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OPA657
SBOS197F – DECEMBER 2001 – REVISED AUGUST 2015
OPA657 1.6-GHz, Low-Noise, FET-Input Operational Amplifier
1 Features
3 Description
•
•
•
•
The OPA657 device combines a high-gain bandwidth,
low-distortion, voltage-feedback operational amplifier
with a low-voltage noise JFET-input stage to offer a
very high dynamic range amplifier for high-precision
ADC (analog-to-digital converter) driving or wideband
transimpedance applications. Photodiode applications
see improved noise and bandwidth using this
decompensated, high-gain bandwidth amplifier.
1
•
•
•
•
•
•
High Gain Bandwidth Product: 1.6 GHz
High Bandwidth 275 MHz (G = 10)
Slew Rate 700 V/µs (G = 10, 1-V Step)
Available in High Grade With Improved DC
Specifications
Operating Temperature Range: –40°C to 85°C
Low-Input Offset Voltage: ±250 µV
Low-Input Bias Current: 2 pA
Low-Input Voltage Noise: 4.8 nV/√Hz
High-Output Current: 70 mA
Fast Overdrive Recovery
2 Applications
•
•
•
•
•
•
Wideband Photodiode Amplifier
Wafer Scanning Equipment
ADC Input Amplifier
Test and Measurement Front End
High Gain Precision Amplifier
Optical Time Domain Reflectometry (OTDR)
Very low level signals can be significantly amplified in
a single OPA657 gain stage with exceptional
bandwidth and accuracy. Having a high 1.6-GHz gain
bandwidth product gives greater than 10-MHz signal
bandwidths up to gains of 160 V/V (44 dB). The very
low input bias current and capacitance supports this
performance even for relatively high source
impedances.
Broadband photodetector applications benefit from
the low-voltage noise JFET inputs for the OPA657.
The JFET input contributes virtually no current noise
while for broadband applications, a low voltage noise
is also required. The low 4.8 nV/√Hz input voltage
noise provides exceptional input sensitivity for higher
bandwidth applications. The example shown below
gives a total equivalent input noise current of 1.8
pA/√Hz over a 10-MHz bandwidth.
Device Information(1)
PART NUMBER
PACKAGE
OPA657
BODY SIZE (NOM)
SOT-23 (5)
2.90 mm × 1.60 mm
SOIC (8)
4.90 mm × 3.91 mm
(1) For all available packages, see the orderable addendum at
the end of the data sheet.
Frequency Response of 200-kΩ Transimpedance
Amplifier
Wideband Photodiode Transimpedance Amplifier
0.1pF
116
Transimpedance Gain (dB)
10MHz Bandwidth
200kΩ
106
96
l
OPA657
86
-Vb
76
66
100k
VO
(12pF)
1M
10M
50M
Frequency (Hz)
1
An IMPORTANT NOTICE at the end of this data sheet addresses availability, warranty, changes, use in safety-critical applications,
intellectual property matters and other important disclaimers. PRODUCTION DATA.
OPA657
SBOS197F – DECEMBER 2001 – REVISED AUGUST 2015
www.ti.com
Table of Contents
1
2
3
4
5
6
7
Features ..................................................................
Applications ...........................................................
Description .............................................................
Revision History.....................................................
Related Operational Amplifier Products..............
Pin Configuration and Functions .........................
Specifications.........................................................
1
1
1
2
3
3
4
7.1
7.2
7.3
7.4
7.5
7.6
Absolute Maximum Ratings ...................................... 4
ESD Ratings.............................................................. 4
Recommended Operating Conditions....................... 4
Thermal Information .................................................. 4
Electrical Characteristics: VS = ±5 V......................... 5
Electrical Characteristics: VS - ±5 V, High-Grade DC
Specifications ............................................................. 7
7.7 Typical Characteristics: VS = ±5 V ............................ 8
8
Detailed Description ............................................ 13
8.1 Overview ................................................................. 13
8.2 Feature Description................................................. 13
8.3 Device Functional Modes........................................ 13
9
Application and Implementation ........................ 14
9.1 Application Information............................................ 14
9.2 Typical Application .................................................. 21
10 Power Supply Recommendations ..................... 24
11 Layout................................................................... 24
11.1 Layout Guidelines ................................................. 24
11.2 Layout Example .................................................... 26
11.3 Thermal Considerations ........................................ 26
12 Device and Documentation Support ................. 28
12.1
12.2
12.3
12.4
Community Resources..........................................
Trademarks ...........................................................
Electrostatic Discharge Caution ............................
Glossary ................................................................
28
28
28
28
13 Mechanical, Packaging, and Orderable
Information ........................................................... 28
4 Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Revision E (December 2008) to Revision F
Page
•
Added ESD Ratings table, Feature Description section, Device Functional Modes, Application and Implementation
section, Power Supply Recommendations section, Layout section, Device and Documentation Support section, and
Mechanical, Packaging, and Orderable Information section ................................................................................................. 1
•
Deleted lead temperature parameter from Absolute Maximum Ratings table ....................................................................... 4
•
Added Power Supply, Minimum Operating Voltage specification to ±5V Electrical Characteristics table ............................. 6
Changes from Revision D (March 2006) to Revision E
•
2
Page
Changed minimum storage temperature range from –40°C to –65°C ................................................................................... 4
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OPA657
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SBOS197F – DECEMBER 2001 – REVISED AUGUST 2015
5 Related Operational Amplifier Products
DEVICE
VS (V)
BW (MHz)
SLEW RATE
(V/μs)
VOLTAGE NOISE
(nV/√Hz)
OPA657
±5
1600
700
4.8
Gain of +7 stable FET Input
OPA656
±5
230
290
7
Unity-Gain Stable FET-Input
OPA659
±6
350
2550
8.9
Unity-Gain Stable FET-Input
LMH6629
5
4000
1600
0.69
Gain of +10 stable Bipolar Input
THS4631
±15
210
1000
7
Unity-Gain Stable FET-Input
OPA857
5
4750
220
—
Programmable Gain (5 kΩ / 20 kΩ)
Transimpedance Amplifier
AMPLIFIER DESCRIPTION
6 Pin Configuration and Functions
D Package
8-Pin SOIC Surface-Mount
Top View
DBV Package
5-Pin SOT-23
Top View
1
8
NC
VIN–
2
7
+VS
VIN+
3
6
VOUT
-VS
4
5
NC
VOUT
1
-VS
2
VIN+
3
5
+VS
4
VIN–
4
5
NC
3
2
1
A57
Pin Orientation/Package Marking
Pin Functions
PIN
NAME
SOIC
SOT-23
I/O
DESCRIPTION
1
NC
5
—
—
No Connection
I
Inverting Input
Noninverting Input
8
VIN–
2
4
VIN+
3
3
I
–VS
4
2
POW
VOUT
6
1
O
+VS
7
5
POW
Negative Power Supply
Output of amplifier
Positive Power Supply
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OPA657
SBOS197F – DECEMBER 2001 – REVISED AUGUST 2015
www.ti.com
7 Specifications
7.1 Absolute Maximum Ratings
over operating free-air temperature range (unless otherwise noted) (1)
MIN
Supply voltage (Total Bipolar Supplies)
Internal power dissipation
MAX
UNIT
±6.5
V
See Thermal Information
Differential input voltage
–VS
+VS
V
Input voltage
–VS
+VS
V
175
°C
125
°C
Junction temperature (TJ)
Storage temperature
(1)
–65
Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings
only, which do not imply functional operation of the device at these or any other conditions beyond those indicated under Recommended
Operating Conditions. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
7.2 ESD Ratings
VALUE
V(ESD)
(1)
(2)
Electrostatic discharge
Human-body model (HBM), per ANSI/ESDA/JEDEC JS-001 (1)
±2000
Charged-device model (CDM), per JEDEC specification JESD22C101 (2)
±500
Machine Model (MM)
±200
UNIT
V
JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.
