High-Speed, Single-Supply, Rail-to-Rail
OPERATIONAL AMPLIFIERS
Micro
Amplifier
™ Series
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
Out D
–In D
+In D
–V
+In C
–In C
Out C
NC
Out A
–In A
+In A
+V
+In B
–In B
Out B
NC
OPA4350
SSOP-16
AD
BC
1
2
3
4
5
6
7
14
13
12
11
10
9
8
Out D
–In D
+In D
V–
+In C
–In C
Out C
Out A
–In A
+In A
V+
+In B
–In B
Out B
OPA4350
SO-14
AD
BC
APPLICATIONS
● CELL PHONE PA CONTROL LOOPS
● DRIVING A/D CONVERTERS
● VIDEO PROCESSING
● DATA ACQUISITION
● PROCESS CONTROL
● AUDIO PROCESSING
●COMMUNICATIONS
●ACTIVE FILTERS
●TEST EQUIPMENT
DESCRIPTION
OPA350 series rail-to-rail CMOS operational amplifi-
ers are optimized for low voltage, single-supply opera-
tion. Rail-to-rail input/output, low noise (5nV/√Hz),
and high speed operation (38MHz, 22V/µs) make them
ideal for driving sampling analog-to-digital converters.
They are also well suited for cell phone PA control
loops and video processing (75Ω drive capability) as
well as audio and general purpose applications. Single,
dual, and quad versions have identical specifications
for maximum design flexibility.
The OPA350 series operates on a single supply as low as
2.5V with an input common-mode voltage range that
extends 300mV below ground and 300mV above the
positive supply. Output voltage swing is to within 10mV
of the supply rails with a 10kΩ load. Dual and quad
designs feature completely independent circuitry for low-
est crosstalk and freedom from interaction.
The single (OPA350) and dual (OPA2350) come in
the miniature MSOP-8 surface mount, SO-8 surface
mount, and 8-pin DIP packages. The quad (OPA4350)
packages are the space-saving SSOP-16 surface mount
and SO-14 surface mount. All are specified from
–40°C to +85°C and operate from –55°C to +125°C.
®
© 1998 Burr-Brown Corporation PDS-1470B Printed in U.S.A. March, 1999
OPA350
OPA2350
OPA4350
FEATURES
●RAIL-TO-RAIL INPUT
●RAIL-TO-RAIL OUTPUT (within 10mV)
●WIDE BANDWIDTH: 38MHz
●HIGH SLEW RATE: 22V/µs
●LOW NOISE: 5nV/√Hz
● LOW THD+NOISE: 0.0006%
● UNITY-GAIN STABLE
●
Micro
SIZE PACKAGES
●SINGLE, DUAL, AND QUAD
International Airport Industrial Park • Mailing Address: PO Box 11400, Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd., Tucson, AZ 85706 • Tel: (520) 746-1111
Twx: 910-952-1111 • Internet: http://www.burr-brown.com/ • Cable: BBRCORP • Telex: 066-6491 • FAX: (520) 889-1510 • Immediate Product Info: (800) 548-6132
1
2
3
4
8
7
6
5
NC
V+
Output
NC
NC
–In
+In
V–
OPA350
8-Pin DIP, SO-8, MSOP-8
1
2
3
4
8
7
6
5
V+
Out B
–In B
+In B
Out A
–In A
+In A
V–
OPA2350
8-Pin DIP, SO-8, MSOP-8
A
B
OPA350
OPA2350
OPA4350
OPA4350
For most current data sheet and other product
information, visit www.burr-brown.com
SPICE Model available at www.burr-brown.com
SBOS099
2
®
OPA350, 2350, 4350
OPA350EA, UA, PA
OPA2350EA, UA, PA
OPA4350EA, UA
SPECIFICATIONS: VS = 2.7V to 5.5V
At TA = +25°C, RL = 1kΩ connected to VS/ 2 and VOUT = VS/ 2, unless otherwise noted.
Boldface limits apply over the specified temperature range, TA = –40°C to +85°C. VS = 5V.
