TEXAS INSTRUMENTS OPA691 Technical data

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OPERATIONAL AMPLIFIER With Disable
O
P
A
6
9
1
O
P
A
6
9
1
SBOS226A – DECEMBER 2001– REVISED SEPTEMBER 2002
Wideband, Current Feedback
OPA691
FEATURES
FLEXIBLE SUPPLY RANGE:
+5V to +12V Single-Supply
±
2.5V to ±6V Dual-Supply
UNITY-GAIN STABLE: 280MHz (G = 1)
HIGH OUTPUT CURRENT: 190mA
OUTPUT VOLTAGE SWING: ±4.0V
HIGH SLEW RATE: 2100V/µs
LOW dG/dφ: 0.07%/0.02°
LOW SUPPLY CURRENT: 5.1mA
LOW DISABLED CURRENT: 150µA
WIDEBAND +5V OPERATION: 190MHz (G = +2)
The OPA691 sets a new level of performance for broadband current feedback op amps. Operating on a very low 5.1mA supply current, the OPA691 offers a slew rate and output power normally associated with a much higher supply cur­rent. A new output stage architecture delivers a high output current with minimal voltage headroom and crossover distor­tion. This gives exceptional single-supply operation. Using a single +5V supply, the OPA691 can deliver a 1V to 4V output swing with over 150mA drive current and 190MHz band­width. This combination of features makes the OPA691 an ideal RGB line driver or single-supply Analog-to-Digital Con­verter (ADC) input driver.
The OPA691’s low 5.1mA supply current is precisely trimmed at 25°C. This trim, along with low drift over-temperature,
APPLICATIONS
xDSL LINE DRIVER
BROADBAND VIDEO BUFFERS
HIGH-SPEED IMAGING CHANNELS
PORTABLE INSTRUMENTS
ADC BUFFERS
ACTIVE FILTERS
WIDEBAND INVERTING SUMMING
HIGH SFDR IF AMPLIFIER
ensures lower maximum supply current than competing products. System power may be further reduced by using the optional disable control pin. Leaving this disable pin open, or holding it HIGH, gives normal operation. If pulled LOW, the OPA691 supply current drops to less than 150µA while the output goes into a high impedance state. This feature may be used for power savings.
OPA691 RELATED PRODUCTS
SINGLES DUALS TRIPLES
Voltage Feedback OPA690 OPA2690 OPA3690 Current Feedback OPA681 OPA2691 OPA3691 Fixed Gain OPA692 OPA3692
+5V
DIS
50
V
1
50
V
2
50
V
3
50
V
4
50
V
5
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
PRODUCTION DATA information is current as of publication date. Products conform to specifications per the terms of Texas Instruments standard warranty. Production processing does not necessarily include testing of all parameters.
50
OPA691
30
100
–5V
RG-58
100MHz, –1dB Compression = 15dBm
200MHz RF Summing Amplifier
Copyright © 2001, Texas Instruments Incorporated
www.ti.com
VO = –(V1 + V2 + V3 + V4 + V5)
50
ABSOLUTE MAXIMUM RATINGS
Power Supply .............................................................................. ±6.5VDC
Internal Power Dissipation
Differential Input Voltage .................................................................. ±1.2V
Input Voltage Range ............................................................................ ±V
Storage Temperature Range: ID, IDBV ......................... –40°C to +125°C
Lead Temperature (soldering, 10s) .............................................. +300°C
Junction Temperature (T ESD Performance:
HBM .............................................................................................. 2000V
CDM.............................................................................................. 1500V
NOTES:: (1) Stresses above these ratings may cause permanent damage. Exposure to absolute maximum conditions for extended periods may degrade device reliability. (2) Packages must be derated based on specified Maximum T
must be observed.
J
(2)
............................ See Thermal Information
) ........................................................... +175°C
J
(1)
ELECTROSTATIC DISCHARGE SENSITIVITY
S
θ
JA
This integrated circuit can be damaged by ESD. Texas Instru­ments recommends that all integrated circuits be handled with appropriate precautions. Failure to observe proper handling and installation procedures can cause damage.
ESD damage can range from subtle performance degradation to complete device failure. Precision integrated circuits may be more susceptible to damage because very small parametric changes could cause the device not to meet its published
.
specifications.
PACKAGE/ORDERING INFORMATION
PRODUCT PACKAGE-LEAD DESIGNATOR
PACKAGE TEMPERATURE PACKAGE ORDERING TRANSPORT
OPA691ID SO-8 D –40°C to +85°C OPA691 OPA691ID Rails, 100
(1)
"" " ""OPA691IDR Tape and Reel, 2500
OPA691IDBV SOT23-6 DBV –40°C to +85°C OAFI OPA691IDBVT Tape and Reel, 250
"" " ""OPA691IDBVR Tape and Reel, 3000
NOTE: (1) For the most current specifications and package information, refer to our web site at www.ti.com.
SPECIFIED
RANGE MARKING NUMBER MEDIA, QUANTITY
PIN CONFIGURATION
Top View SO
NC
Inverting Input
Noninverting Input
–V
1
2
3
4
S
NC = No Connection
8
7
6
5
DIS
+V
S
Output
NC
Top View SOT
Output
–V
Noninverting Input
1
2
S
3
654
6
+V
S
5
DIS
4
Inverting Input
OAFI
123
Pin Orientation/Package Marking
2
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OPA691
SBOS226A
ELECTRICAL CHARACTERISTICS: VS = ±5V
Boldface limits are tested at +25°C.
RF = 402, RL = 100, and G = +2, (see Figure 1 for AC performance only), unless otherwise noted.
OPA691ID, IDBV
TYP MIN/MAX OVER-TEMPERATURE
0°C to –40°C to
PARAMETER CONDITIONS +25°C +25°C
(1)
70°C
(2)
+85°C
(2)
UNITS MAX
AC PERFORMANCE (see Figure 1)
Small-Signal Bandwidth (V
Bandwidth for 0.1dB Gain Flatness G = +2, V Peaking at a Gain of +1 R Large-Signal Bandwidth G = +2, VO = 5Vp-p 200 MHz typ C
= 0.5Vp-p) G = +1, RF = 453 280 MHz typ C
O
G = +2, RF = 402 225 200 190 180 MHz min B G = +5, R
G = +10, R
= 453, VO = 0.5Vp-p 0.2 1 1.5 2 dB max B
F
= 261 210 MHz typ C
F
= 180 200 MHz typ C
F
= 0.5Vp-p 90 40 35 20 MHz min B
O
Slew Rate G = +2, 4V Step 2100 1400 1375 1350 V/µsminB Rise-and-Fall Time G = +2, VO = 0.5V Step 1.6 ns typ C
G = +2, 5V Step 1.9 ns typ C
Settling Time to 0.02% G = +2, VO = 2V Step 12 ns typ C
0.1% G = +2, VO = 2V Step 8 ns typ C
Harmonic Distortion G = +2, f = 5MHz, V
2nd-Harmonic R 3rd-Harmonic RL = 100 –74 –72 –70 –68 dBc max B
L
R
L
= 2Vp-p
O
= 100 –70 –63 –60 –58 dBc max B 500 79 –70 –67 –65 dBc max B
RL 500 93 –87 –82 –78 dBc max B Input Voltage Noise f > 1MHz 1.7 2.5 2.9 3.1 nV/√HZ max B Noninverting Input Current Noise f > 1MHz 12 14 15 15 pA/√HZ max B Inverting Input Current Noise f > 1MHz 15 17 18 19 pA/√HZ max B Differential Gain G = +2, NTSC, VO = 1.4Vp, RL = 150 0.07 % typ C
RL = 37.5 0.17 % typ C
Differential Phase G = +2, NTSC, VO = 1.4Vp, RL = 150 0.02 deg typ C
RL = 37.5 0.07 deg typ C
DC PERFORMANCE
Open-Loop Transimpedance Gain (ZOL)
(4)
VO = 0V, RL = 100 225 125 110 100 kΩ min A Input Offset Voltage VCM = 0V ±0.5 ±2.5 ±3.2 ±3.9 mV max A Average Offset Voltage Drift VCM = 0V ±12 ±20 µV/°CmaxB Noninverting Input Bias Current VCM = 0V +15 +35 +43 +45 µAmaxA Average Noninverting Input Bias Current Drift VCM = 0V –300 –300 nA/°CmaxB Inverting Input Bias Current VCM = 0V ±5 ±25 ±30 ±40 µAmaxA Average Inverting Input Bias Current Drift V
INPUT
Common-Mode Input Range
(5)
= 0V ±90 ±200 nA°/C max B
CM
±3.5 ±3.4 ±3.3 ±3.2 V min A
Common-Mode Rejection VCM = 0V 56 52 51 50 dB min A Noninverting Input Impedance 100 || 2 k || pF typ C Inverting Input Resistance (RI)
Open-Loop 35 typ C
OUTPUT
Voltage Output Swing No Load ±4.0
±3.8 ±3.7 ±3.6 V min A
100 Load ±3.9 ±3.7 ±3.6 ±3.3 V min A
Current Output, Sourcing VO = 0 +190 +160 +140 +100 mA min A Current Output, Sinking VO = 0 –190 160 –140 –100 mA min A Short-Circuit Current VO = 0 ±250 mA typ C Closed-Loop Output Impedance G = +2, f = 100kHz 0.03 typ C
DISABLE (Disabled LOW)
Power-Down Supply Current (+V Disable Time VIN = 1V 400 ns typ C
)V
S
= 0 –150 300 –350 –400 µAmaxA
DIS
Enable Time VIN = 1V 25 ns typ C Off Isolation G = +2, 5MHz 70 dB typ C Output Capacitance in Disable 4 pF typ C Turn On Glitch G = +2, RL = 150, VIN = 0 ±50 mV typ C Turn Off Glitch G = +2, RL = 150, VIN = 0 ±20 mV typ C Enable Voltage 3.3 3.5 3.6 3.7 V min A Disable Voltage 1.8 1.7 1.6 1.5 V max A Control Pin Input Bias Current (DIS) V
= 0 75 130 150 160 µAmaxA
DIS
POWER SUPPLY
Specified Operating Voltage ±5 V typ C Maximum Operating Voltage Range
±6 ±6 ±6VmaxA
Max Quiescent Current VS = ±5V 5.1 5.3 5.5 5.7 mA max A Min Quiescent Current VS = ±5V 5.1 4.9 4.7 4.5 mA min A Power-Supply Rejection Ratio (–PSRR) Input Referred 58 52 50 49 dB min A
TEMPERATURE RANGE
Specification: D, DBV Thermal Resistance,
DSO-8 125 °C/W typ C
θ
JA
Junction-to-Ambient
–40 to +85
DBV SOT23-6 150 °C/W typ C
NOTES: (1) Junction temperature = ambient for 25°C specifications. (2) Junction temperature = ambient at low temperature limit: junction temperature = ambient +10°C at high temperature limit for over-temperature specifications. (3) 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. (4) Current is considered positive out-of-node. V voltage. (5) Tested < 3dB below minimum specified CMRR at ± CMIR limits.