JEDEC document JEP157 states that 250-V CDM allows safe manufacturing with a standard ESD control process.
7.3 Recommended Operating Conditions
over operating free-air temperature range (unless otherwise noted)
VS
Total supply voltage
TA
Ambient temperature
MIN
NOM
MAX
8
10
12
UNIT
V
–40
25
85
°C
7.4 Thermal Information
OPA657
THERMAL METRIC
(1)
D (SOIC)
DBV (SOT-23)
8 PINS
5 PINS
UNIT
RθJA
Junction-to-ambient thermal resistance
125
150
°C/W
RθJC(top)
Junction-to-case (top) thermal resistance
85.2
140.8
°C/W
RθJB
Junction-to-board thermal resistance
75.9
62.8
°C/W
ψJT
Junction-to-top characterization parameter
26.2
24.4
°C/W
ψJB
Junction-to-board characterization parameter
75.4
61.8
°C/W
RθJC(bot)
Junction-to-case (bottom) thermal resistance
—
—
°C/W
(1)
4
For more information about traditional and new thermal metrics, see the Semiconductor and IC Package Thermal Metrics application
report, SPRA953.
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OPA657
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SBOS197F – DECEMBER 2001 – REVISED AUGUST 2015
7.5 Electrical Characteristics: VS = ±5 V
At RF = +453 Ω, RL = +100 Ω, and G = +10 V/V, unless otherwise noted. See Figure 29 for AC performance.
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
TEST
LEVEL (1)
MHz
C
AC PERFORMANCE (see Figure 29)
G = +7 V/V, VO = 200 mVPP
TJ = 25°C
350
G = +10 V/V, VO = 200 mVPP
TJ = 25°C
275
G = +20 V/V, VO = 200 mVPP
TJ = 25°C
90
Gain-bandwidth product
G > +40 V/V
TJ = 25°C
1600
dB
C
Bandwidth for 0.1dB Flatness
G = +10 V/V, 2 VPP
TJ = 25°C
30
MHz
C
TJ = 25°C
7
dB
C
Small-signal bandwidth
Peaking at a Gain of +7
Large-Signal Bandwidth
G = +10 V/V, 2 VPP
TJ = 25°C
180
MHz
C
Slew Rate
G = +10 V/V, 1-V Step
TJ = 25°C
700
V/μs
C
Rise-and-Fall Time
0.2-V Step
TJ = 25°C
1
ns
C
Settling Time to 0.02%
G = +10 V/V, VO = 2-V Step
TJ = 25°C
20
ns
C
Harmonic Distortion
G = +10 V/V, f = 5 MHz, VO = 2 VPP
C
RL = 200 Ω
TJ = 25°C
–70
dBc
C
RL > 500 Ω
TJ = 25°C
–74
dBc
C
RL = 200 Ω
TJ = 25°C
–99
dBc
C
RL > 500 Ω
TJ = 25°C
–106
dBc
C
Input Voltage Noise
f > 100 kHz
TJ = 25°C
4.8
nV/√Hz
C
Input Current Noise
f > 100 kHz
TJ = 25°C
1.3
fA/√Hz
C
dB
A
mV
A
μV/°C
A
pA
A
pA
A
V
A
V
A
dB
A
2nd-Harmonic
3rd-Harmonic
DC PERFORMANCE (2)
Open-Loop Voltage Gain (AOL)
VCM = 0 V, RL = 100 Ω
TJ = 25°C
65
TJ = 0°C to 70°C (3)
64
TJ = –40°C to 85°C (3)
63
TJ = 25°C
Input Offset Voltage
Average Offset Voltage Drift
VCM = 0 V
VCM = 0 V
±0.25
VCM = 0 V
±2.2
TJ = –40°C to 85°C (3)
±2.6
TJ = 25°C
±12
TJ = 0°C to 70°C (3)
±12
TJ = –40°C to 85°C (3)
±12
±2
±20
±1800
(3)
±5000
TJ = 25°C
VCM = 0 V
±2
TJ = 0°C to 70°C (3)
TJ = –40°C to 85°C
Input Offset Current
±1.8
TJ = 0°C to 70°C (3)
TJ = 25°C
Input Bias Current
70
±1
TJ = 0°C to 70°C (3)
±10
±900
TJ = –40°C to 85°C (3)
±2500
INPUT
TJ = 25°C
Most Positive Input Voltage (4)
Most Negative Input Voltage (4)
Common-Mode Rejection Ratio (CMRR)
VCM = ±0.5 V
2
TJ = 0°C to 70°C (3)
1.9
TJ = –40°C to 85°C (3)
1.8
TJ = 25°C
–3.5
TJ = 0°C to 70°C (3)
–3.4
TJ = –40°C to 85°C (3)
–3.3
TJ = 25°C
83
TJ = 0°C to 70°C (3)
81
TJ = –40°C to 85°C (3)
79
2.5
–4
89
Input Impendance
(1)
(2)
(3)
(4)
Test Levels: (A) 100% tested at +25°C. Over-temperature limits by characterization and simulation. (B) Limits set by characterization
and simulation. (C) Typical value only for information.
Current is considered positive out-of-node. VCM is the input common-mode voltage.
Junction temperature = ambient at low temperature limit: junction temperature = ambient +20°C at high temperature limit for over
temperature specifications.
Tested < 3dB below minimum specified CMRR at ±CMIR limits.
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OPA657
SBOS197F – DECEMBER 2001 – REVISED AUGUST 2015
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Electrical Characteristics: VS = ±5 V (continued)
At RF = +453 Ω, RL = +100 Ω, and G = +10 V/V, unless otherwise noted. See Figure 29 for AC performance.