PARAMETER CONDITION MIN TYP(1) MAX UNITS
OFFSET VOLTAGE
Input Offset Voltage VOS VS = 5V ±150 ±500 µV
TA = –40°C to +85°C±1mV
vs Temperature TA = –40°C to +85°C±4µV/°C
vs Power Supply Rejection Ratio PSRR VS = 2.7V to 5.5V, VCM = 0V 40 150 µV/V
TA = –40°C to +85°CV
S = 2.7V to 5.5V, VCM = 0V 175 µV/V
Channel Separation (dual, quad) dc 0.15 µV/V
INPUT BIAS CURRENT
Input Bias Current IB±0.5 ±10 pA
vs Temperature See Typical Performance Curve
Input Offset Current IOS ±0.5 ±10 pA
NOISE
Input Voltage Noise, f = 100Hz to 400kHz 4µVrms
Input Voltage Noise Density, f = 10kHz en7 nV/√Hz
f = 100kHz 5 nV/√Hz
Current Noise Density, f = 10kHz in4 fA/√Hz
INPUT VOLTAGE RANGE
Common-Mode Voltage Range VCM TA = –40°C to +85°C –0.1 (V+)+0.1 V
Common-Mode Rejection Ratio CMRR VS = 2.7V, –0.1V < VCM < 2.8V 66 84 dB
VS = 5.5V, –0.1V < VCM < 5.6V 76 90 dB
TA = –40°C to +85°CV
S = 5.5V, –0.1V < VCM < 5.6V 74 dB
INPUT IMPEDANCE
Differential 1013 || 2.5 Ω || pF
Common-Mode 1013 || 6.5 Ω || pF
OPEN-LOOP GAIN
Open-Loop Voltage Gain AOL RL = 10kΩ, 50mV < VO < (V+) –50mV 100 122 dB
TA = –40°C to +85°CR
L = 10kΩ, 50mV < VO < (V+) –50mV 100 dB
RL = 1kΩ, 200mV < VO < (V+) –200mV 100 120 dB
TA = –40°C to +85°CR
L = 1kΩ, 200mV < VO < (V+) –200mV 100 dB
FREQUENCY RESPONSE CL = 100pF
Gain-Bandwidth Product GBW G = 1 38 MHz
Slew Rate SR G = 1 22 V/µs
Settling Time, 0.1% G = ±1, 2V Step 0.22 µs
0.01% G = ±1, 2V Step 0.5 µs
Overload Recovery Time VIN • G = VS0.1 µs
Total Harmonic Distortion + Noise THD+N
RL = 600Ω, VO = 2.5Vp-p(2), G = 1, f = 1kHz
0.0006 %
Differential Gain Error G = 2, RL = 600Ω, VO = 1.4V (3) 0.17 %
Differential Phase Error G = 2, RL = 600Ω, VO = 1.4V (3) 0.17 deg
OUTPUT
Voltage Output Swing from Rail(4) VOUT RL = 10kΩ, AOL ≥ 100dB 10 50 mV
TA = –40°C to +85°CR
L = 10kΩ, AOL ≥ 100dB 50 mV
RL = 1kΩ, AOL ≥ 100dB 25 200 mV
TA = –40°C to +85°CR
L = 1kΩ, AOL ≥ 100dB 200 mV
Output Current IOUT ±40(5) mA
Short-Circuit Current ISC ±80 mA
Capacitive Load Drive CLOAD See Typical Curve
POWER SUPPLY
Operating Voltage Range VSTA = –40°C to +85°C 2.7 5.5 V
Minimum Operating Voltage 2.5 V
Quiescent Current (per amplifier) IQIO = 0 5.2 7.5 mA
TA = –40°C to +85°CIO = 0 8.5 mA
TEMPERATURE RANGE
Specified Range –40 +85 °C
Operating Range –55 +125 °C
Storage Range –55 +125 °C
Thermal Resistance
θ
JA
MSOP-8 Surface Mount 150 °C/W
SO-8 Surface Mount 150 °C/W
8-Pin DIP 100 °C/W
SO-14 Surface Mount 100 °C/W
SSOP-16 Surface Mount 100 °C/W
NOTES: (1) VS = +5V. (2) VOUT = 0.25V to 2.75V. (3) NTSC signal generator used. See Figure 6 for test circuit. (4) Output voltage swings are measured between
the output and power supply rails. (5) See typical performance curve, “Output Voltage Swing vs Output Current.”
