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Low Noise, Precision CMOS Amplifier AD8655/AD8656
FEATURES
Low noise: 2.7 nV/Hz @ f = 10 kHz Low offset voltage: 250 V max over VCM Offset voltage drift: 0.4 V/C typ and 2.3 V/C max Bandwidth: 28 MHz Rail-to-rail input/output Unity gain stable 2.7 V to 5.5 V operation -40C to +125C operation
NC 1 -IN 2 +IN 3 V- 4
PIN CONFIGURATIONS
8
NC V+ OUT
05304-048
OUT A 1 -IN A 2 +IN A 3 V- 4
8
V+ OUT B
05304-059
AD8655
TOP VIEW (Not to Scale)
7 6 5
AD8656
TOP VIEW (Not to Scale)
7 6 5
-IN B +IN B
NC
NC = NO CONNECT
Figure 1. AD8655 8-Lead MSOP (RM-8)
Figure 2. AD8656 8-Lead MSOP (RM-8)
APPLICATIONS
ADC and DAC buffers Audio Industrial controls Precision filters Digital scales Strain gauges PLL filters
NC 1 -IN 2 +IN 3
8
NC V+ OUT
05304-049
OUT A 1 -IN A 2 +IN A 3
8
V+ OUT B
05304-060
AD8655
7 6 5
AD8656
7
TOP VIEW V- 4 (Not to Scale)
NC
6 -IN B TOP VIEW V- 4 (Not to Scale) 5 +IN B
NC = NO CONNECT
Figure 3. AD8655 8-Lead SOIC (R-8)
Figure 4. AD8656 8-Lead SOIC (R-8)
GENERAL DESCRIPTION
The AD8655/AD8656 are the industry's lowest noise, precision CMOS amplifiers. They leverage the Analog Devices DigiTrim(R) technology to achieve high dc accuracy. The AD8655/AD8656 provide low noise (2.7 nV/Hz @ 10 kHz), low THD + N (0.0007%), and high precision performance (250 V max over VCM) to low voltage applications. The ability to swing rail-to-rail at the input and output enables designers to buffer analog-to-digital converters (ADCs) and other wide dynamic range devices in single-supply systems. The high precision performance of the AD8655/AD8656 improves the resolution and dynamic range in low voltage applications. Audio applications, such as microphone pre-amps and audio mixing consoles, benefit from the low noise, low distortion, and high output current capability of the AD8655/ AD8656 to reduce system level noise performance and maintain audio fidelity. The high precision and rail-to-rail input and output of the AD8655/AD8656 benefit data acquisition, process controls, and PLL filter applications. The AD8655/AD8656 are fully specified over the -40C to +125C temperature range. The AD8655/AD8656 are available in Pb-free, 8-lead MSOP and SOIC packages.
Rev. A
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) 2005 Analog Devices, Inc. All rights reserved.
AD8655/AD8656
TABLE OF CONTENTS
Specifications..................................................................................... 3 Absolute Maximum Ratings............................................................ 5 ESD Caution.................................................................................. 5 Typical Performance Characteristics ............................................. 6 Theory of Operation ...................................................................... 15 Applications..................................................................................... 16 Input Overvoltage Protection ................................................... 16 Input Capacitance....................................................................... 16 Driving Capacitive Loads.......................................................... 16 Layout, Grounding, and Bypassing Considerations .................. 18 Power Supply Bypassing ............................................................ 18 Grounding ................................................................................... 18 Leakage Currents........................................................................ 18 Outline Dimensions ....................................................................... 19 Ordering Guide .......................................................................... 19