is the input common-mode
CM
MIN/
TEST
LEVEL
°C typ C
(3)
OPA691
SBOS226A
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3
ELECTRICAL CHARACTERISTICS: VS = +5V
Bolace limits are tested at +25°C.
RF = 453, RL = 100to VS/2, and G = +2, (see Figure 2 for AC performance only), unless otherwise noted.
OPA691ID, IDBV
TYP MIN/MAX OVER-TEMPERATURE
PARAMETER CONDITIONS +25°C +25°C
(1)
70°C
(2)
+85°C
(2)
UNITS MAX
0°C to –40°C to
AC PERFORMANCE (see Figure 2)
Small-Signal Bandwidth (V
Bandwidth for 0.1dB Gain Flatness G = +2, V Peaking at a Gain of +1 R Large-Signal Bandwidth G = +2, V Slew Rate G = +2, 2V Step 850 600 575 550 V/µsminB
= 0.5Vp-p) G = +1, RF = 499 210 MHz typ C
O
G = +2, R G = +5, RF = 340 180 MHz typ C
G = +10, R = 649, VO < 0.5Vp-p 0.2 1 2.5 3.0 dB max B
F
= 453 190 168 160 140 MHz min B
F
= 180 155 MHz typ C
F
< 0.5Vp-p 90 40 30 25 MHz min B
O
= 2Vp-p 210 MHz typ C
O
Rise-and-Fall Time G = +2, VO = 0.5V Step 2.0 ns typ C Settling Time to 0.02% G = +2, V
G = +2, V
0.1% G = +2, V
Harmonic Distortion G = +2, f = 5MHz, V
2nd-Harmonic R
R
3rd-Harmonic RL = 100to VS/2 –71 –68 –67 –65 dBc max B
= 2V Step 2.3 ns typ C
O
= 2V Step 14 ns typ C
O
= 2V Step 10 ns typ C
O
= 2Vp-p
= 100to VS/2 –66 –58 –57 –56 dBc max B
L
500to VS/2 –73 –65 –63 –62 dBc max B
L
O
RL 500to VS/2 –77 –72 –70 –69 dBc max B Input Voltage Noise f > 1MHz 1.7 2.5 2.9 3.1 nV/√Hz typ B Noninverting Input Current Noise f > 1MHz 12 14 15 15 pA/√Hz typ B Inverting Input Current Noise f > 1MHz 15 17 18 19 pA/√Hz typ B
DC PERFORMANCE
Open-Loop Transimpedance Gain (ZOL) Input Offset Voltage V Average Offset Voltage Drift V Noninverting Input Bias Current V Average Noninverting Input Bias Current Drift V Inverting Input Bias Current V Average Inverting Input Bias Current Drift V
INPUT
Least Positive Input Voltage Most Positive Input Voltage Common-Mode Rejection Ratio (CMRR)
(4)
VO = VS/2, RL = 100to VS/2 200 100 90 80 k min A
= 2.5V ±0.5 ±3 ±3.6 ±4.3 mV max A
CM
= 2.5V ±12 ±20 µV/°CmaxB
CM
= 2.5V +20 +40 +46 +56 µAmaxA
CM
= 2.5V –250 –250 nA/°CmaxB
CM
= 2.5V ±5 ±20 ±25 ±35 µAmaxA
CM
= 2.5V ±112 ±250 nA/°Cmax B
CM
(5)
(5)
1.5 1.6 1.7 1.8 V max A
3.5 3.4 3.3 3.2 V min A
VCM = VS/2 54 50 49 48 dB min A Noninverting Input Impedance 100 || 2 k|| pF typ C Inverting Input Resistance (RI)
Open-Loop 38 typ C
OUTPUT
Most Positive Output Voltage No Load 4 3.8 3.7 3.5 V min A
R
= 100 to VS/2 3.9 3.7 3.6 3.4 V min A
Least Positive Output Voltage No Load 1 1.2 1.3 1.5 V max A Current Output, Sourcing V
Current Output, Sinking V Short-Circuit Current V Closed-Loop Output Impedance G = +2, f = 100kHz 0.03 typ C
L
= 100 to VS/2 1.1 1.3 1.4 1.6 V max A
R
L
= VS/2 +160 +120 +100 +80 mA min A
O
= VS/2 –160 120 100 –80 mA min A
O
= VS/2 250 mA typ C
O
DISABLE (Disabled LOW)
Power-Down Supply Current (+V Off Isolation G = +2, 5MHz 65 dB typ C
)V
S
= 0 –150 300 –350 –400 µAmaxA
DIS
Output Capacitance in Disable 4 pF typ C Turn On Glitch G = +2, R Turn Off Glitch G = +2, R Enable Voltage 3.3 3.5 3.6 3.7 V min A
= 150, VIN = VS /2 ±50 mV typ C
L
= 150, VIN = VS /2 ±20 mV typ C
L
Disable Voltage 1.8 1.7 1.6 1.5 V max A Control Pin Input Bias Current (DIS) V
= 0 75 130 150 160 µA typ C
DIS
POWER SUPPLY
Specified Single-Supply Operating Voltage 5 V typ C Max Single-Supply Operating Voltage 12 12 12 V max A Max Quiescent Current V Min Quiescent Current V Power-Supply Rejection Ratio (–PSRR) Input Referred 55 dB typ C
= +5V 4.5 4.8 5.0 5.2 mA max A
S
= +5V 4.5 4.1 4.0 3.8 mA min A
S
TEMPERATURE RANGE
Specification: D, DBV Thermal Resistance,
DSO-8 125 °C/W typ C
θ
JA
Junction-to-Ambient
–40 to +85
DBV SOT23-6 150 °C/W typ C
NOTES: (3) 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. (1) Junction temperature = ambient for 25°C specifications. (2) Junction temperature = ambient at low temperature limit: junction temperature = ambient +10°C at high temperature limit for over-temperature specifications. (3) 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. (4) Current is considered positive out-of-node.
is the input common-mode voltage. (5) Tested < 3dB below minimum specified CMRR at ±CMIR limits.
V
CM
MIN/
TEST
LEVEL
°C typ C
(3)
4
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OPA691
SBOS226A
TYPICAL CHARACTERISTICS: VS = ±5V
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
Frequency (25MHz/div)
0 250MHz125MHz
LARGE-SIGNAL FREQUENCY RESPONSE
Gain (0.5dB/div)
G = +2, RL = 100
4Vp-p
7Vp-p
1Vp-p
2Vp-p
G = +2, RF = 402, and RL = 100, unless otherwise noted (see Figure 1).
1
SMALL-SIGNAL FREQUENCY RESPONSE
0
123456
Normalized Gain (1dB/div)
–7
VO = 0.5Vp-p
–8
0 250MHz125MHz
SMALL-SIGNAL PULSE RESPONSE
+400 +300 +200 +100
0
100200
Output Voltage (100mV/div)
300400
G = +1, RF = 453
G = +5, RF = 261
G = +10, RF = 180
Frequency (25MHz/div)
Time (5ns/div)
G = +2,
R
F
G = +2
V
= 0.5Vp-p
O
= 402
+4
LARGE-SIGNAL PULSE RESPONSE
+3 +2 +1
0
12
Output Voltage (1V/div)
34
Time (5ns/div)
G = +2
V
= 5Vp-p
O
dG/dP (%/°)
OPA691
SBOS226A
0.2
0.18
0.16
0.14
+5
Video
In
402
OPA691
–5
402
Optional 1.3k
Pull-Down
Video Loads
0.12
0.1
0.08
0.06
0.04
0.02
COMPOSITE VIDEO dG/dP
0
12
Number of 150 Loads
No Pull-Down With 1.3k Pull-Down
dG
dG
dP
dP
34
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–45
V
= 0
–50
DIS
5560657075
DISABLED FEEDTHROUGH vs FREQUENCY
–80
Reverse
–85
Feedthrough (5dB/div)
9095
100
Forward
10.3 10 100 Frequency (MHz)
5
TYPICAL CHARACTERISTICS: VS = ±5V (Cont.)