3
®
OPA350, 2350, 4350
PACKAGE SPECIFIED
DRAWING TEMPERATURE PACKAGE ORDERING TRANSPORT
PRODUCT PACKAGE NUMBER(1) RANGE MARKING NUMBER(2) MEDIA
Single
OPA350EA MSOP-8 Surface Mount 337 –40°C to +85°C C50 OPA350EA/250 Tape and Reel
"""""OPA350EA/2K5 Tape and Reel
OPA350UA SO-8 Surface-Mount 182 –40°C to +85°C OPA350UA OPA350UA Rails
"""""OPA350UA/2K5 Tape and Reel
OPA350PA 8-Pin DIP 006 –40°C to +85°C OPA350PA OPA350PA Rails
Dual
OPA2350EA MSOP-8 Surface-Mount 337 –40 °C to +85°C D50 OPA2350EA/250 Tape and Reel
"""""OPA2350EA/2K5 Tape and Reel
OPA2350UA SO-8 Surface-Mount 182 –40°C to +85°C OPA2350UA OPA2350UA Rails
"""""OPA2350UA/2K5 Tape and Reel
OPA2350PA 8-Pin DIP 006 –40°C to +85°C OPA2350PA OPA2350PA Rails
Quad
OPA4350EA SSOP-16 Surface-Mount 322 –40°C to +85°C OPA4350EA OPA4350EA/250 Tape and Reel
"""""OPA4350EA/2K5 Tape and Reel
OPA4350UA SO-14 Surface Mount 235 –40°C to +85°C OPA4350UA OPA4350UA Rails
"""""OPA4350UA/2K5 Tape and Reel
NOTES: (1) For detailed drawing and dimension table, please see end of data sheet, or Appendix C of Burr-Brown IC Data Book. (2) Models with a slash (/) are
available only in Tape and Reel in the quantities indicated (e.g., /2K5 indicates 2500 devices per reel). Ordering 2500 pieces of “OPA2350EA/2K5” will get a single
2500-piece Tape and Reel. For detailed Tape and Reel mechanical information, refer to Appendix B of Burr-Brown IC Data Book.
PACKAGE/ORDERING INFORMATION
Supply Voltage ................................................................................... 5.5V
Signal Input Terminals, Voltage(2) .................. (V–) – 0.3V to (V+) + 0.3V
Current(2) .................................................... 10mA
Output Short Circuit(3) .............................................................. Continuous
Operating Temperature .................................................. –55°C to +125°C
Storage Temperature .....................................................–55°C to +125°C
Junction Temperature...................................................................... 150°C
Lead Temperature (soldering, 10s)................................................. 300°C
NOTES: (1) Stresses above these ratings may cause permanent damage.
Exposure to absolute maximum conditions for extended periods may de-
grade device reliability. (2) Input terminals are diode-clamped to the power
supply rails. Input signals that can swing more than 0.3V beyond the supply
rails should be current-limited to 10mA or less. (3) Short circuit to ground,
one amplifier per package.
ABSOLUTE MAXIMUM RATINGS(1)
The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes no responsibility
for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change without notice. No patent rights or
licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant any BURR-BROWN product for use in life support
devices and/or systems.
ELECTROSTATIC
DISCHARGE SENSITIVITY
This integrated circuit can be damaged by ESD. Burr-Brown
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 degrada-
tion 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.
4
®
OPA350, 2350, 4350
TYPICAL PERFORMANCE CURVES
At TA = +25°C, VS = +5V, and RL = 1kΩ connected to VS/2, unless otherwise noted.