PARAMETER
TEST CONDITIONS
Differential
MIN
MAX
1012 || 0.7
TJ = 25°C
Common-Mode
TYP
12
TJ = 25°C
10
|| 4.5
UNIT
TEST
LEVEL (1)
Ω || pF
C
Ω || pF
C
V
A
V
B
mA
A
mA
A
A
OUTPUT
No load
Voltage Output Swing
RL = 100 Ω
Current Output, Sourcing
Current Output, Sinking
TJ = 25°C
±3.9
TJ = 0°C to 70°C (3)
±3.7
TJ = 25°C
±3.3
TJ = 0°C to 70°C (3)
±3.2
TJ = –40°C to 85°C (3)
±3.1
TJ = 25°C
50
TJ = 0°C to 70°C (3)
48
TJ = –40°C to 85°C (3)
46
TJ = 25°C
–50
TJ = 0°C to 70°C (3)
–48
TJ = –40°C to 85°C (3)
–46
±3.5
70
–70
TJ = 25°C
0.02
Ω
Specified Operating Voltage
TJ = 25°C
±5
V
A
Minimum Operating Voltage
TJ = 25°C
±4
V
C
V
A
mA
A
mA
A
dB
A
dB
A
Closed-Loop Output Impedance
G = +10 V/V, f = 0.1 MHz
POWER SUPPLY
Maximum Operating Voltage Range
TJ = 25°C
±6
TJ = 0°C to 70°C (3)
±6
TJ = –40°C to 85°C (3)
±6
TJ = 25°C
Maximum Quiescent Current
Minimum Quiescent Current
Power-Supply Rejection Ratio (+PSRR)
Power-Supply Rejection Ratio (–PSRR)
+VS = 4.5 V to 5.5 V
–VS = 4.5 V to 5.5 V
14
16
TJ = 0°C to 70°C (3)
16.2
TJ = –40°C to 85°C (3)
16.3
TJ = 25°C
11.7
TJ = 0°C to 70°C (3)
11.4
TJ = –40°C to 85°C (3)
11.1
TJ = 25°C
76
TJ = 0°C to 70°C (3)
74
TJ = –40°C to 85°C (3)
72
TJ = 25°C
62
TJ = 0°C to 70°C (3)
60
TJ = –40°C to 85°C (3)
58
14
80
68
TEMPERATURE RANGE
Specified Operating Range: U, N
Package
TJ = 25°C
–40 to +85
U: SO-8
TJ = 25°C
125
°C/W
N: SOT23-5
TJ = 25°C
150
°C/W
Thermal Resistance, RθJA
6
°C
Junction-to-Ambient
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SBOS197F – DECEMBER 2001 – REVISED AUGUST 2015
7.6 Electrical Characteristics: VS - ±5 V, High-Grade DC Specifications
At RF = 453 Ω, RL = 100 Ω, and G = +10 V/V, unless otherwise noted.
PARAMETER
(1)
TEST CONDITIONS
MIN
TJ = 25°C
Input Offset Voltage
VCM = 0 V
TJ = 0°C to 70°C (4)
TJ = 0°C to 70°C (4)
TJ = 0°C to 70°C
TJ = 0°C to 70°C
±0.5
TJ = –40°C to 85°C (4)
+VS = 4.5 V to 5.5 V
TJ = 0°C to 70°C
TJ = –40°C to 85°C (4)
(1)
(2)
(3)
(4)
–VS = –4.5 V to –5.5 V
TJ = 0°C to 70°C
(4)
TJ = –40°C to 85°C (4)
pA
A
pA
A
dB
A
dB
A
dB
A
±5 (3)
98
89
87
82
76
74
68 (3)
TJ = 25°C
Power-Supply Rejection
Ratio (–PSRR)
±5 (3)
±450
78 (3)
(4)
A
±1250
91 (3)
(4)
TJ = 25°C
Power-Supply Rejection
Ratio (+PSRR)
±6 (3)
±450
(4)
TJ = 25°C
VCM = ±0.5 V
A
±1250
TJ = –40°C to 85°C (4)
Common-Mode Rejection
Ratio (CMRR)
mV
2)
±6
±1
TJ = 25°C
VCM = 0 V
TEST
LEVEL (
±6 μV/°C
(4)
TJ = –40°C to 85°C (4)
Input Offset Current
UNIT
±0.9
TJ = 25°C
VCM = 0 V
±0.6
±0.85
±2
TJ = –40°C to 85°C
Input Bias Current
±0.1
(3)
(4)
TJ = 25°C
VCM = 0 V
MAX
TJ = 0°C to 70°C (4)
TJ = –40°C to 85°C
Input Offset Voltage Drift
TYP
66
74
64
All other specifications are the same as the standard-grade.
Test Levels: (A) 100% tested at +25°C. Over temperature limits by characterization and simulation.
Junction temperature = ambient for +25°C specifications.
Junction temperature = ambient at low temperature limit: junction temperature = ambient +20°C at high temperature limit for over
temperature specifications.
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OPA657
SBOS197F – DECEMBER 2001 – REVISED AUGUST 2015
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7.7 Typical Characteristics: VS = ±5 V
At TA = 25°C, G = 10 V/V, RF = 453 Ω, and RL = 100 Ω, unless otherwise noted.
9
9
VO = 0.2VPP
6
0
-3
G = +10
-6
-9
G = +20
-12
-15
See Figure 28
0
-3
-9
-12
G = -50
See Figure 29
-18
-21
-21
0.5
1
10
Frequency (MHz)
100
500
0.5
Figure 1. Noninverting Small-Signal Frequency Response
26
20
26
8
20
2
VO = 1VPP
17
14
VO = 5VPP
5
500
VO = 1VPP
23
VO = 2VPP
11
100
VO = 0.2VPP
G = -20
RF = 1kW
29
Gain (dB)
14
10
Frequency (MHz)
Figure 2. Inverting Small-Signal Frequency Response
VO = 1VPP
17
1
32
VO = 0.2VPP
G = +10
23
Gain (dB)
G = -20
-6
-15
G = +50
-18
VO = 5VPP
11
8
See Figure 28
See Figure 29
5
-1
2
-4
0.5
1
10
Frequency (MHz)
100
500
0.5
1.2
0.4
0.8
Large-Signal Right Scale
0.2
0.4
Small-Signal Left Scale
0
0
-0.2
-0.4
-0.4
-0.8
-0.6
-1.2
See Figure 28
-0.8
-1.6
10
Frequency (MHz)
100
500
0.8
0.6
1.6
G = -20
1.2
0.4
Large-Signal Right Scale
0.2
0.8
0.4
Small-Signal Left Scale
0
0
-0.2
-0.4
-0.4
-0.8
-0.6
-1.2
See Figure 29
-0.8
Time (10ns/div)
-1.6
Large-Signal Output Voltage (400mV/div)
1.6
G = +10
0.6
Large-Signal Output Voltage (400mV/div)
0.8
1
Figure 4. Inverting Large-Signal Frequency Response
Small-Signal Output Voltage (200mV/div)
Figure 3. Noninverting Large-Signal Frequency Response
Time (10ns/div)
Figure 5. Noninverting Pulse Response
8
G = -12
3
Normalized Gain (dB)
Normalized Gain (dB)
3
Small-Signal Output Voltage (200mV/div)
VO = 0.2VPP
RG = 50W
6
G = +7
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Figure 6. Inverting Pulse Response
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SBOS197F – DECEMBER 2001 – REVISED AUGUST 2015
Typical Characteristics: VS = ±5 V (continued)
At TA = 25°C, G = 10 V/V, RF = 453 Ω, and RL = 100 Ω, unless otherwise noted.