G = +2, RF = 402, and RL = 100, unless otherwise noted (see Figure 1).
60657075808590
Harmonic Distortion (dBc)
95
100
50
60
70
80
HARMONIC DISTORTION vs LOAD RESISTANCE
VO = 2Vp-p
f = 5MHz
2nd-Harmonic
3rd-Harmonic
100 1000
Load Resistance (Ω)
HARMONIC DISTORTION vs FREQUENCY
dBc = dB Below Carrier
VO = 2Vp-p R
= 100
L
2nd-Harmonic
3rd-Harmonic
60
65
70
75
Harmonic Distortion (dBc)
80
85
65
70
75
HARMONIC DISTORTION vs SUPPLY VOLTAGE
2nd-Harmonic
3rd-Harmonic
2.5 3 3.5 4 4.5 65.55 Supply Voltage (V)
HARMONIC DISTORTION vs OUTPUT VOLTAGE
RL = 100 f = 5MHz
VO = 2Vp-p
R
f = 5MHz
2nd-Harmonic
3rd-Harmonic
= 100
L
–90
Harmonic Distortion (dBc)
–100
0.1 1 10 20 Frequency (MHz)
HARMONIC DISTORTION vs NONINVERTING GAIN
–50
VO = 2Vp-p
R
= 100
L
f = 5MHz
60
70
80
Harmonic Distortion (dBc)
–90
110
2nd-Harmonic
3rd-Harmonic
Gain (V/V)
–80
Harmonic Distortion (dBc)
–85
0.1 1 5 Output Voltage Swing (Vp-p)
50
60
70
80
Harmonic Distortion (dBc)
–90
HARMONIC DISTORTION vs INVERTING GAIN
VO = 2Vp-p
R
= 100
L
f = 5MHz
R
= 402
F
110
2nd-Harmonic
3rd-Harmonic
Inverting Gain (V/V)
6
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OPA691
SBOS226A
TYPICAL CHARACTERISTICS: VS = ±5V (Cont.)
9
6
3
0
3
6
9
Frequency (25MHz/div)
0 250MHz125MHz
FREQUENCY RESPONSE vs CAPACITIVE LOAD
Normalized Gain to Capacitive Load (dB)
OPA691
R
S
V
IN
V
O
C
L
1k
402
402
1k is optional.
CL = 22pF
CL = 10pF
CL = 47pF
CL = 100pF
G = +2, RF = 402, and RL = 100, unless otherwise noted (see Figure 1).
INPUT VOLTAGE AND CURRENT NOISE DENSITY
100
Inverting Input Current Noise (15pA/Hz)
10
Noninverting Input Current Noise (12pA/Hz)
Current Noise (pA/Hz)
Voltage Noise (nV/Hz)
Voltage Noise (1.7nV/Hz)
1
100 1k 10k 100k 1M 10M
Frequency (Hz)
70
RECOMMENDED R
vs CAPACITIVE LOAD
S
60
50
40
(Ω)
S
R
30
2-TONE, 3RD-ORDER
INTERMODULATION SPURIOUS
–30
dBc = dB below carriers
50MHz
40
50
60
10MHz
70
80
3rd-Order Spurious Level (dBc)
–90
Load Power at Matched 50 Load
–8 –6 –4 –20246810
Single-Tone Load Power (dBm)
20MHz
20
10
0
1101001k
65 60 55 50 45 40 35 30 25
Power-Supply Rejection Ratio (dB)
Common-Mode Rejection Ratio (dB)
20
1k 10k 100k 1M 10M 100M
OPA691
SBOS226A
Capacitive Load (pF)
CMRR AND PSRR vs FREQUENCY
–PSRR
Frequency (Hz)
+PSRR
CMRR
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120
OPEN-LOOP TRANSIMPEDANCE GAIN/PHASE
100
80
Z
|ZOL|
OL
60
40
20
Transimpedance Gain (20dB/div)
0
10k 100k 1M 10M 100M 1G
Frequency (Hz)
0
40
80
120
160
200
240
7
Transimpedance Phase (40°/div)
TYPICAL CHARACTERISTICS: VS = ±5V (Cont.)
30 20 10
0
102030
DISABLE/ENABLE GLITCH
Time (20ns/div)
Output Voltage (10mV/div)
6.0
4.0
2.0 0
V
DIS
(2V/div)
V
DIS
Output Voltage
(0V Input)
VIN = 0V
G = +2, RF = 402, and RL = 100, unless otherwise noted (see Figure 1).
SUPPLY AND OUTPUT CURRENT vs TEMPERATURE
10
Sourcing Output Current
8
Sinking Output Current
6
4
Supply Current (2mA/div)
2
0
–50 –25 0 25 50 75 100 125
Quiescent Supply Current
Ambient Temperature (°C)
TYPICAL DC DRIFT OVER TEMPERATURE
2
1.5 1
Noninverting Input Bias Current (IB+)
0.5 0
0.5
1
Input Offset Voltage (mV)
–1.5
Inverting Input
Bias Current (I
)
B–
Input Offset
Voltage (V
2
50 25 0 25 50 75 100 125
Ambient Temperature (°C)
OUTPUT VOLTAGE AND CURRENT LIMITATIONS
250
200
5 4 3
1W Internal
Power Limit
Output Current Limit
2
150
100
Output Current (50mA/div)
50
0
1
(V)
0
O
V
1234
Output Current Limit
–5
25
Load Line
50Load Line
100Load Line
1W Internal Power Limit
–150–200–250–300 –50–100 0 +100+50 +200+150 +250 +300
I
(mA)
O
CLOSED-LOOP OUTPUT IMPEDANCE
Z
402
vs FREQUENCY
O
40 30 20 10 0
1020
Input Bias Currents (µA)
–30
)
OS
–40
10
50
+5
OPA691
1
–5
402
0.1
Output Impedance ()
0.01 10k 100M100k 1M 10M
Frequency (Hz)
2.0
1.6
1.2
0.8
0.4
Output Voltage (400mV/div)
0
8
LARGE-SIGNAL DISABLE/ENABLE RESPONSE
V
DIS
Output Voltage
VIN = +1V
Time (200ns/div)
6.0
4.0 (2V/div)
2.0
DIS
V
0
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OPA691
SBOS226A
TYPICAL CHARACTERISTICS: VS = +5V
4.1
3.7
3.3
2.9
2.5
2.1
1.7
1.3
0.9
LARGE-SIGNAL PULSE RESPONSE
Time (5ns/div)
Output Voltage (400mV/div)
G = +2
V
O
= 2Vp-p
9
6
3
0
3
6
9
FREQUENCY RESPONSE vs CAPACITIVE LOAD
Frequency (25MHz/div)
0 250MHz125MHz
Normalized Gain to Capacitive Load (dB)
CL = 22pF
CL = 10pF
CL = 47pF
CL = 100pF
OPA691
453
453
57.6806
806
1k
V
I
+5V
0.1µF
V
O
R
S
C
L
0.1µF
1k is optional.
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
Frequency (25MHz/div)
0 250MHz125MHz
LARGE-SIGNAL FREQUENCY RESPONSE
Gain (0.5dB/div)
VO = 0.5Vp-p
VO = 2Vp-p
G = +2 R
L
= 100 to 2.5V
VO = 1Vp-p
G = +2, RF = 453, and RL = 100 to +2.5V, unless otherwise noted (see Figure 2).
1
SMALL-SIGNAL FREQUENCY RESPONSE
0
123456
Normalized Gain (1dB/div)
–7
VO = 0.5Vp-p
–8
0 250MHz125MHz
Frequency (25MHz/div)
SMALL-SIGNAL PULSE RESPONSE
2.9
2.8
2.7
2.6
2.5
2.4
2.3
Output Voltage (100mV/div)
2.2
2.1
G = +5,
= 340
R
F
Time (5ns/div)
G = +10,
= 180
R
F
G = +1,
= 499
R
F
G = +2,
= 453
R
F
G = +2
V
= 0.5Vp-p
O
60
50
40
(Ω)
30
S
R
20
10
0
1 10 100
OPA691
SBOS226A
RECOMMENDED R
Capacitive Load (pF)
vs CAPACITIVE LOAD
S
www.ti.com
9
TYPICAL CHARACTERISTICS: VS = +5V (Cont.)
G = +2, RF = 453, and RL = 100 to +2.5V, unless otherwise noted (see Figure 2).