POWER SUPPLY AND COMMON-MODE
REJECTION RATIO vs FREQUENCY
100
90
80
70
60
50
40
30
20
10
0
PSRR, CMRR (dB)
Frequency (Hz)
10 100 1k 10k 100k 1M 10M
PSRR
CMRR
(VS = +5V
VCM = –0.1V to 5.1V)
INPUT VOLTAGE AND CURRENT NOISE
SPECTRAL DENSITY vs FREQUENCY
100k
10k
1k
100
10
1
10k
1k
100
10
1
0.1
Voltage Noise (nV√Hz)
Frequency (Hz)
10 100 1k 10k 100k 1M 10M
Current Noise (fA√Hz)
Voltage Noise
Current Noise
CHANNEL SEPARATION vs FREQUENCY
Frequency (Hz)
Channel Separation (dB)
140
130
120
110
100
90
80
70
60 10010 1k 1M100k10k 10M
Dual and quad devices.
HARMONIC DISTORTION + NOISE vs FREQUENCY
1
(–40dBc)
0.1
(–60dBc)
0.01
(–80dBc)
0.001
(–100dBc)
0.0001
(–120dBc)
Harmonic Distortion (%)
Frequency (Hz)
1k 10k 100k 1M
G = 1
V
O
= 2.5Vp-p
R
L
= 600Ω
3rd Harmonic 2nd Harmonic
OPEN-LOOP GAIN/PHASE vs FREQUENCY
0.1 1
160
140
120
100
80
60
40
20
0
Voltage Gain (dB)
0
–45
–90
–135
–180
Phase (°)
Frequency (Hz)
10 100 1k 10k 100k 1M 10M 100M
G
φ
TOTAL HARMONIC DISTORTION + NOISE
vs FREQUENCY
1
0.1
0.01
0.001
0.0001
THD+N (%)
Frequency (Hz)
10 100 1k 10k 100k
R
L
= 600Ω
G = 100, 3Vp-p (V
O
= 1V to 4V)
G = 10, 3Vp-p (V
O
= 1V to 4V)
G = 1, 3Vp-p (V
O
= 1V to 4V)
Input goes through transition region
G = 1, 2.5Vp-p (V
O
= 0.25V to 2.75V)
Input does NOT go through transition region
5
®
OPA350, 2350, 4350
TYPICAL PERFORMANCE CUR VES (CONT)
At TA = +25°C, VS = +5V, and RL = 1kΩ connected to VS/2, unless otherwise noted.
OPEN-LOOP GAIN vs TEMPERATURE
130
125
120
115
110
Open-Loop Gain (dB)
Temperature (°C)
–75 –50 –25 0 25 50 75 100 125
R
L
= 600Ω
R
L
= 1kΩ
R
L
= 10kΩ
SLEW RATE vs TEMPERATURE
Temperature (°C)
Slew Rate (V/µs)
40
35
30
25
20
15
10
5
0–75 –50 –25 0 25 50 75 100 125
Negative Slew Rate
Positive Slew Rate
DIFFERENTIAL GAIN/PHASE vs RESISTIVE LOAD
0.5
0.4
0.3
0.2
0.1
0
Differential Gain (%)
Differential Phase (°)
Resistive Load (Ω)
0 100 200 300 500400 600 800700 900 1000
G = 2
V
O
= 1.4V
NTSC Signal Generator
See Figure 6 for test circuit.