-60
VO = 2VPP
f = 5MHz
-75
3rd Harmonic
-80
-85
-90
-95
-100
See Figure 28
-105
f = 5MHz
RL = 200W
2nd Harmonic
See Figure 28
3rd Harmonic
-65
-70
Harmonic Distortion (dBc)
Harmonic Distortion (dBc)
-60
2nd Harmonic
-65
-70
-75
-80
-85
-90
-95
-100
-110
-105
100
1k
0.5
1
Output Voltage Swing (VPP)
Resistance (W)
Figure 7. Harmonic Distortion vs Load Resistance
Figure 8. Harmonic Distortion vs Output Voltage (5 MHz)
-70
VO = 2VPP
RL = 200W
-60
-70
2nd Harmonic
-80
-90
3rd Harmonic
-100
2nd Harmonic
-80
-85
-90
-95
3rd Harmonic
-100
See Figure 28
-105
See Figure 28
-110
-110
0.2
1
Frequency (MHz)
10
20
0.5
Figure 9. Harmonic Distortion vs Frequency
1
Output Voltage Swing (VPP)
5
Figure 10. Harmonic Distortion vs Output Voltage (1 MHz)
-40
-40
VO = 2VPP
f = 5MHz
RL = 200W
-60
VO = 2VPP
RG = 50W
f = 5MHz
RL = 200W
-50
Harmonic Distortion (dBc)
-50
Harmonic Distortion (dBc)
f = 1MHz
RL = 200W
-75
Harmonic Distortion (dBc)
Harmonic Distortion (dBc)
-50
5
2nd Harmonic
-70
-80
3rd Harmonic
-90
-60
2nd Harmonic
-70
-80
3rd Harmonic
-90
-100
-100
See Figure 28, RG Adjusted
See Figure 29, RF Adjusted
-110
-110
5
10
50
10
50
Gain (V/V)
Gain (V/V)
Figure 11. Harmonic Distortion vs Noninverting Gain
Figure 12. Harmonic Distortion vs Inverting Gain
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Typical Characteristics: VS = ±5 V (continued)
At TA = 25°C, G = 10 V/V, RF = 453 Ω, and RL = 100 Ω, unless otherwise noted.
-50
PI
3rd-Order Spurious Level (dBc)
10
Input Voltage Noise 4.8nV/ÖHz
Input Current Noise 1.3fA/ÖHz
20MHz
PO
50W
15MHz
453W
-70
50W
-80
10MHz
-90
5MHz
1
-100
10
100
1k
10k
100k
Frequency (Hz)
1M
10M
-10
Figure 13. Input Current And Voltage Noise Density
-6
-4
-2
0
2
4
Single-Tone Load Power (dBm)
-8
6
Open Loop Gain - Magnitude (dB)
CMRR
100
90
+PSRR
80
70
-PSRR
60
50
40
30
45
0
80
20log10(AOL)
60
–45
40
–90
ÐA OL
20
–135
0
–180
Aol Magnitude
–20
–225
Aol Phase
–40
–270
100
20
1k
10k
100k
1M
10M
8
Figure 14. 2-Tone, 3rd-Order IMD Spurious
100
110
CMRR, PSRR (dB)
50W
50W OPA657
-60
Open Loop Gain - Phase (°)
en (nV/ÖHz), in (fA/ÖHz)
100
1k
10k
100M
100k
1M
Frequency (Hz)
10M
100M
1G
D001
Frequency (Hz)
Figure 16. Open-Loop Gain and Phase
Figure 15. Common-Mode Rejection Ratio and PowerSupply Rejection Ratio vs Frequency
Normalized Gain to Capacitive Load (dB)
RS (W)
100
10
For Maximally-Flat Frequency Response
1
10
100
1k
23
CL = 10pF
20
CL = 22pF
17
CL = 100pF
14
VO
50W OPA657
CL
8
1kW
453W
5
50W
2
1
10
100
500
Frequency (MHz)
Capacitive Load (pF)
Figure 17. Recommended RS vs Capacitive Load
10
RS
VI
11
Figure 18. Frequency Response vs Capacitive Load
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Typical Characteristics: VS = ±5 V (continued)
At TA = 25°C, G = 10 V/V, RF = 453 Ω, and RL = 100 Ω, unless otherwise noted.
2
1
Input Bias Current (pA)
Input Offset Voltage (mV)
1.5
0.5
0
-0.5
1
0.5
0
-0.5
-1
-1.5
-1
-2
-50
-25
0
25
50
75
100
125
-3
-2
Figure 19. Typical Input Offset Voltage Over Temperature
1
2
3
Figure 20. Typical Input Bias Current vs Common-Mode
Input Voltage
1000
150
18
Supply Current
800
700
600
500
400
300
200
Right Scale
125
15
Left Scale
100
Sourcing Current
12
75
9
50
6
Left Scale
Sinking Current
25
3
Supply Current (3mA/div)
Output Current (25mA/div)
900
Input Bias Current (pA)
0
-1
Common-Mode Input Voltage (V)
Ambient Temperature (°C)
100
0
0
-50
-25
0
25
50
75
100
125
0
-50
-25
Figure 21. Typical Input Bias Current Over Temperature
Input Voltage
Right Scale
1
Output Voltage
Left Scale
0
-1
Figure 22. Supply And Output Current vs Temperature
4
0.3
3
0.15
0.2
2
0.1
1
0.05
0
0
0.1
-0.1
0.25
G = -20
-1
-2
-0.2
-3
-0.3
-3
-4
-0.4
-4
-5
-0.5
-5
See Figure 28
125
0.4
0
G = +10
100
Input Voltage
Right Scale
-0.05
Output Voltage
Left Scale
-2
0.2
-0.1
Input Voltage (V)
2
75
5
Input Voltage (V)
Output Voltage (V)
3
50
0.5
Output Voltage (V)
4
25
Ambient Temperature (°C)
Ambient Temperature (°C)
5
0
-0.15
-0.2
See Figure 29
-0.25
Time (20ns/div)
Time (20ns/div)
Figure 23. Noninverting Input Overdrive Recovery
Figure 24. Inverting Input Overdrive Recovery
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Typical Characteristics: VS = ±5 V (continued)
At TA = 25°C, G = 10 V/V, RF = 453 Ω, and RL = 100 Ω, unless otherwise noted.
4
10
1W Internal Power
OPA657
3
RL = 100W
2
VO (V)
Output Impedance (W)
5
RL = 50W
1
0
RL = 25W
-1
-2
-3
-4
-5
-100 -80
1W Internal Power
-60 -40 -20
0
20
40
60
80
ZO
453 Ω
1
50 Ω
0.1
0.01
100
0.1
1
IO (mA)
10
100
Frequency (MHz)
Figure 26. Closed-Loop Output Impedance vs Frequency
Figure 25. Output Voltage and Current Limitations
CMRR (dB)
110
90
70
50
-5
-4
-3
-2
-1
0
1
2
3
4
5
Common-Mode Input Voltage (V)
Figure 27. Common-Mode Rejection Ratio vs Common-Mode Input Voltage
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8 Detailed Description
8.1 Overview
The OPA657 is high gain-bandwidth, voltage feedback operational amplifier featuring a low noise JFET input
stage. The OPA657 has been decompensated to allow for optimized Bandwidth to Quiescent current
performance and better optimize its voltage noise performance. The OPA657 finds wide use in optical front-end
applications and in test and measurement systems that require high input impedance. It is built using a very high
speed complementary bipolar process.