REVISION HISTORY
6/05--Rev. 0 to Rev. A Added AD8656 ...................................................................Universal Added Figure 2 and Figure 4........................................................... 1 Changes to Specifications ................................................................ 3 Changed Caption of Figure 12 and Added Figure 13 .................. 7 Replaced Figure 16 ........................................................................... 7 Changed Caption of Figure 37 and Added Figure 38 ................ 11 Replaced Figure 47 ......................................................................... 13 Added Figure 55.............................................................................. 14 Changes to Ordering Guide .......................................................... 18 4/05--Revision 0: Initial Version
Rev. A | Page 2 of 20
AD8655/AD8656
SPECIFICATIONS
VS = 5.0 V, VCM = VS/2, TA = 25C, unless otherwise specified. 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 OUTPUT CHARACTERISTICS Output Voltage High Output Voltage Low Output Current POWER SUPPLY Power Supply Rejection Ratio Supply Current/Amplifier INPUT CAPACITANCE Differential Common-Mode NOISE PERFORMANCE Input Voltage Noise Density Total Harmonic Distortion + Noise FREQUENCY RESPONSE Gain Bandwidth Product Slew Rate Settling Time Phase Margin Symbol VOS VOS/T IB IOS -40C TA +125C CMRR AVO VCM = 0 V to 5 V VO = 0.2 V to 4.8 V, RL = 10 k, VCM = 0 V -40C TA +125C IL = 1 mA; -40C TA +125C IL = 1 mA; -40C TA +125C VOUT = 0.5 V VS = 2.7 V to 5.0 V VO = 0 V -40C TA +125C 0 85 100 95 4.97 100 110 Conditions VCM = 0 V to 5 V -40C TA +125C -40C TA +125C -40C TA +125C Min Typ 50 0.4 1 Max 250 550 2.3 10 500 10 500 5 Unit V V V/C pA pA pA pA V dB dB dB V mV mA dB mA mA pF pF nV/Hz nV/Hz % MHz V/s ns degrees
VOH VOL IOUT PSRR ISY CIN
4.991 8 220 105 3.7
30
88
4.5 5.3
9.3 16.7 en THD + N GBP SR ts f = 1 kHz f = 10 kHz G = 1, RL = 1 k, f = 1 kHz, VIN = 2 V p-p 4 2.7 0.0007 28 11 370 69
RL = 10 k To 0.1%, VIN = 0 V to 2 V step, G = +1 CL = 0 pF
Rev. A | Page 3 of 20
AD8655/AD8656
VS = 2.7 V, VCM = VS/2, TA = 25C, unless otherwise specified. 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 OUTPUT CHARACTERISTICS Output Voltage High Output Voltage Low Output Current POWER SUPPLY Power Supply Rejection Ratio Supply Current/Amplifier INPUT CAPACITANCE Differential Common-Mode NOISE PERFORMANCE Input Voltage Noise Density Total Harmonic Distortion + Noise FREQUENCY RESPONSE Gain Bandwidth Product Slew Rate Settling Time Phase Margin Symbol VOS VOS/T IB IOS -40C TA +125C CMRR AVO VCM = 0 V to 2.7 V VO = 0.2 V to 2.5 V, RL = 10 k, VCM = 0 V -40C TA +125C IL = 1 mA; -40C TA +125C IL = 1 mA; -40C TA +125C VOUT = 0.5 V VS = 2.7 V to 5.0 V VO = 0 V -40C TA +125C 0 80 98 90 2.67 98 Conditions VCM = 0 V to 2.7 V -40C TA +125C -40C TA +125C -40C TA +125C Min Typ 44 0.4 1 Max 250 550 2.0 10 500 10 500 2.7 Unit V V V/C pA pA pA pA V dB dB dB V mV mA dB mA mA pF pF nV/Hz nV/Hz % MHz V/s ns degrees
VOH VOL IOUT PSRR ISY CIN
2.688 10 75 105 3.7
30
88
4.5 5.3
9.3 16.7 en THD + N GBP SR ts f = 1 kHz f = 10 kHz G = 1, RL = 1k, f = 1 kHz, VIN = 2 V p-p 4.0 2.7 0.0007 27 8.5 370 54
RL = 10 k To 0.1%, VIN = 0 to 1 V step, G = +1 CL = 0 pF
Rev. A | Page 4 of 20
AD8655/AD8656
ABSOLUTE MAXIMUM RATINGS
Table 3.