Phase
Gain
COMMON-MODE AND POWER SUPPLY REJECTION RATIO
vs TEMPERATURE
100
90
80
70
60
CMRR (dB)
110
100
90
80
70
PSRR (dB)
Temperature (°C)
–75 –50 –25 0 25 50 75 100 125
CMRR, V
S
= 5.5V
(V
CM
= –0.1V to +5.6V)
CMRR, V
S
= 2.7V
(V
CM
= –0.1V to +2.8V)
PSRR
QUIESCENT CURRENT AND
SHORT-CIRCUIT CURRENT vs TEMPERATURE
Temperature (°C)
Quiescent Current (mA)
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
100
90
80
70
60
50
40
30
Short-Circuit Current (mA)
–75 –50 –25 0 25 50 75 100 125
I
Q
+I
SC
–I
SC
QUIESCENT CURRENT vs SUPPLY VOLTAGE
Supply Voltage (V)
Quiescent Current (mA)
6.0
5.5
5.0
4.5
4.0
3.5
3.02.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
Per Amplifier
6
®
OPA350, 2350, 4350
TYPICAL PERFORMANCE CUR VES (CONT)
At TA = +25°C, VS = +5V, and RL = 1kΩ connected to VS/2, unless otherwise noted.
INPUT BIAS CURRENT vs TEMPERATURE
Input Bias Current (pA)
Temperature (°C)
–75 –50 –25 0 25 50 75 100 125
1k
100
10
1
0.1
INPUT BIAS CURRENT
vs INPUT COMMON-MODE VOLTAGE
Common-Mode Voltage (V)
Input Bias Current (pA)
1.5
1.0
0.5
0.0
–0.5
–0.5 0.0 0.5 1.0 2.01.5 2.5 3.0 3.5 4.0 5.04.5 5.5
CLOSED-LOOP OUTPUT IMPEDANCE vs FREQUENCY
Frequency (Hz)
Output Impedance (Ω)
100
10
1
0.1
0.01
0.001
0.0001 1 10 100 1k 10k 100k 1M 10M 100M
G = 100
G = 10
G = 1
MAXIMUM OUTPUT VOLTAGE vs FREQUENCY
100M1M 10M
Frequency (Hz)
100k
6
5
4
3
2
1
0
Output Voltage (Vp-p)
Maximum output
voltage without
slew rate-induced
distortion.
V
S
= 2.7V
V
S
= 5.5V
OUTPUT VOLTAGE SWING vs OUTPUT CURRENT
Output Current (mA)
Output Voltage (V)
V+
(V+)–1
(V+)–2
(V–)+2
(V–)+1
(V–) 0±10 ±20 ±30 ±40
+25°C
+125°C–55°C
–55°C
+125°C+25°C
Depending on circuit configuration
(including closed-loop gain) performance
may be degraded in shaded region.
OPEN-LOOP GAIN vs OUTPUT VOLTAGE SWING
140
130
120
110
100
90
80
70
60
Open-Loop Gain (dB)
Output Voltage Swing from Rails (mV)
0 20 40 60 10080 120 160140 180 200
I
OUT
= 4.2mA
I
OUT
= 250µAI
OUT
= 2.5mA
7
®
OPA350, 2350, 4350
TYPICAL PERFORMANCE CUR VES (CONT)
At TA = +25°C, VS = +5V, and RL = 1kΩ connected to VS/2, unless otherwise noted.
SMALL-SIGNAL STEP RESPONSE
CL = 100pF
100ns/div
50mV/div
SETTLING TIME vs CLOSED-LOOP GAIN
10
1
0.1
Settling Time (µs)
Closed-Loop Gain (V/V)
–1 –10 –100
0.1%
0.01%
LARGE-SIGNAL STEP RESPONSE
CL = 100pF
200ns/div
1V/div
SMALL-SIGNAL OVERSHOOT vs LOAD CAPACITANCE
1M100 1k 10k 100k
Load Capacitance (pF)
10
80
70
60
50
40
30
20
10
0
Overshoot (%)
G = 1
G = –1
G = ±10
Offset Voltage (µV)
OFFSET VOLTAGE
PRODUCTION DISTRIBUTION
18
16
14
12
10
8
6
4
2
0
Percent of Amplifiers (%)
–500
–450
–400
–350
–300
–250
–200
–150
–100
–50
0
50
100
150
200
250
300
350
400
450
500
Typical distribution of
packaged units.
Offset Voltage Drift (µV/°C)
OFFSET VOLTAGE DRIFT
PRODUCTION DISTRIBUTION
20
18
16
14
12
10
8
6
4
2
00123456789101112131415
Percent of Amplifiers (%)
Typical production
distribution of
packaged units.