8.2 Feature Description
8.2.1 Input and ESD Protection
The internal junction breakdown voltages are relatively low for these very small geometry devices. These
breakdowns are reflected in the Absolute Maximum Ratings table. All device pins are protected with internal ESD
protection diodes to the power supplies, as shown in Figure 28.
These diodes provide moderate protection to input overdrive voltages above the supplies as well. The protection
diodes can typically support 30-mA continuous current. Where higher currents are possible (that is, in systems
with ±12-V supply parts driving into the OPA657), current limiting series resistors should be added into the two
inputs. Keep these resistor values as low as possible because high values degrade both noise performance and
frequency response.
+VCC
External
Pin
Internal
Circuitry
-VCC
Figure 28. Internal ESD Protection
8.3 Device Functional Modes
8.3.1 Split-Supply Operation (±4-V to ±6-V)
To facilitate testing with common lab equipment, the OPA657 may be configured to allow for split-supply
operation. This configuration eases lab testing because the mid-point between the power rails is ground, and
most signal generators, network analyzers, oscilloscopes, spectrum analyzers and other lab equipment reference
their inputs and outputs to ground. Figure 29 and Figure 30 show the OPA657 configured in a simple
noninverting and inverting configuration respectively with ±5-V supplies. The input and output will swing
symmetrically around ground. Due to its ease of use, split-supply operation is preferred in systems where signals
swing around ground, but it requires generation of two supply rails.
8.3.2 Single-Supply Operation (8-V to 12-V)
Many newer systems use single power supply to improve efficiency and reduce the cost of the extra power
supply. The OPA657 is designed for use with split-supply configuration; however, it can be used with a singlesupply with no change in performance, as long as the input and output are biased within the linear operation of
the device. To change the circuit from split supply to single supply, level shift all the voltages by 1/2 the
difference between the power supply rails. An additional advantage of configuring an amplifier for single-supply
operation is that the effects of -PSRR will be minimized because the low supply rail has been grounded.
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9 Application and Implementation
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
9.1 Application Information
9.1.1 Wideband, Noninverting Operation
The OPA657 provides a unique combination of low-input voltage noise, very high-gain bandwidth, and the DC
precision of a trimmed JFET-input stage to give an exceptional high input impedance, high-gain stage amplifier.
Its very high gain bandwidth product (GBP) can be used to either deliver high-signal bandwidths at high gains, or
to extend the achievable bandwidth or gain in photodiode-transimpedance applications. To achieve the full
performance of the OPA657, careful attention to printed circuit board (PCB) layout and component selection is
required as discussed in the following sections of this data sheet.
Figure 29 shows the noninverting gain of +10-V/V circuit used as the basis for most of the Typical
Characteristics: VS = ±5 V. Most of the curves are characterized using signal sources with 50-Ω driving
impedance, and with measurement equipment presenting a 50-Ω load impedance. In Figure 29, the 50-Ω shunt
resistor at the VI terminal matches the source impedance of the test generator, while the 50-Ω series resistor at
the VO terminal provides a matching resistor for the measurement equipment load. Generally, data sheet voltage
swing specifications are at the output pin (VO in Figure 29) while output power specifications are at the matched
50-Ω load. The total 100-Ω load at the output combined with the 500-Ω total feedback network load presents the
OPA657 with an effective output load of 83 Ω for the circuit of Figure 29.
+5V
+VS
0.1mF
6.8mF
+
50W Source
50W Load
VI
VO
50W
50W
OPA657
RF
453W
RG
50W
+
6.8mF
0.1mF
-VS
-5V
Figure 29. Noninverting G = +10 V/V Specifications and Test Circuit
Voltage-feedback operational amplifiers, unlike current-feedback amplifiers, can use a wide range of resistor
values to set the gain. To retain a controlled frequency response for the noninverting voltage amplifier of
Figure 29, the parallel combination of RF || RG should always be less than 150 Ω. In the noninverting
configuration, the parallel combination of RF || RG forms a pole with the parasitic input capacitance at the
inverting node of the OPA657 (including layout parasitics). For best performance, this pole should be at a
frequency greater than the closed-loop bandwidth for the OPA657. For lower noninverting gains than the
minimum recommended gain of +7 for the OPA657, consider the unity-gain stable JFET input OPA656 or high
slew rate, low gain stable OPA659.
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Application Information (continued)
9.1.2 Wideband, Inverting Gain Operation
There can be significant benefits to operating the OPA657 as an inverting amplifier. This is particularly true when
a matched input impedance is required. Figure 30 shows the inverting gain circuit used as a starting point for the
Typical Characteristics showing inverting-mode performance.
Driving this circuit from a 50-Ω source, and constraining the gain resistor (RG) to equal 50 Ω gives both a signal
bandwidth and noise advantage. RG in this case is acting as both the input termination resistor and the gain
setting resistor for the circuit. Although the signal gain for the circuit of Figure 30 is double that for Figure 29, the
noise gains are equal when the 50-Ω source resistor is included. This has the interesting effect of doubling the
equivalent GBP for the amplifier. This can be seen in comparing the G = +10 V/V and G = –20 V/V small-signal
frequency response curves. Both show about 250-MHz bandwidth, but the inverting configuration of Figure 30 is
giving 6-dB higher signal gain. If the signal source is actually the low-impedance output of another amplifier, RG
should be increased to the minimum value allowed at the output of that amplifier and RF adjusted to get the
desired gain. It is critical for stable operation of the OPA657 that this driving amplifier show a very low output
impedance through frequencies exceeding the expected closed-loop bandwidth for the OPA657.
+5V
+VS
+
0.1mF
6.8mF
VO
50W
50W Load
OPA657
50W Source
RG
50W
RF
1kW
VI
0.1mF
+
6.8mF
-VS
-5V
Figure 30. Inverting G = –20 V/V Specifications and Test Circuit
Figure 30 also shows the noninverting input tied directly to ground. Often, a bias current canceling resistor to
ground is included here to null out the DC errors caused by the input bias currents. This is only useful when the
input bias currents are matched. For a JFET part like the OPA657, the input bias currents do not match but are
so low to begin with (< 5 pA) that DC errors as a result of input bias currents are negligible. Hence, no resistor is
recommended at the noninverting input for the inverting signal gain condition.
9.1.3 Low-Gain Compensation
Where a low gain is desired, and inverting operation is acceptable, a new external compensation technique may
be used to retain the full slew rate and noise benefits of the OPA657 while maintaining the increased loop gain
and the associated improvement in distortion offered by the decompensated architecture. This technique shapes
the loop gain for good stability while giving an easily-controlled second-order low-pass frequency response.