Parameter Supply Voltage Input Voltage Differential Input Voltage Output Short-Circuit Duration to GND Electrostatic Discharge (HBM) Storage Temperature Range R, RM Packages Junction Temperature Range R, RM Packages Lead Temperature (Soldering, 10 sec) Rating 6V VSS - 0.3 V to VDD + 0.3 V 6 V Indefinite 3.0 kV -65C to +150C -65C to +150C 260C
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. Table 4.
Package Type 8-Lead MSOP (RM) 8-Lead SOIC (R)
1
JA1 210 158
JC 45 43
Unit C/W C/W
JA is specified for worst-case conditions; that is, JA is specified for a device soldered in the circuit board for surface-mount packages.
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.
Rev. A | Page 5 of 20
AD8655/AD8656
TYPICAL PERFORMANCE CHARACTERISTICS
60 VS = 2.5V 50
NUMBER OF AMPLIFIERS
20 VS = 2.5V 10
40
VOS (V)
05304-001
0
30
20
-10
10
-20
05304-004
0 -150 -100 -50 0 50 VOS (V) 100 150
-30 0 1 2 3 4 COMMON-MODE VOLTAGE (V) 5 6
Figure 5. Input Offset Voltage Distribution
150.0 VS = 2.5V 100.0
Figure 8. Input Offset Voltage vs. Common-Mode Voltage
250 VS = 2.5V 200
50.0
VOS (V)
150 0.0
IB (pA)
100 -50.0
-100.0
05304-002
50
05304-005
-150.0 -50
0 0 20 40 60 80 100 TEMPERATURE (C) 120
0
50 TEMPERATURE (C)
100
150
140
Figure 6. Input Offset Voltage vs. Temperature
60 VS = 2.5V 50
NUMBER OF AMPLIFIERS
Figure 9. Input Bias Current vs. Temperature
4.0 VS = 2.5V 3.5
SUPPLY CURRENT (mA)
05304-003
3.0 2.5 2.0 1.5 1.0
05304-006
40
30
20
10
0.5 0 0 1 2 3 4 SUPPLY VOLTAGE (V) 5 6
0 0 0.2 0.4 0.6 0.8 1.0 1.2 |TCVOS| (V/C) 1.4 1.6
Figure 7. |TCVOS | Distribution
Figure 10. Supply Current vs. Supply Voltage
Rev. A | Page 6 of 20
AD8655/AD8656
4.5 VS = 2.5V
4.996 4.994 VS = 2.5V LOAD CURRENT = 1mA
4.0
SUPPLY CURRENT (mA)
4.992
3.5
VOH (V)
05304-007
4.990
3.0
4.988
4.986
2.5
05304-009
4.984
2.0 -50
0
50 TEMPERATURE (C)
100
150
4.982 -50
0
50 TEMPERATURE (C)
100
150
Figure 11. Supply Current vs. Temperature
2500 VS = 2.5V
Figure 14. Output Voltage Swing High vs. Temperature
12 LOAD CURRENT = 1mA VS = 2.5V 10
DELTA SWING FROM SUPPLY (mV)
2000
VOH 1000 VOL 500
05304-008
VOL (mV)
1500
8
6
4
05304-010
0 0 50 100 150 CURRENT LOAD (mA) 200
250
2 -50
0
50 TEMPERATURE (C)
100
150
Figure 12. AD8655 Output Voltage to Supply Rail vs. Current Load
10000 VS = 2.5V 120
Figure 15. Output Voltage Swing Low vs. Temperature
DELTA SWING FROM SUPPLY (mV)
100 1000 80
VS = 2.5V VIN = 28mV RL = 1M CL = 47pF
CMRR (dB)
VOL
100
60
40 10
20 VOH 1 0.1 1 10 CURRENT LOAD (mA) 100
05304-056
05304-011
1000
0 100
1k
10k 100k FREQUENCY (Hz)
1M
10M
Figure 13. AD8656 Output Swing vs. Current Load
Figure 16. CMRR vs. Frequency
Rev. A | Page 7 of 20
AD8655/AD8656
110.00
VOLTAGE NOISE DENSITY (nV/Hz 1/2)
100
VS = 2.5V VCM = 0V 107.00
VS = 2.5V
104.00
CMRR (dB)
101.00
10
98.00
05304-012
92.00 -50
1 1 10 100 1k FREQUENCY (Hz) 10k
0
50 TEMPERATURE (C)
100
150
100k
Figure 17. Large Signal CMRR vs. Temperature
100 +PSRR VS = 2.5V VIN = 50mV RL = 1M CL = 47pF
Figure 20. Voltage Noise Density vs. Frequency
VS = 2.5V Vn (p-p) = 1.23V
80
-PSRR
PSRR (dB)
500nV/DIV
60
1
40
20
05304-013 05304-020
0 100
1k
10k 100k 1M FREQUENCY (Hz)
10M
100M
1s/DIV
Figure 18. Small Signal PSSR vs. Frequency
110.00 VS = 2.5V 108.00
Figure 21. Low Frequency Noise (0.1 Hz to 10 Hz).