60
65
70
75
Harmonic Distortion (dBc)
80
60
65
70
75
Harmonic Distortion (dBc)
–80
HARMONIC DISTORTION vs LOAD RESISTANCE
VO = 2Vp-p f
= 5MHz
2nd-Harmonic
3rd-Harmonic
100 1000
Resistance (Ω)
HARMONIC DISTORTION vs OUTPUT VOLTAGE
RL = 100 to 2.5V
f = 5MHz
0.1 1 3 Output Voltage Swing (Vp-p)
2nd-Harmonic
3rd-Harmonic
50
60
70
80
Harmonic Distortion (dBc)
90
30
40
50
60
70
80
90
3rd-Order Spurious Level (dBc)
100
14 12 10 8 6 4 20 2
HARMONIC DISTORTION vs FREQUENCY
VO = 2Vp-p R
= 100to 2.5V
L
2nd-Harmonic
3rd-Harmonic
0.1 1 10 20 Frequency (MHz)
2-TONE, 3RD-ORDER
INTERMODULATION SPURIOUS
dBc = dB below carriers
50MHz
20MHz
10MHz
Load Power at Matched 50 Load
Single-Tone Load Power (dBm)
10
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OPA691
SBOS226A
APPLICATIONS INFORMATION
OPA691
+5V
DIS
VS/2
806
100V
O
V
I
+V
S
57.6
806
R
F
453
R
G
453
0.1µF
0.1µF
6.8µF
+
0.1µF
WIDEBAND CURRENT FEEDBACK OPERATION
The OPA691 gives the exceptional AC performance of a wideband current feedback op amp with a highly linear, high power output stage. Requiring only 5.1mA quiescent current, the OPA691 will swing to within 1V of either supply rail and deliver in excess of 160mA at room temperature. This low output headroom requirement, along with supply voltage independent biasing, gives remarkable single (+5V) supply operation. The OPA691 will deliver greater than 200MHz bandwidth driving a 2Vp-p output into 100 on a single +5V supply. Previous boosted output stage amplifiers have typi­cally suffered from very poor crossover distortion as the output current goes through zero. The OPA691 achieves a comparable power gain with much better linearity. The pri­mary advantage of a current feedback op amp over a voltage feedback op amp is that AC performance (bandwidth and distortion) is relatively independent of signal gain. For similar AC performance at low gains, with improved DC accuracy, consider the high slew rate, unity-gain stable, voltage feed­back OPA690.
Figure 1 shows the DC-coupled, gain of +2, dual power­supply circuit configuration used as the basis of the ±5V Electrical Characteristic tables and Typical Characteristic curves. For test purposes, the input impedance is set to 50 with a resistor to ground and the output impedance is set to 50 with a series output resistor. Voltage swings reported in the specifications are taken directly at the input and output pins while load powers (dBm) are defined at a matched 50 load. For the circuit of Figure 1, the total effective load will be 100 || 804 = 89. The disable control line ( typically left open to ensure normal amplifier operation. One optional component is included in Figure 1. In addition to the usual power-supply de-coupling capacitors to ground, a
0.1µF capacitor is included between the two power-supply
DIS
) is
pins. In practical PC board layouts, this optional added capacitor will typically improve the 2nd-harmonic distortion performance by 3dB to 6dB.
Figure 2 shows the AC-coupled, gain of +2, single-supply circuit configuration used as the basis of the +5V Electrical Characteristic tables and Typical Characteristic curves. Though not a rail-to-rail design, the OPA691 requires mini­mal input and output voltage headroom compared to other very wideband current feedback op amps. It will deliver a 3Vp-p output swing on a single +5V supply with greater than 150MHz bandwidth. The key requirement of broadband single­supply operation is to maintain input and output signal swings within the usable voltage ranges at both the input and the output. The circuit of Figure 2 establishes an input midpoint bias using a simple resistive divider from the +5V supply (two 806 resistors). The input signal is then AC­coupled into this midpoint voltage bias. The input voltage can swing to within 1.5V of either supply pin, giving a 2Vp-p input signal range centered between the supply pins. The input impedance matching resistor (57.6) used for testing is adjusted to give a 50 input match when the parallel combi­nation of the biasing divider network is included. The gain resistor (R
) is AC-coupled, giving the circuit a DC gain of
G
+1which puts the input DC bias voltage (2.5V) on the output as well. The feedback resistor value has been ad­justed from the bipolar supply condition to re-optimize for a flat frequency response in +5V, gain of +2, operation (see Setting Resistor Values to Optimize Bandwidth). Again, on a single +5V supply, the output voltage can swing to within 1V of either supply pin while delivering more than 120mA output current. A demanding 100 load to a mid-point bias is used in this characterization circuit. The new output stage used in the OPA691 can deliver large bipolar output currents into this mid-point load with minimal crossover distortion, as shown by the +5V supply, 3rd-harmonic distortion plots.
+5V
+V
–V
S
S
+
OPA691
402
–5V
R
F
6.8µF
+
DIS
O
50V
0.1µF
50 Load
FIGURE 2. AC-Coupled, G = +2, Single-Supply Specification
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and Test Circuit.
11
0.1µF 6.8µF
50 Source
V
I
50
FIGURE 1. DC-Coupled, G = +2, Bipolar Supply, Specifica-
OPA691
SBOS226A
tion and Test Circuit.
R
402
0.1µF
G
SINGLE-SUPPLY ADC INTERFACE
Most modern, high performance ADCs (such as the Texas Instruments ADS8xx and ADS9xx series) operate on a single +5V (or lower) power supply. It has been a considerable challenge for single-supply op amps to deliver a low distor­tion input signal at the ADC input for signal frequencies exceeding 5MHz. The high slew rate, exceptional output swing, and high linearity of the OPA691 make it an ideal single-supply ADC driver. Figure 3 shows an example input interface to a very high performance, 10-bit, 60MSPS CMOS converter.
The OPA691 in the circuit of Figure 3 provides > 180MHz bandwidth operating at a signal gain of +4 with a 2Vp-p output swing. One of the primary advantages of the current feedback internal architecture used in the OPA691 is that high bandwidth can be maintained as the signal gain is increased. The noninverting input bias voltage is referenced to the midpoint of the ADC signal range by dividing off the top and bottom of the internal ADC reference ladder. With the gain resistor (R
) AC-coupled, this bias voltage has a gain of
G
+1 to the output, centering the output voltage swing as well. Tested performance at a 20MHz analog input frequency and a 60MSPS clock rate on the converter gives > 58dBc SFDR.
WIDEBAND INVERTING SUMMING AMPLIFIER
Since the signal bandwidth for a current feedback op amp may be controlled independently of the noise gain (NG, which is normally the same as the noninverting signal gain), very broadband inverting summing stages may be implemented using the OPA691. The circuit on the front page of this data sheet shows an example inverting summing amplifier where the resistor values have been adjusted to maintain both maximum bandwidth and input impedance matching. If each RF signal is assumed to be driven from a 50 source, the NG for this circuit will be (1 + 100/(100/5)) = 6. The total feedback impedance (from V
to the inverting error current) is
O
the sum of R
+ (RI NG) where RI is the impedance looking
F
into the inverting input from the summing junction (see the Setting Resistor Values to Optimize Performance section). Using 100 feedback (to get a signal gain of –2 from each input to the output pin) requires an additional 30 in series with the inverting input to increase the feedback impedance. With this resistor added to the typical internal R
= 35, the
I
total feedback impedance is 100 + (65 6) = 490, which is equal to the required value to get a maximum bandwidth flat frequency response for NG = 6. Tested performance shows more than 200MHz small-signal bandwidth and a –1dBm compression of 15dBm at the matched 50 load through 100MHz.
WIDEBAND VIDEO MULTIPLEXING
One common application for video speed amplifiers which include a disable pin is to wire multiple amplifier outputs together, then select which one of several possible video inputs to source onto a single line. This simple Wired-OR Video Multiplexer can be easily implemented using the OPA691, see Figure 4.
Typically, channel switching is performed either on sync or retrace time in the video signal. The two inputs are approxi­mately equal at this time. The make-before-break disable characteristic of the OPA691 ensures that there is always one amplifier controlling the line when using a wired-OR circuit like that presented in Figure 4. Since both inputs may be on for a short period during the transition between channels, the outputs are combined through the output impedance matching resistors (82.5 in this case). When one channel is disabled, its feedback network forms part of the output impedance and slightly attenuates the signal in getting out onto the cable. The gain and output matching resistor have been slightly increased to get a signal gain of +1 at the matched load and provide a 75 output impedance to the cable. The video multiplexer connection (see Figure 4)
+5V
R
F
360
R
120
0.1µF
G
OPA691
+2.5V DC Bias
2Vp-p
DIS
0.1µF
0.5Vp-p
FIGURE 3. Wideband, AC-Coupled, Single-Supply ADC Driver.
12
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50
2k
2k
Clock
22pF
+3.5V
0.1µF
+1.5V
0.1µF
+5V
Input Input
CM
REFT
REFB
ADS823
10-Bit
60MSPS
OPA691
SBOS226A
also ensures that the maximum differential voltage across the inputs of the unselected channel do not exceed the rated ±1.2V maximum for standard video signal levels.