8
®
OPA350, 2350, 4350
0
5V
APPLICATIONS INFORMATION
OPA350 series op amps are fabricated on a state-of-the-art
0.6 micron CMOS process. They are unity-gain stable and
suitable for a wide range of general purpose applications.
Rail-to-rail input/output make them ideal for driving sam-
pling A/D converters. They are also well suited for control-
ling the output power in cell phones. These applications
often require high speed and low noise. In addition, the
OPA350 series offers a low cost solution for general purpose
and consumer video applications (75Ω drive capability).
Excellent ac performance makes the OPA350 series well
suited for audio applications. Their bandwidth, slew rate,
low noise (5nV/√Hz), low THD (0.0006%), and small pack-
age options are ideal for these applications. The class AB
output stage is capable of driving 600Ω loads connected to
any point between V+ and ground.
Rail-to-rail input and output swing significantly increases
dynamic range, especially in low voltage supply applica-
tions. Figure 1 shows the input and output waveforms for
FIGURE 2. Simplified Schematic.
VS = +5, G = +1, RL = 1kΩ
VIN
1.25V/div
FIGURE 1. Rail-to-Rail Input and Output.
V
BIAS1
V
BIAS2
V
IN
+V
IN
–
Class AB
Control
Circuitry V
O
V–
(Ground)
V+
Reference
Current
5V
0
VOUT
the OPA350 in unity-gain configuration. Operation is
from a single +5V supply with a 1kΩ load connected to
VS/2. The input is a 5Vp-p sinusoid. Output voltage swing
is approximately 4.95Vp-p.
Power supply pins should be bypassed with 0.01µF ceramic
capacitors.
OPERATING VOLTAGE
OPA350 series op amps are fully specified from +2.7V to
+5.5V. However, supply voltage may range from +2.5V to
+5.5V. Parameters are guaranteed over the specified supply
range—a unique feature of the OPA350 series. In addition,
many specifications apply from –40°C to +85°C. Most
behavior remains virtually unchanged throughout the full
operating voltage range. Parameters which vary signifi-
cantly with operating voltage or temperature are shown in
the typical performance curves.
RAIL-TO-RAIL INPUT
The guaranteed input common-mode voltage range of the
OPA350 series extends 100mV beyond the supply rails. This
is achieved with a complementary input stage—an
N-channel input differential pair in parallel with a P-channel
differential pair (see Figure 2). The N-channel pair is active
for input voltages close to the positive rail, typically
(V+) – 1.8V to 100mV above the positive supply, while the
P-channel pair is on for inputs from 100mV below the
negative supply to approximately (V+) – 1.8V. There is a
small transition region, typically (V+) – 2V to (V+) – 1.6V, in
which both pairs are on. This 400mV transition region can
vary ±400mV with process variation. Thus, the transition
region (both input stages on) can range from (V+) – 2.4V to
(V+) – 2.0V on the low end, up to (V+) – 1.6V to (V+) – 1.2V
on the high end.
9
®
OPA350, 2350, 4350
OPA350 series op amps are laser-trimmed to reduce offset
voltage difference between the N-channel and
P-channel input stages, resulting in improved common-
mode rejection and a smooth transition between the
N-channel pair and the P-channel pair. However, within the
400mV transition region PSRR, CMRR, offset voltage,
offset drift, and THD may be degraded compared to opera-
tion outside this region.
A double-folded cascode adds the signal from the two input
pairs and presents a differential signal to the class AB output
stage. Normally, input bias current is approximately 500fA.
However, large inputs (greater than 300mV beyond the
supply rails) can turn on the OPA350’s input protection
diodes, causing excessive current to flow in or out of the
input pins. Momentary voltages greater than 300mV beyond
the power supply can be tolerated if the current on the input
pins is limited to 10mA. This is easily accomplished with an
input resistor as shown in Figure 3. Many input signals are
inherently current-limited to less than 10mA, therefore, a
limiting resistor is not required.
performance curve “Small-Signal Overshoot vs Capacitive
Load” shows performance with a 1kΩ resistive load. In-
creasing load resistance improves capacitive load drive ca-
pability.
