
R1 11k VIN R2 11k V+ C2 1nF V-
04807-0-068
C1 2nF
USING THE AD8698 IN ACTIVE FILTER DESIGNS
The AD8698 is recommended for unity gain filter designs with a corner frequency of up to 100 kHz, one tenth of the op amp's unity gain bandwidth.
Figure 57. Two Pole Low-Pass Filter
The circuit of Figure 57 has a bandwidth of 10 kHz and a maximally flat response. In this case, the damping factor is controlled by the ratio of the capacitors and the gain is unity.
Rev. 0 | Page 16 of 20
AD8698 OUTLINE DIMENSIONS
5.00 (0.1968) 4.80 (0.1890)
8 5 4
4.00 (0.1574) 3.80 (0.1497) 1
6.20 (0.2440) 5.80 (0.2284)
1.27 (0.0500) BSC 0.25 (0.0098) 0.10 (0.0040)
1.75 (0.0688) 1.35 (0.0532)
0.50 (0.0196) x 45 0.25 (0.0099)
0.51 (0.0201) COPLANARITY SEATING 0.31 (0.0122) 0.10 PLANE
8 0.25 (0.0098) 0 1.27 (0.0500) 0.40 (0.0157) 0.17 (0.0067)
COMPLIANT TO JEDEC STANDARDS MS-012AA 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 58. 8-Lead Small Outline IC [SOIC] (R-8)--Dimensions shown in millimeters
3.00 BSC
8
5
3.00 BSC
4
4.90 BSC
PIN 1 0.65 BSC 0.15 0.00 0.38 0.22 COPLANARITY 0.10 1.10 MAX 8 0 0.80 0.60 0.40
0.23 0.08 SEATING PLANE
COMPLIANT TO JEDEC STANDARDS MO-187AA
Figure 59. 8-Lead Small Outline IC [SOIC] (RM-8)--Dimensions shown in millimeters
ORDERING GUIDE
Model AD8698ARM-R2 AD8698ARM-REEL AD8698AR AD8698AR-REEL AD8698AR-REEL7 Temperature Package -40C to +85C -40C to +85C -40C to +85C -40C to +85C -40C to +85C Package Description MSOP MSOP SOIC SOIC SOIC Package Option RM-8 RM-8 R-8 R-8 R-8 Branding A02 A02
Rev. 0 | Page 17 of 20
AD8698 NOTES
Rev. 0 | Page 18 of 20
AD8698 NOTES
Rev. 0 | Page 19 of 20
AD8698 NOTES
(c) 2004 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D04807-0-4/04(0)
Rev. 0 | Page 20 of 20
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