VIN
T VS = 2.5V CL = 50pF GAIN = +1
VOUT 106.00
1V/DIV
PSRR (dB)
2
104.00
102.00
05304-014 05304-021
100.00 -50
0
50 TEMPERATURE (C)
100
150
20s/DIV
Figure 19. Large Signal PSSR vs. Temperature
Figure 22. No Phase Reversal
Rev. A | Page 8 of 20
05304-019
95.00
AD8655/AD8656
120 100 80 60 40 20 0 -20 -40 10k -225 100M
05304-015
-45 VS = 2.5V CLOAD = 11.5pF PHASE MARGIN = 69
6 VS = 2.5V VIN = 5V G = +1
5
PHASE SHIFT (Degrees)
-90
4
OUTPUT (V)
GAIN (dB)
-135
3
2
-180
1
05304-018
100k
1M FREQUENCY (Hz)
10M
0 10k
100k 1M FREQUENCY (Hz)
10M
Figure 23. Open-Loop Gain and Phase vs. Frequency
140.00 VS = 2.5V RL = 10k 130.00
Figure 26. Maximum Output Swing vs. Frequency
VS = 2.5V CL = 100pF GAIN = +1 VIN = 4V
VOUT (1V/DIV)
T
AVO (dB)
120.00
2
110.00
100.00
05304-016
05304-022
90.00 -50
0
50 TEMPERATURE (C)
100
150
TIME (10s/DIV)
Figure 24. Large Signal Open-Loop Gain vs. Temperature
50 40
CLOSED-LOOP GAIN (dB)
Figure 27. Large Signal Response
VS = 2.5V RL = 1M CL = 47pF
T VS = 2.5V CL = 100pF G = +1
30 20 VOUT (100mV/DIV)
05304-017
2
10 0
-10 -20 1k
10k
100k 1M FREQUENCY (Hz)
10M
100M
TIME (1s/DIV)
Figure 25. Closed-Loop Gain vs. Frequency
Figure 28. Small Signal Response
Rev. A | Page 9 of 20
05304-023
AD8655/AD8656
30 VS = 2.5V VIN = 200mV OUTPUT IMPEDANCE ()
100 VS = 2.5V
25
G = +100 10
G = +10
G = +1
OVERSHOOT %
20 -OS 15
10 +OS 5
05304-024
1
0 0 50 100 150 200 250 CAPACITANCE (pF) 300
350
0.1 100
1k
10k 100k FREQUENCY (Hz)
1M
10M
100M
Figure 29. Small Signal Overshoot vs. Load Capacitance
80
T 300mV VIN 0V 1 NUMBER OF AMPLIFIERS
Figure 32. Output Impedance vs. Frequency
VS = 1.35V 70 60 50 40 30 20
05304-028
0V 2 VOUT VS = 2.5V VIN = 300mV GAIN = -10 RECOVERY TIME = 240ns
05304-025
-2.5V
10 0 -150 -125 -100 -75 -50 -25 0 25 VOS (V) 50 75 100 125 150
400ns/DIV
Figure 30. Negative Overload Recovery Time
60
T 0V 1 VIN -300mV VS = 2.5V VIN = 300mV GAIN = -10 RECOVERY TIME = 240ns
Figure 33. Input Offset Voltage Distribution
VS = 1.35V 40
2.5V VOUT
VOS (V)
20
0
-20
05304-026 05304-029
0V 2
400ns/DIV
-40 -50
0
50 TEMPERATURE (C)
100
150
Figure 31. Positive Overload Recovery Time
Figure 34. Input Offset Voltage vs. Temperature
Rev. A | Page 10 of 20
05304-027
AD8655/AD8656
80 VS = 1.35V 70
NUMBER OF AMPLIFIERS
DELTA OUTPUT FROM SUPPLY (mV)
10000 VS = 1.35V
60 50 40 30 20
05304-030
1000
100
10
VOL
05304-057
10 0 0 0.2 0.4 0.6 0.8 1.0 1.2 |TCVOS| (V/C) 1.4 1.6
VOH 1 0.1 1 10 CURRENT LOAD (mA)