FEEDBACK CAPACITOR IMPROVES RESPONSE
For optimum settling time and stability with high-imped-
ance feedback networks, it may be necessary to add a
feedback capacitor across the feedback resistor, RF, as
shown in Figure 4. This capacitor compensates for the zero
created by the feedback network impedance and the
OPA350’s input capacitance (and any parasitic layout
capacitance). The effect becomes more significant with
higher impedance networks.
FIGURE 3. Input Current Protection for Voltages Exceeding
the Supply Voltage.
RAIL-TO-RAIL OUTPUT
A class AB output stage with common-source transistors is
used to achieve rail-to-rail output. For light resistive loads
(>10kΩ), the output voltage swing is typically a ten milli-
volts from the supply rails. With heavier resistive loads
(600Ω to 10kΩ), the output can swing to within a few tens
of millivolts from the supply rails and maintain high open-
loop gain. See the typical performance curves “Output
Voltage Swing vs Output Current” and “Open-Loop Gain vs
Output Voltage.”
CAPACITIVE LOAD AND STABILITY
OPA350 series op amps can drive a wide range of capacitive
loads. However, all op amps under certain conditions may
become unstable. Op amp configuration, gain, and load
value are just a few of the factors to consider when determin-
ing stability. An op amp in unity gain configuration is the
most susceptible to the effects of capacitive load. The
capacitive load reacts with the op amp’s output impedance,
along with any additional load resistance, to create a pole in
the small-signal response which degrades the phase margin.
In unity gain, OPA350 series op amps perform well with
very large capacitive loads. Increasing gain enhances the
amplifier’s ability to drive more capacitance. The typical
FIGURE 4. Feedback Capacitor Improves Dynamic Perfor-
mance.
5kΩ
OPAx350
10mA max
V+
V
IN
V
OUT
I
OVERLOAD
It is suggested that a variable capacitor be used for the
feedback capacitor since input capacitance may vary be-
tween op amps and layout capacitance is difficult to
determine. For the circuit shown in Figure 4, the value of
the variable feedback capacitor should be chosen so that
the input resistance times the input capacitance of the
OPA350 (typically 9pF) plus the estimated parasitic layout
capacitance equals the feedback capacitor times the feed-
back resistor:
RIN • CIN = RF • CF
where CIN is equal to the OPA350’s input capacitance
(sum of differential and common-mode) plus the layout
capacitance. The capacitor can be varied until optimum
performance is obtained.
DRIVING A/D CONVERTERS
OPA350 series op amps are optimized for driving medium
speed (up to 500kHz) sampling A/D converters. However,
they also offer excellent performance for higher speed
converters. The OPA350 series provides an effective means
of buffering the A/D’s input capacitance and resulting
charge injection while providing signal gain.
OPA350
V+
V
OUT
V
IN
R
IN
R
IN
• C
IN
= R
F
•
C
F
R
F
C
L
C
IN
C
IN
C
F
Where C
IN
is equal to the OPA350’s input
capacitance (approximately 9pF) plus any
parastic layout capacitance.
10
®
OPA350, 2350, 4350
FIGURE 5. OPA4350 Driving Sampling A/D Converter.
Figure 5 shows the OPA350 driving an ADS7861. The
ADS7861 is a dual, 500kHz 12-bit sampling converter in
the tiny SSOP-24 package. When used with the miniature
package options of the OPA350 series, the combination is
ideal for space-limited applications. For further informa-
tion, consult the ADS7861 data sheet.
OUTPUT IMPEDANCE
The low frequency open-loop output impedance of the
OPA350’s common-source output stage is approximately
1kΩ. When the op amp is connected with feedback, this
value is reduced significantly by the loop gain of the op
amp. For example, with 122dB of open-loop gain, the
output impedance is reduced in unity-gain to less than
0.001Ω. For each decade rise in the closed-loop gain, the
loop gain is reduced by the same amount which results in
a ten-fold increase in effective output impedance (see the
typical performance curve, “Output Impedance vs Fre-
quency”).