Considering only the noise gain for the circuit of Figure 31, the low-frequency noise gain, (NG1) is set by the
resistor ratios while the high frequency noise gain (NG2) is set by the capacitor ratios. The capacitor values set
both the transition frequencies and the high-frequency noise gain. If this noise gain, determined by NG2 = 1 +
CS/CF, is set to a value greater than the recommended minimum stable gain for the operational amplifier and the
noise gain pole, set by 1/RFCF, is placed correctly, a very well controlled second-order low-pass frequency
response results.
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Application Information (continued)
+5V
OPA657
RG
250W
VO = -2 ´ VI
RF
500W
VI
CS
27pF
CF
2.9pF
-5V
Figure 31. Broadband Low-Gain Inverting External Compensation
To choose the values for both CS and CF, two parameters and only three equations must be solved. The first
parameter is the target high-frequency noise gain NG2, which should be greater than the minimum stable gain for
the OPA657. Here, a target NG2 of 10.5 V/V is used. The second parameter is the desired low-frequency signal
gain, which also sets the low-frequency noise gain NG1. To simplify this discussion, target a maximally-flat
second-order low-pass Butterworth frequency response (Q = 0.707). The signal gain of –2 V/V shown in
Figure 31 sets the low-frequency noise gain to NG1 = 1 + RF/RG (= 3 in this example). Then, using only these two
gains and the GBP for the OPA657 (1600 MHz), the key frequency in the compensation can be determined as:
ZO =
GBP
NG
2
1
1-
NG1
NG2
-
1-2
NG1
NG2
(1)
Physically, this ZO (10.6 MHz for the values shown above) is set by 1/(2π × RF(CF + CS)) and is the frequency at
which the rising portion of the noise gain would intersect unity gain if projected back to 0-dB gain. The actual
zero in the noise gain occurs at NG1 × ZO and the pole in the noise gain occurs at NG2 × ZO. Because GBP is
expressed in Hz, multiply ZO by 2π and use this to get CF by solving:
1
CF =
(= 2.86pF)
2p ´ RFZONG2
(2)
Finally, because CS and CF set the high-frequency noise gain, determine CS by using [NG2 = 10.5]:
CS = (NG2 - 1)CF
(= 27.2pF)
(3)
The resulting closed-loop bandwidth is approximately equal to:
f-3dB @
ZO GBP
(= 130MHz)
(4)
For the values shown in Figure 31, the f–3 dB is approximately 130 MHz. This is less than that predicted by simply
dividing the GBP product by NG1. The compensation network controls the bandwidth to a lower value while
providing the full slew rate at the output and an exceptional distortion performance due to increased loop gain at
frequencies below NG1 × ZO. The capacitor values shown in Figure 31 are calculated for NG1 = 3 and NG2 =
10.5 with no adjustment for parasitics.
Figure 32 shows the gain of –2 V/V (6 dB) measured frequency response for the circuit of Figure 31. The
amplifier displays exceptional gain flatness through 70 MHz and a –3-dB bandwidth of 170 MHz.
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Application Information (continued)
12
9
6
Gain (3dB/div)
3
0
170MHz
-3
-6
-9
-12
-15
-18
1
10
100
500
Frequency (MHz)
Figure 32. G = –2 Frequency Response With External Compensation
The real benefit to this compensation is to allow a high slew rate, exceptional DC precision operational amplifier
to provide a low overshoot, fast settling pulse response. For a 1-V output step, the 700-V/μs slew rate of the
OPA657 allows a rise time limited edge rate (2ns for a 170-MHz bandwidth). While unity-gain stable operational
amplifiers may offer comparable bandwidths, the lower slew rates extend the settling time for larger steps. For
instance, the OPA656 can also provide a 150-MHz gain of –2-V/V bandwidth implying a 2.3-ns transition time.
However, the lower slew rate of this unity-gain stable amplifier (290 V/μs) limits a 1-V step transition to 3.5 ns
and delays the settling time as the slewing transition is recovered. The combination of higher slew rate and
exceptional DC precision for the OPA657 can yield one of the fastest, most precise, pulse amplifiers using the
circuit of Figure 31.
An added benefit to the compensation of Figure 31 is to increase the loop gain above that achievable at
comparable gains by internally-compensated amplifiers. The circuit of Figure 31 has lower harmonic distortion
through 10 MHz than the OPA656 operated at a gain of –2 V/V.
9.1.4 Operating Suggestions
9.1.4.1 Setting Resistor Values to Minimize Noise
The OPA657 provides a very low input noise voltage while requiring a low 14 mA of quiescent current. To take
full advantage of this low input noise, careful attention to the other possible noise contributors is required.
Figure 33 shows the operational amplifier noise analysis model with all the noise terms included. In this model,
all the noise terms are taken to be noise voltage or current density terms in either nV/√Hz or pA/√Hz.
ENI
*
EO
OPA657
RS
ERS
IBN
*
RF
Ö4kTRS
4kT
RG
*
RG
Ö4kTRF
IBI
4kT = 1.6E -20J
at 290°K
Figure 33. Operational Amplifier Noise Analysis Model
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Application Information (continued)
The total output spot noise voltage can be computed as the square root of the squared contributing terms to the
output noise voltage. This computation is adding all the contributing noise powers at the output by superposition,
then taking the square root to get back to a spot noise voltage. Equation 5 shows the general form for this output
noise voltage using the terms shown in Figure 28:
EO =
ENI2 + (IBNRS)2 + 4kTRS NG2 + (IBIRF)2 + 4kTRFNG
(5)
Dividing this expression by the noise gain (GN = 1 + RF/RG) gives the equivalent input referred spot noise voltage
at the noninverting input, as shown in Equation 6:
EN =
ENI2 + (IBNRS)2 + 4kTRS +
IBIRF
NG
2
+
4kTRF
NG
(6)
Putting high resistor values into Equation 6 can quickly dominate the total equivalent input referred noise. A
source impedance on the noninverting input of 1.6 kΩ adds a Johnson voltage noise term equal to just that for
the amplifier itself (5 nV/√Hz). While the JFET input of the OPA657 is ideal for high source impedance
applications, both the overall bandwidth and noise may be limited by these higher source impedances in the
noninverting configuration of Figure 29.
9.1.4.2 Frequency Response Control
Voltage-feedback operational amplifiers exhibit decreasing closed-loop bandwidth as the signal gain is increased.