100
Figure 35. |TCVOS| Distribution
4.5 VS = 1.35V 4.0 SUPPLY CURRENT (mA)
2.698
Figure 38. AD8656 Output Swing vs. Current Load
VS = 1.35V LOAD CURRENT = 1mA
2.694
VOH (V)
3.5
2.690
2.686
3.0
2.682
2.5
05304-031
2.678
05304-032
2.0 -50
0
50 TEMPERATURE (C)
100
150
2.674 -50
0
50 TEMPERATURE (C)
100
150
Figure 36. Supply Current vs. Temperature
1400 VS = 1.35V 1200
Figure 39. Output Voltage Swing High vs. Temperature
14 VS = 1.35V LOAD CURRENT = 1mA 12
1000
(VSY-VOUT) (mV)
VOH 800
10 VOL (mV)
8
600 400 200 VOL
6
4
05304-050
0 0 20 40 60 80 LOAD CURRENT (mA) 100
120
2 -50
0
50 TEMPERATURE (C)
100
150
Figure 37. AD8655 Output Voltage to Supply Rail vs. Load Current
Figure 40. Output Voltage Swing Low vs. Temperature
Rev. A | Page 11 of 20
05304-033
AD8655/AD8656
35
T VS = 1.35V G = +1 CL = 50pF
30
VS = 1.35V VIN = 200mV -OS
VIN
25
VOUT 1V/DIV 2
OVERSHOOT %
20
15 +OS 10 5
05304-047
0 0 50 100 150 200 250 CAPACITANCE (pF) 300
20s/DIV
350
Figure 41. No Phase Reversal
Figure 44. Small Signal Overshoot vs. Load Capacitance
T 200mV VIN 0V 1
VS = 1.35V CL = 50pF GAIN = +1
T
VOUT (500mV/DIV)
2
0V 2 VOUT -1.35V VS = 1.35V VIN = 200mV GAIN = -10 RECOVERY TIME = 180ns 400ns/DIV
TIME (10s/DIV)
Figure 42. Large Signal Response
05304-042
Figure 45. Negative Overload Recovery Time
T
T VS = 1.35V CL = 100pF GAIN = +1
0V 1
VIN -200mV VS = 1.35V VIN = 200mV GAIN = -10 RECOVERY TIME = 200ns
VOUT (100mV/DIV)
2
1.35V VOUT 0V 2
05304-046
05304-043
TIME (1s/DIV)
400ns/DIV
Figure 43. Small Signal Response
Figure 46. Positive Overload Recovery Time
Rev. A | Page 12 of 20
05304-045
05304-044
AD8655/AD8656
120 VS = 1.35V VIN = 28mV RL = 1M CL = 47pF
120 100 80 60
GAIN (dB)
-45 VS = 1.35V CLOAD = 11.5pF PHASE MARGIN = 54
PHASE SHIFT (Degrees)
05304-037
100
-90
80
CMRR (dB)
60
40 20
-135
40
0
20
05304-034
-180
-20 -40 10k -225 100M
05304-036
0 100
1k
10k FREQUENCY (Hz)
100k
1M
100k
1M FREQUENCY (Hz)
10M
Figure 47. CMRR vs. Frequency
102.00 VS = 1.35V
Figure 50. Open-Loop Gain and Phase vs. Frequency
130.00 VS = 1.35V RL = 10k 120.00
98.00
CMRR (dB)
94.00
AVO (dB)
05304-035
110.00
100.00
90.00
90.00
86.00 -50
0
50 TEMPERATURE (C)
100
150
80.00 -50
0
50 TEMPERATURE (C)
100
150
Figure 48. Large Signal CMRR vs. Temperature
100 +PSRR 80 -PSRR
PSRR (dB)
Figure 51. Large Signal Open-Loop Gain vs. Temperature
50
CLOSED-LOOP GAIN (dB)
VS = 1.35V VIN = 50mV RL = 1M CL = 47pF
40
VS = 1.35V RL = 1M CL = 47pF