At higher frequencies, the output impedance will rise as
the open-loop gain of the op amp drops. However, at these
frequencies the output also becomes capacitive due to
parasitic capacitance. This prevents the output impedance
from becoming too high, which can cause stability prob-
lems when driving capacitive loads. As mentioned previ-
ously, the OPA350 has excellent capacitive load drive
capability for an op amp with its bandwidth.
VIDEO LINE DRIVER
Figure 6 shows a circuit for a single supply, G = 2 com-
posite video line driver. The synchronized outputs of a
composite video line driver extend below ground. As
shown, the input to the op amp should be ac-coupled and
shifted positively to provide adequate signal swing to
account for these negative signals in a single-supply con-
figuration.
The input is terminated with a 75Ω resistor and ac-coupled
with a 47µF capacitor to a voltage divider that provides the
dc bias point to the input. In Figure 6, this point is
approximately (V–) + 1.7V. Setting the optimal bias point
requires some understanding of the nature of composite
video signals. For best performance, one should be careful
to avoid the distortion caused by the transition region of
the OPA350’s complementary input stage. Refer to the
discussion of rail-to-rail input.
1/4
OPA4350
V
IN
B1
2
3
4
2kΩ
2kΩ
C
B1
CH B1+
CH B1–
CH B0+
CH B0–
CH A1+
CH A1–
CH A0+
CH A0–
REF
IN
REF
OUT
SERIAL DATA A
SERIAL DATA B
BUSY
CLOCK
CS
RD
CONVST
A0
M0
M1
2
3
4
5
6
7
8
9
10
11
23
22
21
20
19
18
17
16
15
14
1/4
OPA4350
V
IN
B0
+5V
6
5
2kΩ
2kΩ
C
B0
1/4
OPA4350
V
IN
A1
9
10
12
13
8
7
1
2kΩ
2kΩ
C
A1
1/4
OPA4350
V
IN
A0
14
11
112
2kΩ
2kΩ
C
A0
0.1µF0.1µF
+V
A
+V
D
24 13
Serial
Interface
DGND AGND
ADS7861
V
IN
= 0V to 2.45V for 0V to 4.9V output.
Choose C
B1
, C
B0
, C
A1
, C
A0
to filter high frequency noise.
11
®
OPA350, 2350, 4350
FIGURE 6. Single-Supply Video Line Driver.
FIGURE 8. 10kHz Low-Pass Filter. FIGURE 9. 10kHz High-Pass Filter.
+2.5V
V
IN
R
2
19.6kΩ
R
1
2.74kΩ
–2.5V
C
2
1nF
R
L
20kΩ
OPA350 V
OUT
C
1
4.7nF
+2.5V
V
IN
C
2
270pF
C
1
1830pF
–2.5V
R
2
49.9kΩ
R
L
20kΩ
OPA350 V
OUT
R
1
10.5kΩ
FIGURE 7. Two Op-Amp Instrumentation Amplifier With Improved High Frequency Common-Mode Rejection.
OPA350
+5V
V
OUT
+5V (pin 7)
Video
In
R
OUT
R
L
Cable
R
F
1kΩ
R
G
1kΩ
R
4
5kΩ
R
3
5kΩ
C
3
10µF
0.1µF10µF
+
6
7
4
3
2
C
4
0.1µF
C
5
1000µF
C
2
47µF
R
2
5kΩ
R
1
75Ω
C
1
220µF
1/2
OPA2350
1/2
OPA2350
R
3
25kΩ
R
2
25kΩ
R
G
R
1
100kΩ
R
4
100kΩ
R
L
10kΩ
V
O
50kΩ
G = 5 + 200kΩ
R
G
+5V
+5V
REF1004-2.5
4
8
(2.5V)
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