In theory, this relationship is described by the gain bandwidth product (GBP) shown in the Electrical
Characteristics: VS = ±5 V. Ideally, dividing GBP by the noninverting signal gain (also called the noise gain, or
NG) predicts the closed-loop bandwidth. In practice, this only holds true when the phase margin approaches 90°,
as it does in high-gain configurations. At low gains (increased feedback factors), most high-speed amplifiers
exhibit a more complex response with lower phase margin. The OPA657 is compensated to give a maximallyflat, second-order, Butterworth, closed-loop response at a noninverting gain of +10 V/V (see Figure 29). This
results in a typical gain of +10-V/V bandwidth of 275 MHz, far exceeding that predicted by dividing the 1600-MHz
GBP by 10. Increasing the gain causes the phase margin to approach 90° and the bandwidth to more closely
approach the predicted value of (GBP/NG). At a gain of +50 V/V the OPA657 shows the 32-MHz bandwidth
predicted using the simple formula and the typical GBP of 1600 MHz. Inverting operation offers some interesting
opportunities to increase the available gain-bandwidth product. When the source impedance is matched by the
gain resistor (see Figure 30), the signal gain is –(RF/RG) while the noise gain for bandwidth purposes is (1 +
RF/RG). This cuts the noise gain in half, increasing the minimum stable gain for inverting operation under these
conditions to –12 V/V and the equivalent gain bandwidth product to 3.2 GHz.
9.1.4.3 Driving Capacitive Loads
One of the most demanding and yet very common load conditions for an operational amplifier is capacitive
loading. Often, the capacitive load is the input of an ADC—including additional external capacitance which may
be recommended to improve A/D linearity. A high-speed, high open-loop gain amplifier such as the OPA657 can
be very susceptible to decreased stability and closed-loop response peaking when a capacitive load is placed
directly on the output pin. When the amplifier open-loop output resistance is considered, this capacitive load
introduces an additional pole in the signal path that can decrease the phase margin. Several external solutions to
this problem have been suggested. When the primary considerations are frequency response flatness, pulse
response fidelity, and/or distortion, the simplest and most effective solution is to isolate the capacitive load from
the feedback loop by inserting a series isolation resistor between the amplifier output and the capacitive load.
This does not eliminate the pole from the loop response, but rather shifts it and adds a zero at a higher
frequency. The additional zero acts to cancel the phase lag from the capacitive load pole, thus increasing the
phase margin and improving stability.
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Application Information (continued)
The Typical Characteristics illustrate the Recommended RS vs Capacitive Load (Figure 17) and the resulting
frequency response at the load. In this case, a design target of a maximally-flat frequency response is used.
Lower values of RS may be used if some peaking can be tolerated. Also, operating at higher gains (than the +10
V/V used in the Typical Characteristics) require lower values of RS for a minimally-peaked frequency response.
Parasitic capacitive loads greater than 2 pF can begin to degrade the performance of the OPA657. Long PC
board traces, unmatched cables, and connections to multiple devices can easily cause this value to be
exceeded. Always consider this effect carefully, and add the recommended series resistor as close as possible to
the OPA657 output pin (see the Layout Guidelines section).
9.1.4.4 Distortion Performance
The OPA657 is capable of delivering a low-distortion signal at high frequencies over a wide range of gains. The
distortion plots in the Typical Characteristics: VS = ±5 V show the typical distortion under a wide variety of
conditions.
Generally, until the fundamental signal reaches very high frequencies or powers, the second-harmonic dominates
the distortion with a negligible third-harmonic component. Focusing then on the second-harmonic, increasing the
load impedance improves distortion directly. Remember that the total load includes the feedback network—in the
noninverting configuration this is sum of RF + RG, while in the inverting configuration this is just RF (see
Figure 29). Increasing output voltage swing increases harmonic distortion directly. A 6-dB increase in output
swing generally increases the second-harmonic 12 dB and the third-harmonic 18 dB. Increasing the signal gain
also increases the second-harmonic distortion. Again, a 6-dB increase in gain increases the second- and thirdharmonic by approximately 6 dB, even with a constant output power and frequency. And finally, the distortion
increases as the fundamental frequency increases due to the roll-off in the loop gain with frequency. Conversely,
the distortion improves going to lower frequencies down to the dominant open-loop pole at approximately 100
kHz. Starting from the –70-dBc second-harmonic for a 5-MHz, 2 VPP fundamental into a 200-Ω load at G = +10
V/V (from the Typical Characteristics: VS = ±5 V), the second-harmonic distortion for frequencies lower than 100
kHz is approximately less than –90 dBc.
The OPA657 has an extremely low third-order harmonic distortion. This also shows up in the two-tone, thirdorder, intermodulation spurious (IM3) response curves. The third-order spurious levels are extremely low (< –80
dBc) at low output power levels. The output stage continues to hold them low even as the fundamental power
reaches higher levels. As shown in Typical Characteristics: VS = ±5 V, the spurious intermodulation powers do
not increase as predicted by a traditional intercept model. As the fundamental power level increases, the
dynamic range does not decrease significantly. For two tones centered at 10 MHz, with 4 dBm/tone into a
matched 50-Ω load (that is, 1 VPP for each tone at the load, which requires 4 VPP for the overall two-tone
envelope at the output pin), the Typical Characteristics: VS = ±5 V show a 82-dBc difference between the test
tone and the third-order intermodulation spurious levels. This exceptional performance improves further when
operating at lower frequencies and/or higher load impedances.
9.1.4.5 DC Accuracy and Offset Control
The OPA657 can provide excellent DC accuracy due to its high open-loop gain, high common-mode rejection,
high power-supply rejection, and its trimmed input offset voltage (and drift) along with the negligible errors
introduced by the low input bias current. For the best DC precision, a high-grade version (OPA657UB or
OPA657NB) screens the key DC parameters to an even tighter limit. Both standard- and high-grade versions
take advantage of a new final test technique to 100% test input offset voltage drift over temperature. This
discussion uses the high-grade typical and min/max Electrical Characteristics: VS = ±5 V for illustration; however,
an identical analysis applies to the standard-grade version.
The total output DC offset voltage in any configuration and temperature is the combination of a number of
possible error terms. In a JFET part such as the OPA657, the input bias current terms are typically quite low but
are unmatched. Using bias-current cancellation techniques, more typical in bipolar input amplifiers, does not
improve output DC offset errors. Errors due to the input bias current only become dominant at elevated
temperatures. The OPA657 shows the typical 2× increase in every 10°C common to JFET-input stage amplifiers.
Using the 5-pA maximum tested value at +25°C, and a +20°C internal self heating (see thermal analysis), the
maximum input bias current at +85°C ambient is 5 pA × 2(105 – 25)/10 = 1280 pA. For noninverting configurations,
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Application Information (continued)
this term only begins to be a significant term versus the input offset voltage for source impedances greater than
750 kΩ. This would also be the feedback resistor value for transimpedance applications (see Figure 34) where
the output DC error due to inverting input bias current is on the order of that contributed by the input offset
voltage. In general, except for these extremely high-impedance values, the output DC errors due to the input bias
current may be neglected.
After the input offset voltage itself, the most significant term contributing to output offset voltage is the PSRR for
the negative supply. This term is modeled as an input offset voltage shift due to changes in the negative powersupply voltage (and similarly for the +PSRR). The high-grade test limit for –PSRR is 68 dB. This translates into
0.4-mV/V input offset voltage shift = 10(–68/20). This low sensitivity to the negative supply voltage requires a 1.5-V
change in the negative supply to match the ±0.6mV input offset voltage error. The +PSRR is tested to a
minimum value of 78 dB. This translates into 10(–78/20) = 0.125 mV/V sensitivity for the input offset voltage to
positive power-supply changes.