30 20
60
40
10
0 -10 -20 1k
20
05304-040
0 100
1k
10k 100k 1M FREQUENCY (Hz)
10M
100M
10k
100k 1M FREQUENCY (Hz)
10M
100M
Figure 49. Small Signal PSSR vs. Frequency
Figure 52. Closed-Loop Gain vs. Frequency
Rev. A | Page 13 of 20
05304-038
AD8655/AD8656
3.0
0
R1 10k V- VOUT B V+
2.5
CHANNEL SEPERATION (dB)
-20
+2.5V V+ VIN 50mV p-p + - A V- -2.5V
R2 100
VS = 2.5V VIN = 50mV
2.0
OUTPUT (V)
VS = 1.35V VIN = 2.7V G = +1 NO LOAD
-40
-60 -80 -100
1.5
1.0
0.5
05304-039
0 10k
-140 10 100 1k 10k 100k FREQUENCY (Hz) 1M 10M
100k 1M FREQUENCY (Hz)
10M
100M
Figure 53. Maximum Output Swing vs. Frequency
1000 VS = 1.35V
Figure 55. Channel Separation vs. Frequency
OUTPUT IMPEDANCE ()
100
G = +100 G = +10 10 G = +1
1
05304-041
0.1 100
1k
10k 100k 1M FREQUENCY (Hz)
10M
100M
Figure 54. Output Impedance vs. Frequency
Rev. A | Page 14 of 20
05304-058
-120
AD8655/AD8656
THEORY OF OPERATION
The AD8655/AD8656 amplifiers are voltage feedback, rail-torail input and output precision CMOS amplifiers, which operate from 2.7 V to 5.0 V of power supply voltage. These amplifiers use the Analog Devices DigiTrim technology to achieve a higher degree of precision than is available from most CMOS amplifiers. DigiTrim technology, used in a number of ADI amplifiers, is a method of trimming the offset voltage of the amplifier after it is packaged. The advantage of post-package trimming is that it corrects any offset voltages caused by the mechanical stresses of assembly. The AD8655/AD8656 are available in standard op amp pinouts, making DigiTrim completely transparent to the user. The input stage of the amplifiers is a true rail-to-rail architecture, allowing the input common-mode voltage range of the amplifiers to extend to both positive and negative supply rails. The openloop gain of the AD8655/AD8656 with a load of 10 k is typically 110 dB. The AD8655/AD8656 can be used in any precision op amp application. The amplifier does not exhibit phase reversal for common-mode voltages within the power supply. The AD8655/AD8656 are great choices for high resolution data acquisition systems with voltage noise of 2.7 nV/Hz and THD + Noise of -103 dB for a 2 V p-p signal at 10 kHz. Their low noise, sub-pA input bias current, precision offset, and high speed make them superb preamps for fast filter applications. The speed and output drive capability of the AD8655/AD8656 also make them useful in video applications.