The section on Disable Operation shows the turn-on and turn-off switching glitches using a grounded input for a single channel is typically less than ±50mV. Where two outputs are switched (see Figure 6), the output line is always under the
+5V
V
DIS
Video 1
2k
75
control of one amplifier or the other due to the make-before­break disable timing. In this case, the switching glitches for two 0V inputs drop to < 20mV.
4-CHANNEL FREQUENCY CHANNELIZER
The circuit of Figure 5 is a 4-channel multiplexer. In this circuit the OPA691 provides the drive for all 4 channels.
+5V
DIS
OPA691
Video 2
FIGURE 4. 2-Channel Video Multiplexer.
75
75
+5V
OPA691
–5V
75
–5V
402340
402340
+5V
OPA691
75
2k
#1
75
#2
75
#3
75
–5V
402
402
DIS
+5V
OPA691
–5V
402
+5V
OPA691
–5V
402
+5V
OPA691
–5V
402
DIS 1
DIS 2
DIS 3
59
R
59
R
59
R
O
O
O
82.5
82.5
75Cable
RG-59
75Cable
RG-59
75Ω Load
V
OUT
75
FIGURE 5. 4-Channel Frequency Channelizer.
OPA691
SBOS226A
402
+5V
#4
75
OPA691
402
402
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–5V
DIS 4
59
R
O
13
Each channel includes a bandpass filter. Each bandpass filter is set for a different frequency band. This allows the channelizing part of this circuit. The role of the channelizers OPA691s is to provide impedance isolation. This is done through the use of four matching resistances (59Ω in this case). These matching resistors ensure that the signals will combine during the transition between channels. They have been used to get a gain of +1 at the load.
This circuit may be used with a different number of channels. Its limitation comes from the drive requirement for each channel as well as the minimum acceptable return loss.
The output resistor value (R
) to keep a gain of +1 at the load
O
depends on the number of channels. For the OPA691 with a gain of 2 using R
= 402 and RG = 402, Equation 1 is:
F
(1)
n
75 1 804
ΩΩ
[]
R
=
O
+
(
)
2
•+
1
241200
ΩΩ
75 1 804
[]
+
n
(
)
1
SINGLE-SUPPLY “IF” AMPLIFIER
The high bandwidth provided by the OPA691 while operating on a single +5V supply lends itself well to IF amplifier applications. One of the advantages of using an op amp like the OPA691 as an IF amplifier is that precise signal gain is achieved along with much lower 3rd-order intermodulation versus quiescent power dissipation. In addition, the OPA691 in the SOT23-6 package offers a very small package with a power shutdown feature for portable applications. One con­cern with using op amps for an IF amplifier is their relatively high noise figures. It is sometimes suggested that an opti­mum source resistance can be used to minimize op amp noise figures. Adding a resistor to reach this optimum value may improve the noise figure, but will actually decrease the signal-to-noise ratio. A more effective way to move towards an optimum source impedance is to bring the signal in through an input transformer. Figure 6 shows an example that is particularly useful for the OPA691.
+5V
Power-supply
decoupling not shown.
5k
5k1µF
OPA691
50 Source
V
I
1:2
0.1µF
R
200
G
R
600
V
FIGURE 6. Low-Noise, Single-Supply IF Amplifier.
F
O
= 3V/V (9.54dB)
V
I
DIS
50
50 Load
V
O
Bringing the signal in through a step-up transformer to the inverting input gain resistor has several advantages for the OPA691. First, the decoupling capacitor on the noninverting input eliminates the contribution of the noninverting input current noise to the output noise. Secondly, the noninverting input noise voltage of the op amp is actually attenuated if reflected to the input side of R
. Using the 1:2 (turns ratio) step-up transformer
G
reflects the 50 source impedance at the primary through to the secondary as a 200 source impedance (and the 200Ω R resistor is reflected through to the transformer primary as a 50 input matching impedance). The noise gain to the amplifier output is then 1 + 600/400 = 2.5V/V. Taking the op amp’s
2.2nV/
Hz
input voltage noise times this noise gain to the output, then reflecting this noise term to the input side of the R resistor, divides it by 3. This gives a net gain of 0.833 for the noninverting input voltage noise when reflected to the input point for the op amp circuit. This is further reduced when referred back to the transformer primary.
The relatively low-gain IF amplifier circuit of Figure 6 gives a 12dB noise figure at the input of the transformer. Increasing the R
resistor to 600 (once RG is set to 200 for input
F
impedance matching) will slightly reduce the bandwidth. Measured results show 150MHz small-signal bandwidth for the circuit of Figure 6 with exceptional flatness through 30MHz. Although the OPA691 does not show an intercept characteristic for the 2-tone, 3rd-order intermodulation distor­tion, it does hold a very high Spurious-Free Dynamic Range (SFDR) through high output powers and frequencies. The maximum single-tone power at the matched load for the single-supply circuit of Figure 6 is 1dBm (this requires a
2.8Vp-p swing at the output pin of the OPA691 for the 2-tone envelope). Measured 2-tone SFDR at this maximum load power for the circuit of Figure 6 exceeds 55dBc for frequen­cies to 20MHz.
DESIGN-IN TOOLS
DEMONSTRATION BOARDS
Several PC boards are available to assist in the initial evaluation of circuit performance using the OPA691 in its two package styles. All of these are available free as an unpopulated PC board delivered with descriptive documen­tation. The summary information for these boards is shown in the table below.
BOARD LITERATURE
PRODUCT PACKAGE NUMBER NUMBER
OPA691ID SO-8 DEM-OPA68xU SBOU009 OPA691IDBV SOT23-6 DEM-OPA6xxN SBOU010
To request any of these boards, check the Texas Instru­ments web site at www.ti.com.
MACROMODELS AND APPLICATIONS SUPPORT
Computer simulation of circuit performance using SPICE is often useful when analyzing the performance of analog circuits and systems. This is particularly true for video and RF
PART REQUEST
G
G
14
www.ti.com
OPA691
SBOS226A
amplifier circuits where parasitic capacitance and inductance can have a major effect on circuit performance. A SPICE model for the OPA691 is available through the TI web site (www.ti.com). These models do a good job of predicting small-signal AC and transient performance under a wide variety of operating conditions. They do not do as well in predicting the harmonic distortion or dG/dφ characteristics. These models do not attempt to distinguish between the package types in their small-signal AC performance.
OPERATING SUGGESTIONS
R
, the buffer output impedance, is a critical portion of the
I
bandwidth control equation. The OPA691 is typically about 35Ω. A current feedback op amp senses an error current in the
inverting node (as opposed to a differential input error volt­age for a voltage feedback op amp) and passes this on to the output through an internal frequency dependent transimped­ance gain. The Typical Characteristics show this open-loop transimpedance response. This is analogous to the open­loop voltage gain curve for a voltage feedback op amp. Developing the transfer function for the circuit of Figure 7 gives Equation 1:
SETTING RESISTOR VALUES TO OPTIMIZE BANDWIDTH
A current feedback op amp like the OPA691 can hold an almost constant bandwidth over signal gain settings with the proper adjustment of the external resistor values. This is shown in the Typical Characteristic curves; the small-signal bandwidth decreases only slightly with increasing gain. Those curves also show that the feedback resistor has been changed for each gain setting. The resistor values on the inverting side of the circuit for a current feedback op amp can be treated as frequency response compensation elements while their ratios set the signal gain. Figure 7 shows the small­signal frequency response analysis circuit for the OPA691.
V
I
α
V
O
R
I
i
ERR
R
G
FIGURE 7. Recommended Feedback Resistor versus Noise Gain.
The key elements of this current feedback op amp model are: α Buffer gain from the noninverting input to the inverting input
Buffer output impedance
R
I
Feedback error current signal
i
ERR
Z(s) → Frequency dependent open-loop transimpedance gain from i The buffer gain is typically very close to 1.00 and is normally
neglected from signal gain considerations. It will, however, set the CMRR for a single op amp differential ampli­fier configuration. For a buffer gain α < 1.0, the CMRR = –20 • log (1– α) dB.
R
Z
(S) iERR
F
ERR
to V
αα1
RR
FI
+
1
V
O
=
V
I
R
F
+
R
G
R
Z
1
S
()
F
R
G
++
=
 
  
NG
+
RRNG
FI
+
1
=+
NG
1
This is written in a loop-gain analysis format where the errors arising from a non-infinite open-loop gain are shown in the denominator. If Z
were infinite over all frequencies, the
(S)
denominator of Equation 1 would reduce to 1 and the ideal desired signal gain shown in the numerator would be achieved. The fraction in the denominator of Equation 1 determines the frequency response. Equation 2 shows this as the loop-gain equation:
Z
S
()
=
+
RRNG
FI
Loop Gain
If 20 log (RF + NG • RI) were drawn on top of the open-loop transimpedance plot, the difference between the two would be the loop gain at a given frequency. Eventually, Z to equal the denominator of Equation 2 at which point the loop gain has reduced to 1 (and the curves have intersected). This point of equality is where the amplifiers closed-loop frequency response given by Equation 1 will start to roll off, and is exactly analogous to the frequency at which the noise gain equals the open-loop voltage gain for a voltage feed­back op amp. The difference here is that the total impedance in the denominator of Equation 2 may be controlled some­what separately from the desired signal gain (or NG).