As an example, compute the worst-case output DC error for the transimpedance circuit of Figure 34 at 25°C and
then the shift over the 0°C to 70°C range given the following assumptions.
Negative Power Supply
= –5 V ±0.2 V with a ±5 mV/°C worst-case shift
Positive Power Supply
= +5 V ±0.2 V with a ±5 mV/°C worst-case shift
Initial 25°C Output DC Error Band
= ±0.6 mV (OPA657 high-grade input offset voltage limit)
±0.08 mV (due to the –PSRR = 0.4 mV/V × ±0.2 V)
±0.04 mV (due to the +PSRR = 0.2 mV/V × ±0.2 V)
Total = ±0.72mV
This would be the worst-case error band in volume production at 25°C acceptance testing given the conditions
stated. Over the temperature range (0°C to 70°C), expect the following worst-case shifting from initial value. A
20°C internal junction self-heating is assumed here.
±0.36 mV (OPA657 high-grade input offset drift)
= ±6 μV/°C × (70°C + 20°C – 25°C)
±0.11 mV (–PSRR of 66dB with 5mV × (70°C – 25°C) supply shift)
±0.04 mV (+PSRR of 76dB with 5mV × (70°C – 25°C) supply shift)
Total = ±0.51mV
This would be the worst-case shift from an initial offset over a 0°C to 70°C ambient for the conditions stated.
Typical initial output DC error bands and shifts over temperature are much lower than these worst-case
estimates.
In the transimpedance configuration, the CMRR errors can be neglected because the input common-mode
voltage is held at ground. For noninverting gain configurations (see Figure 29), the CMRR term needs to be
considered but is typically far lower than the input offset voltage term. With a tested minimum of 91 dB (28 μV/V),
the added apparent DC error is no more than ±0.06 mV for a ±2-V input swing to the circuit of Figure 29.
20
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9.2 Typical Application
The high GBP and low input voltage and current noise for the OPA657 make it an ideal wideband
transimpedance amplifier for moderate to high transimpedance gains. Unity-gain stability in the operational
amplifier is not required for application as a transimpedance amplifier.
VB
Supply Decoupling not
shown
+5 V
+
OPA657
CD
49 pF
CPCB
0.3 pF
RS
50
Oscilloscope
with 50
Inputs
-5 V
RF
200 k
CF + CPCB
0.55 pF
Figure 34. Wideband, High-Sensitivity, Transimpedance Amplifier Diagram
9.2.1 Design Requirements
Design a high-bandwidth, high-gain transimpedance amplifier with the design requirements shown in Table 1.
Table 1. Design Requirements
TARGET BANDWIDTH (MHz)
TRANSIMPEDANCE GAIN (kΩ)
PHOTODIODE CAPACITANCE (pF)
1
200
49
9.2.2 Detailed Design Procedure
Designs that require high bandwidth from a large area detector with relatively high transimpedance gain benefit
from the low input voltage noise of the OPA657. This input voltage noise is peaked up over frequency by the
diode source capacitance, and can, in many cases, become the limiting factor to input sensitivity. The key
elements to the design are the expected diode capacitance (CD) with the reverse bias voltage (VB) applied, the
desired transimpedance gain, RF, and the GBP for the OPA657 (1600 MHz). Figure 34 shows a transimpedance
circuit with the parameters as described in Table 1. With these three variables set (and including the parasitic
input capacitance for the OPA657 and the PCB added to CD), the feedback capacitor value (CF) may be set to
control the frequency response. To achieve a maximally-flat second-order Butterworth frequency response, the
feedback pole should be set to:
1
=
2pRFCF
GBP
4pRFCD
(7)
Adding the common-mode and differential mode input capacitance (0.7 + 4.5) pF and the trace PCB capacitance
of approximately 0.3 pF to the 49-pF diode source capacitance of Figure 34, and targeting a 200-kΩ
transimpedance gain using the 1600-MHz GBP for the OPA657 requires a feedback pole set to 3.5 MHz. This
requires a total feedback capacitance of 0.2 pF. Such low capacitance values are difficult to achieve due to
parasitics from the PCB and the surface mount components.
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Equation 8 gives the approximate closed loop bandwidth of the system to be 4.8 MHz. Because the target
bandwidth is only 1 MHz, the feedback capacitance can be increased to a more practical value while maintaining
the bandwidth requirements of the design. A feedback capacitance of 0.550 pf was chosen. This includes the
physical 0.4-pF feedback capacitor in addition to the 0.1-pF parasitic capacitance from the feedback resistor and
around 50-fF capacitance from the PCB traces. Removing the ground and power planes from under the surface
mount components helps to minimize this parasitic capacitance. The simulated closed loop bandwidth of
Figure 34 was 1.7 MHz. The phase margin was close to 82°. This design should result in a system with
negligible overshoot to a pulsed input.
f-3dB = GBP / (2pRFCD ) Hz
(8)
Figure 35 shows the measured output noise of the system which matches the simulated output noise in
Figure 36 very closely. The low-frequency output noise of 60 nV/√Hz gets input-referred to 0.3 pA/√Hz. The
transimpedance gain resistor is the dominant noise source with the operational amplifier itself contributing a
negligible amount, reflecting one of the main benefits in using a JFET input amplifier in a high-gain
transimpedance application. If the total output noise of the TIA is bandlimited to a frequency less than the
feedback pole frequency, a very simple expression for the equivalent output noise voltage can be derived as: see
Equation 9. Figure 37 shows the measured pulse response to a 5-µA input current pulse. The output voltage
measured on the scope is 0.5 V because of the 50-Ω termination to the scope.
where
•
•
•
•
•
•
VOUTN = Equivalent output noise when band limited to F < 1 / (2ΩRfCf)
IN = Input current noise for the operational amplifier inverting input
EN = Input voltage noise for the operational amplifier
CD = Diode capacitance including operational amplifier and PCB parasitic capacitance
F = Band-limiting frequency in Hz (usually a postfilter before further signal processing)
4 kT = 1.6 e – 20 J at T = 290°K
(9)
1000
700
500
1000
700
500
300
300
Output Noise (nV/vHz)
Output Noise (nV/vHz)
9.2.3 Application Curves
200
100
70
50
30
20
10
100
200
100
70
50
30
20
1k
10k
100k
1M
Frequency (Hz)
10M
100M
10
100
D002
Figure 35. Measured Total TIA Noise
22
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1k
10k
100k
1M
Frequency (Hz)
10M
100M
D004
Figure 36. Simulated TIA Output Noise
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0.1
Output Voltage (V)
0
-0.1
-0.2
-0.3
-0.4
-0.5
-0.6
0
5
10
15
Rise Time = 239 ns
20
25
Time (usec)
30
35
40
45
D003
Fall Time = 224 ns
Figure 37. Transient Pulse Response to 5-µA Input Current Pulse
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10 Power Supply Recommendations
The OPA657 is principally intended to work in a supply range of ±4 V to ±6 V. Good power-supply bypassing is
required. Minimize the distance (
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