Rev. A | Page 15 of 20
AD8655/AD8656
APPLICATIONS
INPUT OVERVOLTAGE PROTECTION
The internal protective circuitry of the AD8655/AD8656 allows voltages exceeding the supply to be applied at the input. It is recommended, however, not to apply voltages that exceed the supplies by more than 0.3 V at either input of the amplifier. If a higher input voltage is applied, series resistors should be used to limit the current flowing into the inputs. The input current should be limited to less than 5 mA. The extremely low input bias current allows the use of larger resistors, which allows the user to apply higher voltages at the inputs. The use of these resistors adds thermal noise, which contributes to the overall output voltage noise of the amplifier. For example, a 10 k resistor has less than 12.6 nV/Hz of thermal noise and less than 10 nV of error voltage at room temperature. One simple technique for compensation is a snubber that consists of a simple RC network. With this circuit in place, output swing is maintained, and the amplifier is stable at all gains. Figure 57 shows the implementation of a snubber, which reduces overshoot by more than 30% and eliminates ringing. Using a snubber does not recover the loss of bandwidth incurred from a heavy capacitive load.
VS = 2.5V AV = 1 CL = 500pF
VOLTAGE (100mV/DIV)
INPUT CAPACITANCE
Along with bypassing and ground, high speed amplifiers can be sensitive to parasitic capacitance between the inputs and ground. For circuits with resistive feedback network, the total capacitance, whether it is the source capacitance, stray capacitance on the input pin, or the input capacitance of the amplifier, causes a breakpoint in the noise gain of the circuit. As a result, a capacitor must be added in parallel with the gain resistor to obtain stability. The noise gain is a function of frequency and peaks at the higher frequencies, assuming the feedback capacitor is selected to make the second-order system critically damped. A few picofarads of capacitance at the input reduce the input impedance at high frequencies, which increases the amplifier's gain, causing peaking in the frequency response or oscillations. With the AD8655/AD8656, additional input damping is required for stability with capacitive loads greater than 200 pF with direct input to output feedback. See the Driving Capacitive Loads section.
TIME (2s/DIV)
05304-051
Figure 56. Driving Heavy Capacitive Loads Without Compensation
VCC
+ -
+ -
V- V+ 200 500pF VEE 500pF
05304-052
200mV
Figure 57. Snubber Network
Although the AD8655/AD8656 can drive capacitive loads up to 500 pF without oscillating, a large amount of ringing is present when operating the part with input frequencies above 100 kHz. This is especially true when the amplifiers are configured in positive unity gain (worst case). When such large capacitive loads are required, the use of external compensation is highly recommended. This reduces the overshoot and minimizes ringing, which, in turn, improves the stability of the AD8655/AD8656 when driving large capacitive loads.
VOLTAGE (100mV/DIV)
DRIVING CAPACITIVE LOADS
VS = 2.5V AV = 1 RS = 200 CS = 500pF CL = 500pF
TIME (10s/DIV)
Figure 58. Driving Heavy Capacitive Loads Using a Snubber Network
Rev. A | Page 16 of 20
05304-053
AD8655/AD8656
THD Readings vs. Common-Mode Voltage
Total harmonic distortion of the AD8655/AD8656 is well below 0.0007% with a load of 1 k. This distortion is a function of the circuit configuration, the voltage applied, and the layout, in addition to other factors.
%
+2.5V - VOUT RL
05304-054
1.0 0.5 0.2 0.1 0.05 0.02 0.01 0.005 0.002 0.001 0.0005 0.0002 0.0001 20 50 SWEEP 1 100 200 500 1k Hz 2k 5k 10k 20k 50k 80k
05304-055
SWEEP 1: VIN = 2V p-p RL = 10k
SWEEP 2: VIN = 2V p-p RL = 1k
AD8655
+ VIN -2.5V
SWEEP 2
Figure 59. THD + N Test Circuit
Figure 60. THD + Noise vs. Frequency
Rev. A | Page 17 of 20
AD8655/AD8656
LAYOUT, GROUNDING, AND BYPASSING CONSIDERATIONS
POWER SUPPLY BYPASSING
Power supply pins can act as inputs for noise, so care must be taken to apply a noise-free, stable dc voltage. The purpose of bypass capacitors is to create low impedances from the supply to ground at all frequencies, thereby shunting or filtering most of the noise. Bypassing schemes are designed to minimize the supply impedance at all frequencies with a parallel combination of capacitors with values of 0.1 F and 4.7 F. Chip capacitors of 0.1 F (X7R or NPO) are critical and should be as close as possible to the amplifier package. The 4.7 F tantalum capacitor is less critical for high frequency bypassing, and, in most cases, only one is needed per board at the supply inputs.