The OPA691 is internally compensated to give a maxi­mally flat frequency response for R
= 402 at NG = 2 on
F
±5V supplies. Evaluating the denominator of Equation 2 (which is the feedback transimpedance) gives an optimal target of 472. As the signal gain changes, the contribu­tion of the NG • R will change, but the total can be held constant by adjust-
O
ing R
. Equation 4 gives an approximate equation for
F
optimum R
F
term in the feedback transimpedance
I
over signal gain:
RNGR
= 472
FI
(2)
Z
S
R
F
R
G
(3)
rolls off
(S)
(4)
OPA691
SBOS226A
www.ti.com
15
As the desired signal gain increases, this equation will eventually predict a negative R to this adjustment can also be set by holding R
. A somewhat subjective limit
F
to a
G
minimum value of 20. Lower values will load both the buffer stage at the input and the output stage if R
gets too low
F
actually decreasing the bandwidth. Figure 8 shows the rec­ommended R operation. The values for R
versus NG for both ±5V and a single +5V
F
versus gain shown here are
F
approximately equal to the values used to generate the Typical Characteristics. They differ in that the optimized values used in the Typical Characteristics are also correcting for board parasitics not considered in the simplified analysis leading to Equation 3. The values shown in Figure 8 give a good starting point for design where bandwidth optimization is desired.
600
500
400
+5V
(e.g., integrators, transimpedance, and some filters) should consider the unity-gain stable voltage feedback OPA680, since the feedback resistor is the compensation element for a current feedback op amp. Wideband inverting operation (and especially summing) is particularly suited to the OPA691. See Figure 9 for a typical inverting configuration where the I/O impedances and signal gain from Figure 1 are retained in an inverting circuit configuration.
+5V
Power-supply
decoupling not shown.
50Load
50
V
I
50
Source
R
188
R
M
68.1
DIS
V
OPA691
G
374
O
R
F
300
200
Feedback Resistor (Ω)
100
0
02010 155
±5V
Noise Gain
FIGURE 8. Feedback Resistor vs Noise Gain.
The total impedance going into the inverting input may be used to adjust the closed-loop signal bandwidth. Inserting a series resistor between the inverting input and the summing junction will increase the feedback impedance (denominator of Equation 2), decreasing the bandwidth. This approach to bandwidth control is used for the inverting summing circuit on the front page. The internal buffer output impedance for the OPA691 is slightly influenced by the source impedance looking out of the noninverting input terminal. High source resistors will have the effect of increasing R
, decreasing the
I
bandwidth. For those single-supply applications which de­velop a midpoint bias at the noninverting input through high valued resistors, the decoupling capacitor is essential for power-supply noise rejection, noninverting input noise cur­rent shunting, and to minimize the high frequency value for R
in Figure 7.
I
INVERTING AMPLIFIER OPERATION
Since the OPA691 is a general-purpose, wideband current feedback op amp, most of the familiar op amp application circuits are available to the designer. Those applications that require considerable flexibility in the feedback element
–5V
FIGURE 9. Inverting Gain of –2 with Impedance Matching.
In the inverting configuration, two key design considerations must be noted. The first is that the gain resistor (R
G
becomes part of the signal channel input impedance. If input impedance matching is desired (which is beneficial when­ever the signal is coupled through a cable, twisted-pair, long PC board trace, or other transmission line conductor), it is normally necessary to add an additional matching resistor to ground. R
by itself is normally not set to the required input
G
impedance since its value, along with the desired gain, will determine an R
which may be non-optimal from a frequency
F
response standpoint. The total input impedance for the source becomes the parallel combination of R
and RM.
G
The second major consideration, touched on in the previous paragraph, is that the signal source impedance becomes part of the noise gain equation and will have slight effect on the bandwidth through Equation 1. The values shown in Figure 9 have accounted for this by slightly decreasing R (from Figure 1) to re-optimize the bandwidth for the noise gain of Figure 9 (NG = 2.73) In the example of Figure 9, the R
value combines in parallel with the external 50 source
M
impedance, yielding an effective driving impedance of 50Ω || 68Ω = 28.8. This impedance is added in series with R
for calculating the noise gainwhich gives NG = 2.73.
G
This value, along with the R
of Figure 9 and the inverting
F
input impedance of 35, are inserted into Equation 3 to get a feedback transimpedance nearly equal to the 472Ω opti- mum value.
Note that the noninverting input in this bipolar supply invert­ing application is connected directly to ground. It is often suggested that an additional resistor be connected to ground
)
F
16
www.ti.com
OPA691
SBOS226A
on the noninverting input to achieve bias current error can­cellation at the output. The input bias currents for a current feedback op amp are not generally matched in either magni­tude or polarity. Connecting a resistor to ground on the noninverting input of the OPA691 in the circuit of Figure 9 will actually provide additional gain for that inputs bias and noise currents, but will not decrease the output DC error since the input bias currents are not matched.
OUTPUT CURRENT AND VOLTAGE
The OPA691 provides output voltage and current capabilities that are unsurpassed in a low-cost monolithic op amp. Under no-load conditions at 25°C, the output voltage typically swings closer than 1V to either supply rail; the +25°C swing limit is within 1.2V of either rail. Into a 15 load (the minimum tested load), it is tested to deliver more than ±160mA.
The specifications described above, though familiar in the industry, consider voltage and current limits separately. In many applications, it is the voltage current, or V-I product, which is more relevant to circuit operation. Refer to the Output Voltage and Current Limitations plot in the Typical Characteristics. The X- and Y-axes of this graph show the zero-voltage output current limit and the zero-current output voltage limit, respectively. The four quadrants give a more detailed view of the OPA691s output drive capabilities, noting that the graph is bounded by a Safe Operating Area of 1W maximum internal power dissipation. Superimposing resistor load lines onto the plot shows that the OPA691 can drive ±2.5V into 25 or ±3.5V into 50 without exceeding the output capabilities or the 1W dissipation limit. A 100 load line (the standard test circuit load) shows the full ±3.9V output swing capability, as shown in the Typical Specifica­tions.
The minimum specified output voltage and current over­temperature are set by worst-case simulations at the cold temperature extreme. Only at cold startup will the output current and voltage decrease to the numbers shown in the Electrical Characteristic tables. As the output transistors deliver power, their junction temperatures will increase, de­creasing their V
s (increasing the available output voltage
BE
swing) and increasing their current gains (increasing the available output current). In steady-state operation, the avail­able output voltage and current will always be greater than that shown in the over-temperature specifications since the output stage junction temperatures will be higher than the minimum specified operating ambient.
To protect the output stage from accidental shorts to ground and the power supplies, output short-circuit protection is included in the OPA691. The circuit acts to limit the maximum source or sink current to approximately 250mA.
DRIVING CAPACITIVE LOADS
One of the most demanding and yet very common load conditions for an op amp is capacitive loading. Often, the capacitive load is the input of an ADCincluding additional external capacitance which may be recommended to im-
prove ADC linearity. A high-speed, high open-loop gain amplifier like the OPA691 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 amplifiers 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.
The Typical Characteristics show the recommended R
ver-
S
sus Capacitive Load and the resulting frequency response at the load. Parasitic capacitive loads greater than 2pF can begin to degrade the performance of the OPA691. 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 OPA691 output pin (see Board Layout Guidelines).
DISTORTION PERFORMANCE
The OPA691 provides good distortion performance into a 100 load on ±5V supplies. Relative to alternative solutions, it provides exceptional performance into lighter loads and/or operating on a single +5V supply. Generally, until the funda­mental signal reaches very high frequency or power levels, the 2nd-harmonic will dominate the distortion with a negli­gible 3rd-harmonic component. Focusing then on the 2nd­harmonic, increasing the load impedance improves distortion directly. Remember that the total load includes the feedback networkin the noninverting configuration (see Figure 1) this is the sum of R is just R
F
capacitor (0.1µF) between the supply pins (for bipolar opera­tion) improves the 2nd-order distortion slightly (3dB to 6dB).
In most op amps, increasing the output voltage swing in­creases harmonic distortion directly. The Typical Character­istics show the 2nd-harmonic increasing at a little less than the expected 2x rate while the 3rd-harmonic increases at a little less than the expected 3x rate. Where the test power doubles, the 2nd-harmonic increases by less than the ex­pected 6dB while the 3rd-harmonic increases by less than the expected 12dB. This also shows up in the 2-tone, 3rd-order intermodulation spurious (IM3) response curves. The 3rd-order spurious levels are extremely low at low output power levels. The output stage continues to hold them low even as the fundamental power reaches very high levels. As the Typical Characteristics show, the spurious intermodulation powers do not increase as predicted by a traditional intercept model. As the fundamental power level increases, the
+ RG, while in the inverting configuration it
F
. Also, providing an additional supply decoupling
OPA691
SBOS226A
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17
dynamic range does not decrease significantly. For two tones centered at 20MHz, with 10dBm/tone into a matched 50 load (i.e., 2Vp-p for each tone at the load, which requires 8Vp-p for the overall 2-tone envelope at the output pin), the Typical Characteristics show 48dBc difference between the test-tone power and the 3rd-order intermodulation spurious levels. This exceptional performance improves further when operating at lower frequencies.
