High Precision, Low Noise

OPERATIONAL AMPLIFIERS

FEATURES

LOW NOISE: 3nV/

WIDE BANDWIDTH:

OPA227: 8MHz, 2.3V/

OPA228: 33MHz, 10V/

SETTLING TIME: 5

(significant improvement over OP-27)

HIGH CMRR: 138dB

HIGH OPEN-LOOP GAIN: 160dB

LOW INPUT BIAS CURRENT: 10nA max

LOW OFFSET VOLTAGE: 75

V max

WIDE SUPPLY RANGE:

2.5V to

18V

OPA227 REPLACES OP-27, LT1007, MAX427

OPA228 REPLACES OP-37, LT1037, MAX437

SINGLE, DUAL, AND QUAD VERSIONS

APPLICATIONS

DATA ACQUISITION

TELECOM EQUIPMENT

GEOPHYSICAL ANALYSIS

VIBRATION ANALYSIS

SPECTRAL ANALYSIS

PROFESSIONAL AUDIO EQUIPMENT

ACTIVE FILTERS

POWER SUPPLY CONTROL

OPA227

OPA2227
OPA4227

OPA228

OPA2228
OPA4228

PDS-1494B

Printed in U.S.A. May, 1999

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

OPA4227

OPA227

OPA2

OPA2227

OPA4227

OPA2

227

For most current data sheet and other product

information, visit www.burr-brown.com

DESCRIPTION

The OPA227 and OPA228 series op amps combine
low noise and wide bandwidth with high precision to
make them the ideal choice for applications requiring
both ac and precision dc performance.

The OPA227 is unity gain stable and features high
slew rate (2.3V/

s) and wide bandwidth (8MHz). The

OPA228 is optimized for closed-loop gains of 5 or
greater, and offers higher speed with a slew rate of
10V/

s and a bandwidth of 33MHz.

The OPA227 and OPA228 series op amps are ideal
for professional audio equipment. In addition, low
quiescent current and low cost make them ideal for
portable applications requiring high precision.

The OPA227 and OPA228 series op amps are pin-
for-pin replacements for the industry standard OP-27
and OP-37 with substantial improvements across the
board. The dual and quad versions are available for
space savings and per-channel cost reduction.

The OPA227, OPA228, OPA2227, and OPA2228
are available in DIP-8 and SO-8 packages. The
OPA4227 and OPA4228 are available in DIP-14
and SO-14 packages with standard pin configura-
tions. Operation is specified from 40

C to +85

SPICE Model available for OPA227 at www.burr-brown.com

Trim

V+

Output

Trim

+In

OPA227, OPA228

DIP-8, SO-8

V+

Out B

In B

+In B

Out A

In A

+In A

OPA2227, OPA2228

DIP-8, SO-8

Out D

In D

+In D

+In C

In C

Out C

Out A

In A

+In A

V+

+In B

In B

Out B

OPA4227, OPA4228

DIP-14, SO-14

OPA227, 2227, 4227
OPA228, 2228, 4228

SPECIFICATIONS: V

5V to

15V

OPA227 Series

At T

= +25

C, and R

= 10k

, unless otherwise noted.

Boldface limits apply over the specified temperature range, T

= 40

C to +85

OPA227PA, UA

OPA227P, U

OPA2227PA, UA

OPA2227P, U

OPA4227PA, UA

PARAMETER

CONDITION

MIN

TYP

MAX

MIN

TYP

MAX

UNITS

OFFSET VOLTAGE
Input Offset Voltage

200

= 40

C to +85

ver Temperature

100

200

vs Temperature

/dT

0.1

0.6

0.3

vs Power Supply

PSRR

2.5V to

18V

0.5

V/V

= 40

C to +85

V/V

vs Time

0.2

V/mo

Channel Separation (dual, quad)