LEAKAGE CURRENTS
Poor PC board layout, contaminants, and the board insulator material can create leakage currents that are much larger than the input bias current of the AD8655/AD8656. Any voltage differential between the inputs and nearby traces creates leakage currents through the PC board insulator, for example, 1 V/100 G = 10 pA. Similarly, any contaminants on the board can create significant leakage (skin oils are a common problem). To significantly reduce leakage, put a guard ring (shield) around the inputs and input leads that are driven to the same voltage potential as the inputs. This ensures there is no voltage potential between the inputs and the surrounding area to create any leakage currents. To be effective, the guard ring must be driven by a relatively low impedance source and should completely surround the input leads on all sides, above and below, by using a multilayer board. The charge absorption of the insulator material itself can also cause leakage currents. Minimizing the amount of material between the input leads and the guard ring helps to reduce the absorption. Also, using low absorption materials, such as Teflon(R) or ceramic, may be necessary in some instances.
GROUNDING
A ground plane layer is important for densely packed PC boards to minimize parasitic inductances. This minimizes voltage drops with changes in current. However, an understanding of where the current flows in a circuit is critical to implementing effective high speed circuit design. The length of the current path is directly proportional to the magnitude of parasitic inductances, and, therefore, the high frequency impedance of the path. Large changes in currents in an inductive ground return create unwanted voltage noise. The length of the high frequency bypass capacitor leads is critical, and, therefore, surface-mount capacitors are recommended. A parasitic inductance in the bypass ground trace works against the low impedance created by the bypass capacitor. Because load currents flow from the supplies, the ground for the load impedance should be at the same physical location as the bypass capacitor grounds. For larger value capacitors intended to be effective at lower frequencies, the current return path distance is less critical.
Rev. A | Page 18 of 20
AD8655/AD8656
OUTLINE DIMENSIONS
5.00 (0.1968) 4.80 (0.1890)
8 5
8
3.00 BSC
4.00 (0.1574) 3.80 (0.1497) 1
6.20 (0.2440)
5
4 5.80 (0.2284)
3.00 BSC
1
4.90 BSC
4
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)
PIN 1 0.65 BSC 1.10 MAX 8 0 0.80 0.60 0.40
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)
0.15 0.00 0.38 0.22 COPLANARITY 0.10
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
0.23 0.08 SEATING PLANE
COMPLIANT TO JEDEC STANDARDS MO-187-AA
Figure 61. 8-Lead Standard Small Outline Package [SOIC_N] Narrow Body (R-8) Dimensions shown in millimeters and (inches)
Figure 62. 8-Lead Mini Small Outline Package [MSOP] (RM-8) Dimensions shown in millimeters
ORDERING GUIDE
Model AD8655ARZ 1 AD8655ARZ-REEL1 AD8655ARZ-REEL71 AD8655ARMZ-REEL1 AD8655ARMZ-R21 AD8656ARZ1 AD8656ARZ-REEL1 AD8656ARZ-REEL71 AD8656ARMZ-REEL1 AD8656ARMZ-R21
1
Temperature Range -40C to +125C -40C to +125C -40C to +125C -40C to +125C -40C to +125C -40C to +125C -40C to +125C -40C to +125C -40C to +125C -40C to +125C
Package Description 8-Lead SOIC_N 8-Lead SOIC_N 8-Lead SOIC_N 8-Lead MSOP 8-Lead MSOP 8-Lead SOIC_N 8-Lead SOIC_N 8-Lead SOIC_N 8-Lead MSOP 8-Lead MSOP
Package Option R-8 R-8 R-8 RM-8 RM-8 R-8 R-8 R-8 RM-8 RM-8
Branding
A0D A0D
A0S A0S
Z = Pb-free part.
Rev. A | Page 19 of 20
AD8655/AD8656
NOTES
(c) 2005 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D05304-0-6/05(A)
Rev. A | Page 20 of 20

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