NOISE PERFORMANCE
Wideband current feedback op amps generally have a higher output noise than comparable voltage feedback op amps. The OPA691 offers an excellent balance between voltage and current noise terms to achieve low output noise. The inverting current noise (15pA/ solutions while the input voltage noise (1.7nV/ than most unity-gain stable, wideband, voltage feedback op amps. This low input voltage noise was achieved at the price of higher noninverting input current noise (12pA/ as the AC source impedance looking out of the noninverting node is less than 100, this current noise will not contribute significantly to the total output noise. The op amp input voltage noise and the two input current noise terms combine to give low output noise under a wide variety of operating conditions. Figure 10 shows the op amp noise analysis model with all the noise terms included. In this model, all noise terms are taken to be noise voltage or current density terms in either nV/ or pA/
Hz
.
R
S
E
RS
4kTR
S
4kT
R
G
FIGURE 10. Op Amp Noise Analysis Model.
The total output spot noise voltage can be computed as the square root of the sum of all squared output noise voltage contributors. Equation 5 shows the general form for the output noise voltage using the terms shown in Figure 10.
2
E E I R kTR NG I R kTR NG
=+
O
NI BN
Hz
) is significantly lower than earlier
E
NI
I
BN
(
)
SS
OPA691
R
G
2
+
44
I
BI
 
R
F
4kTR
4kT = 1.6E –20J
at 290°K
2
+
(
BI F F
Hz
Hz
F
2
)
+
) is lower
). As long
Hz
E
O
(5)
Dividing this expression by the noise gain (NG = (1 + R
F/RG
will give the equivalent input-referred spot noise voltage at the noninverting input, as shown in Equation 6.
(6)
E E I R kTR
NNIBN
2
=+
(
2
++
4
)
SS
IR
BI F F
NG
kTR
4
+
NG
2
Evaluating these two equations for the OPA691 circuit and component values (see Figure 1) will give a total output spot noise voltage of 8.0nV/ noise voltage of 4.0nV/ noise voltage is higher than the 1.7nV/
Hz
and a total equivalent input spot
Hz
. This total input-referred spot
Hz
specification for the op amp voltage noise alone. This reflects the noise added to the output by the inverting current noise times the feedback resistor. If the feedback resistor is reduced in high gain configurations (as suggested previously), the total input­referred voltage noise given by Equation 5 will approach just the 1.7nV/ gain of +10 using R noise of 2.1nV/
Hz
of the op amp itself. For example, going to a
= 180 will give a total input-referred
F
Hz
.
DC ACCURACY AND OFFSET CONTROL
A current feedback op amp like the OPA691 provides excep­tional bandwidth in high gains, giving fast pulse settling but only moderate DC accuracy. The Typical Specifications show an input offset voltage comparable to high-speed voltage feedback amplifiers. However, the two input bias currents are somewhat higher and are unmatched. Whereas bias current cancellation techniques are very effective with most voltage feedback op amps, they do not generally reduce the output DC offset for wideband current feedback op amps. Since the two input bias currents are unrelated in both magnitude and polarity, matching the source impedance looking out of each input to reduce their error contribution to the output is ineffec­tive. Evaluating the configuration of Figure 1, using worst-case +25°C input offset voltage and the two input bias currents, gives a worst-case output offset range equal to:
± (NG • V where NG = noninverting signal gain = ± (2 2.5mV) + (35µA 25 2) ± (402 25µA) = ±5mV + 1.75mV ± 10.05mV = –13.3mV +16.8mV A fine-scale, output offset null, or DC operating point adjust-
ment, is sometimes required. Numerous techniques are available for introducing DC offset control into an op amp circuit. Most simple adjustment techniques do not correct for temperature drift. It is possible to combine a lower speed, precision op amp with the OPA691 to get the DC accuracy of the precision op amp along with the signal bandwidth of the OPA691. See Figure 11 for a noninverting G = +10 circuit that holds an output offset voltage less than ±7.5mV over­temperature with > 150MHz signal bandwidth.
This DC-coupled circuit provides very high signal bandwidth using the OPA691. At lower frequencies, the output voltage is attenuated by the signal gain and compared to the original
) + (IBN RS/2 NG) ± (IBI RF)
OS(MAX)
))
18
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OPA691
SBOS226A
input voltage at the inputs of the OPA237 (this is a low-cost, precision voltage feedback op amp with 1.5MHz gain band­width product). If these two dont agree (due to DC offsets introduced by the OPA691), the OPA237 sums in a correc­tion current through the 2.86k inverting summing path. Several design considerations will allow this circuit to be optimized. First, the feedback to the OPA237s noninverting input must be precisely matched to the high-speed signal gain. Making the 2k resistor to ground an adjustable resis­tor would allow the low and high frequency gains to be precisely matched. Secondly, the crossover frequency region where the OPA237 passes control to the OPA691 must occur with exceptional phase linearity. These two issues reduce to designing for pole/zero cancellation in the overall transfer function. Using the 2.86k resistor will nomi­nally satisfy this requirement for the circuit in Figure 11. Perfect cancellation over process and temperature is not possible. This initial resistor setting and precise gain match­ing, however, will minimize long-term pulse settling tails.
+5V
Power supply
V
I
de-coupling not shown
+5V
1.8k
2.86k
OPA237
–5V
2k
OPA691
–5V
20
180
18k
DIS
V
O
FIGURE 11. Wideband, DC Connected Composite Circuit.
DISABLE OPERATION
The OPA691 provides an optional disable feature that may be used to reduce system power. If the unconnected, the OPA691 will operate normally. To disable, the control pin must be asserted LOW. Figure 12 shows a simplified internal circuit for the disable control feature.
In normal operation, base current to Q1 is provided through the 110k resistor while the emitter current through the 15k resistor sets up a voltage drop that is inadequate to turn on the two diodes in Q1s emitter. As V additional current is pulled through the 15k resistor eventu­ally turning on these two diodes ( 75µA). At this point, any further current pulled out of V
DIS
holding the emitter-base voltage of Q1 at approximately 0V. This shuts off the collector current out of Q1, turning the amplifier off. The supply current in the disable mode are only those required to operate the circuit of Figure 12. Additional circuitry ensures that turn-on time occurs faster than turn-off time (make-before-break).
DIS
control pin is left
is pulled LOW,
DIS
goes through those diodes
+V
S
15k
Q1
25k 110k
I
V
DIS
S
Control
–V
S
FIGURE 12. Simplified Disable Control Circuit.
When disabled, the output and input nodes go to a high impedance state. If the OPA691 is operating in a gain of +1, this will show a very high impedance (4pF || 1MΩ) at the output and exceptional signal isolation. If operating at a gain greater than +1, the total feedback network resistance (R
+ RG) will appear as the impedance looking back into the
F
output, but the circuit will still show very high forward and reverse isolation. If configured as an inverting amplifier, the input and output will be connected through the feedback network resistance (R
+ RG) giving relatively poor input-to-
F
output isolation. One key parameter in disable operation is the output glitch
when switching in and out of the disabled mode. Figure 13 shows these glitches for the circuit of Figure 1 with the input signal set to 0V. The glitch waveform at the output pin is plotted along with the
The transition edge rate (dV/dT) of the
DIS
pin voltage.
DIS
control line will influence this glitch. For the plot of Figure 12, the edge rate was reduced until no further reduction in glitch amplitude was observed. This approximately 1V/ns maximum slew rate may be achieved by adding a simple RC filter into the V
DIS
pin from a higher speed logic line. If extremely fast transition logic is used, a 2k series resistor between the logic gate and the using just the parasitic input capacitance on the
DIS
input pin will provide adequate bandlimiting
DIS
pin
while still ensuring an adequate logic level swing.
6.0
4.0
(2V/div)
2.0 0
DIS
V
30 20 10
0
–10
Output Voltage (10mV/div)
2030
V
DIS
Output Voltage
(0V Input)
Time (20ns/div)
FIGURE 13. Disable/Enable Glitch.
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SBOS226A
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19
THERMAL ANALYSIS
Due to the high output power capability of the OPA691, heatsinking or forced airflow may be required under extreme operating conditions. Maximum desired junction temperature will set the maximum allowed internal power dissipation, as described below. In no case should the maximum junction temperature be allowed to exceed 175°C.
Operating junction temperature (T The total internal power dissipation (P quiescent power (P output stage (P
) and additional power dissipated in the
DQ
) to deliver load power. Quiescent power is
DL
) is given by TA + PD
J
) is the sum of
D
θ
JA
simply the specified no-load supply current times the total supply voltage across the part. P
will depend on the
DL
required output signal and load but would, for a grounded resistive load, be at a maximum when the output is fixed at a voltage equal to 1/2 either supply voltage (for equal bipolar supplies). Under this condition P
DL
2
= V
/(4 RL) where R
S
includes feedback network loading. Note that it is the power in the output stage and not in the
load that determines internal power dissipation. As a worst-case example, compute the maximum T
using an
J
OPA691IDBV (SOT23-6 package) in the circuit of Figure 1 operating at the maximum specified ambient temperature of +85°C and driving a grounded 20 load to +2.5V DC:
P
= 10V 5.7mA + 52/(4 (20 || 804)) = 377m
D
Maximum T
= +85°C + (0.377W (150°C/W) = 141.5°C
J
Although this is still well below the specified maximum junction temperature, system reliability considerations may require lower junction temperatures. Remember, this is a worst-case internal power dissipationuse your actual sig­nal and load to computer P
. The highest possible internal
DL
dissipation will occur if the load requires current to be forced into the output for positive output voltages or sourced from the output for negative output voltages. This puts a high current through a large internal voltage drop in the output transistors. The Output Voltage and Current Limitations plot shown in the Typical Characteristics includes a boundary for 1W maximum internal power dissipation under these condi­tions.
BOARD LAYOUT GUIDELINES
Achieving optimum performance with a high-frequency am­plifier like the OPA691 requires careful attention to board layout parasitics and external component types. Recommen­dations that will optimize performance include:
a) Minimize parasitic capacitance to any AC ground for all of the signal I/O pins. Parasitic capacitance on the output and inverting input pins can cause instability: on the noninverting input, it can react with the source impedance to cause unintentional bandlimiting. To reduce unwanted capacitance, a window around the signal I/O pins should be opened in all of the ground and power planes around those pins. Other­wise, ground and power planes should be unbroken else­where on the board.
b) Minimize the distance (< 0.25") from the power-supply pins to high-frequency 0.1µF decoupling capacitors. At the device pins, the ground and power plane layout should not be in close proximity to the signal I/O pins. Avoid narrow power and ground traces to minimize inductance between the pins and the decoupling capacitors. The power-supply connections (on pins 4 and 7) should always be decoupled with these capacitors. An optional supply decoupling capaci-
.
tor across the two power supplies (for bipolar operation) will improve 2nd-harmonic distortion performance. Larger (2.2µF to 6.8µF) decoupling capacitors, effective at lower frequen­cies, should also be used on the main supply pins. These may be placed somewhat farther from the device and may be shared among several devices in the same area of the PC board.
c) Careful selection and placement of external compo­nents will preserve the high-frequency performance of
L
the OPA691. Resistors should be a very low reactance type. Surface-mount resistors work best and allow a tighter overall layout. Metal-film and carbon composition, axially-leaded resistors can also provide good high-frequency performance. Again, keep their leads and PC board trace length as short as possible. Never use wirewound type resistors in a high­frequency application. Since the output pin and inverting input pin are the most sensitive to parasitic capacitance, always position the feedback and series output resistor, if any, as close as possible to the output pin. Other network components, such as noninverting input termination resis­tors, should also be placed close to the package. Where double-side component mounting is allowed, place the feed­back resistor directly under the package on the other side of the board between the output and inverting input pins. The frequency response is primarily determined by the feedback resistor value as described previously. Increasing its value will reduce the bandwidth, while decreasing it will give a more peaked frequency response. The 402 feedback resistor used in the Electrical Characteristic tables at a gain of +2 on ±5V supplies is a good starting point for design. Note that a 453 feedback resistor, rather than a direct short, is recom­mended for the unity-gain follower application. A current feedback op amp requires a feedback resistor even in the unity-gain follower configuration to control stability.
d) Connections to other wideband devices on the board may be made with short, direct traces or through onboard transmission lines. For short connections, consider the trace and the input to the next device as a lumped capacitive load. Relatively wide traces (50mils to 100mils) should be used, preferably with ground and power planes opened up around them. Estimate the total capacitive load and set R plot of recommended R
versus Capacitive Load. Low para-
S
sitic capacitive loads (< 5pF) may not need an R OPA691 is nominally compensated to operate with a 2pF parasitic load. If a long trace is required, and the 6dB signal loss intrinsic to a doubly-terminated transmission line is acceptable, implement a matched impedance transmission line using microstrip or stripline techniques (consult an ECL
from the
S
since the
S
20
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OPA691
SBOS226A
design handbook for microstrip and stripline layout tech­niques). A 50 environment is normally not necessary on board, and in fact, a higher impedance environment will improve distortion, as shown in the Distortion versus Load plots. With a characteristic board trace impedance defined based on board material and trace dimensions, a matching series resistor into the trace from the output of the OPA691 is used as well as a terminating shunt resistor at the input of the destination device. Remember also that the terminating impedance will be the parallel combination of the shunt resistor and the input impedance of the destination device: this total effective impedance should be set to match the trace impedance. The high output voltage and current capa­bility of the OPA691 allows multiple destination devices to be handled as separate transmission lines, each with their own series and shunt terminations. If the 6dB attenuation of a doubly-terminated transmission line is unacceptable, a long trace can be series-terminated at the source end only. Treat the trace as a capacitive load in this case and set the series resistor value as shown in the plot of R
versus Capacitive
S
Load. This will not preserve signal integrity as well as a doubly-terminated line. If the input impedance of the destina­tion device is low, there will be some signal attenuation due to the voltage divider formed by the series output into the terminating impedance.
e) Socketing a high-speed part like the OPA691 is not recommended. The additional lead length and pin-to-pin
capacitance introduced by the socket can create an ex­tremely troublesome parasitic network which can make it almost impossible to achieve a smooth, stable frequency response. Best results are obtained by soldering the OPA691 onto the board.
INPUT AND ESD PROTECTION
The OPA691 is built using a very high speed complementary bipolar process. 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 have limited ESD protection using internal diodes to the power supplies, as shown in Figure 14.
These diodes provide moderate protection to input overdrive voltages above the supplies as well. The protection diodes can typically support 30mA continuous current. Where higher currents are possible (e.g., in systems with ±15V supply parts driving into the OPA691), current-limiting series resistors should be added into the two inputs. Keep these resistor values as low as possible since high values degrade both noise performance and frequency response.
+V
CC
External
Pin
–V
CC
FIGURE 14. Internal ESD Protection.
Internal Circuitry
OPA691
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21
PACKAGE DRAWINGS
D (R-PDSO-G**) PLASTIC SMALL-OUTLINE PACKAGE
8 PINS SHOWN
8 5
1 4
A
0.069 (1,75) MAX
0.020 (0,51)
0.014 (0,35)
0.157 (4,00)
0.150 (3,81)
0.010 (0,25)
0.004 (0,10)
0.244 (6,20)
0.228 (5,80)
0.010 (0,25)0.050 (1,27)
0.008 (0,20) NOM
Gage Plane
0.010 (0,25)
0°– 8°
0.044 (1,12)
0.016 (0,40)
Seating Plane
0.004 (0,10)
PINS **
DIM
A MAX
A MIN
NOTES: A. All linear dimensions are in inches (millimeters).
B. This drawing is subject to change without notice. C. Body dimensions do not include mold flash or protrusion, not to exceed 0.006 (0,15). D. Falls within JEDEC MS-012
8
0.197
(5,00)
0.189
(4,80)
14
0.344 (8,75)
0.337 (8,55)
16
0.394
(10,00)
0.386 (9,80)
4040047/E 09/01
22
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OPA691
SBOS226A
PACKAGE DRAWINGS (Cont.)
DBV (R-PDSO-G6) PLASTIC SMALL-OUTLINE
0,95
1,45 0,95
3,00 2,80
46
31
0,05 MIN
6X
0,50 0,25
1,70 1,50
0,20
3,00 2,60
Seating Plane
M
0,15 NOM
Gage Plane
0,25
0 –8
0,10
0,55 0,35
NOTES: A. All linear dimensions are in millimeters.
B. This drawing is subject to change without notice. C. Body dimensions do not include mold flash or protrusion. D. Leads 1, 2, 3 may be wider than leads 4, 5, 6 for package orientation.
4073253-5/G 01/02
OPA691
SBOS226A
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23
PACKAGE OPTION ADDENDUM
www.ti.com
9-Dec-2004
PACKAGING INFORMATION
Orderable Device Status
(1)
Package
Type
Package Drawing
Pins Package
Qty
Eco Plan
OPA691ID ACTIVE SOIC D 8 100 None CU SNPB Level-3-235C-168 HR
OPA691IDBVR ACTIVE SOT-23 DBV 6 3000 None CU NIPDAU Level-3-220C-168 HR
OPA691IDBVT ACTIVE SOT-23 DBV 6 250 None CU NIPDAU Level-3-220C-168 HR
OPA691IDR ACTIVE SOIC D 8 2500 None CU SNPB Level-3-235C-168 HR
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs. LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect. NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in
a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available. OBSOLETE: TI has discontinued the production of the device.
(2)
Eco Plan - May not be currently available - please check http://www.ti.com/productcontent for the latest availability information and additional
product content details.
None: Not yet available Lead (Pb-Free). Pb-Free (RoHS): TI's terms "Lead-Free" or "Pb-Free" mean semiconductor products that are compatible with the current RoHS requirements
for all 6 substances, including the requirement that lead not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, TI Pb-Free products are suitable for use in specified lead-free processes. Green (RoHS & no Sb/Br): TI defines "Green" to mean "Pb-Free" and in addition, uses package materials that do not contain halogens, including bromine (Br) or antimony (Sb) above 0.1% of total product weight.
(2)
Lead/Ball Finish MSL Peak Temp
(3)
(3)
MSL, Peak Temp. -- The Moisture Sensitivity Level rating according to the JEDECindustry standard classifications, and peak solder
temperature.
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Addendum-Page 1
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