0.2

V/V

f = 1kHz, R

= 5k

110

INPUT BIAS CURRENT
Input Bias Current

2.5

= 40

C to +85

Input Offset Current

2.5

= 40

C to +85

NOISE
Input Voltage Noise, f = 0.1Hz to 10Hz

nVp-p

nVrms

Input Voltage Noise Density, f = 10Hz e

3.5

nV/

f = 100Hz

nV/

f = 1kHz

nV/

Current Noise Density, f = 1kHz

0.4

pA/

INPUT VOLTAGE RANGE
Common-Mode Voltage Range

(V)+2

(V+)2

Common-Mode Rejection

CMRR

= (V)+2V

to (V+)2V

120

138

= 40

C to +85

120

INPUT IMPEDANCE
Differential

|| 12

|| pF

Common-Mode

= (V)+2V

to (V+)2V

|| 3

|| pF

OPEN-LOOP GAIN
Open-Loop Voltage Gain

= (V)+2V to (V+)2V, R

= 10k

132

160

= 40

C to +85

132

= (V)+3.5V to (V+)3.5V, R

= 600

132

160

= 40

C to +85

132

FREQUENCY RESPONSE
Gain Bandwidth Product

GBW

MHz

Slew Rate

2.3

Settling Time: 0.1%

G = 1, 10V Step, C

= 100pF

0.01%

G = 1, 10V Step, C

= 100pF

5.6

Overload Recovery Time

· G = V

1.3

Total Harmonic Distortion + Noise THD+N

f = 1kHz, G = 1, V

= 3.5Vrms

0.00005

OUTPUT
Voltage Output

= 10k

(V)+2

(V+)2

= 40

C to +85

= 10k

(V)+2

(V+)2

= 600

(V)+3.5

(V+)3.5

= 40

C to +85

= 600

(V)+3.5

(V+)3.5

Short-Circuit Current

Capacitive Load Drive

LOAD

See Typical Curve

POWER SUPPLY
Specified Voltage Range

Operating Voltage Range

2.5

Quiescent Current (per amplifier)

= 0

3.7

3.8

= 40

C to +85

= 0

4.2

TEMPERATURE RANGE
Specified Range

+85

Operating Range

+125

Storage Range

+150

Thermal Resistance

SO-8 Surface Mount

150

C/W

DIP-8

100

C/W

DIP-14

C/W

SO-14 Surface Mount

100

C/W

Specifications same as OPA227P, U.

OPA227, 2227, 4227
OPA228, 2228, 4228

OPA228PA, UA

OPA228P, U

OPA2228PA, UA

OPA2228P, U

OPA4228PA, UA

PARAMETER

CONDITION

MIN

TYP

MAX

MIN

TYP

MAX

UNITS

OFFSET VOLTAGE
Input Offset Voltage

200

= 40

C to +85

ver Temperature

100

200

vs Temperature

/dT

0.1

0.6

0.3

vs Power Supply

PSRR

2.5V to

18V

0.5

V/V

= 40

C to +85

V/V

vs Time

0.2

V/mo

Channel Separation (dual, quad)

0.2

V/V

f = 1kHz, R

= 5k

110

INPUT BIAS CURRENT
Input Bias Current

2.5

= 40

C to +85

Input Offset Current

2.5

= 40

C to +85

NOISE
Input Voltage Noise, f = 0.1Hz to 10Hz

nVp-p

nVrms

Input Voltage Noise Density, f = 10Hz e

3.5

nV/

f = 100Hz

nV/

f = 1kHz

nV/

Current Noise Density, f = 1kHz

0.4

pA/

INPUT VOLTAGE RANGE
Common-Mode Voltage Range

(V)+2

(V+)2

Common-Mode Rejection

CMRR

= (V)+2V

to (V+)2V

120

138

= 40

C to +85

120

INPUT IMPEDANCE
Differential

|| 12

|| pF

Common-Mode

= (V)+2V

to (V+)2V

|| 3

|| pF

OPEN-LOOP GAIN
Open-Loop Voltage Gain

= (V)+2V to (V+)2V, R

= 10k

132

160

= 40

C to +85

132

= (V)+3.5V to (V+)3.5V, R

= 600

132

160

= 40

C to +85

132

FREQUENCY RESPONSE
Minimum Closed-Loop Gain

V/V

Gain Bandwidth Product

GBW

MHz

Slew Rate

Settling Time: 0.1%

G = 5, 10V Step, C

= 100pF, C

=12pF

1.5

0.01%

G = 5, 10V Step, C

= 100pF, C

=12pF

Overload Recovery Time

· G = V

0.6

Total Harmonic Distortion + Noise THD+N

f = 1kHz, G = 5, V

= 3.5Vrms

0.00005

OUTPUT
Voltage Output

= 10k

(V)+2

(V+)2

= 40

C to +85

= 10k

(V)+2

(V+)2

= 600

(V)+3.5

(V+)3.5

= 40

C to +85

= 600

(V)+3.5

(V+)3.5

Short-Circuit Current

Capacitive Load Drive

LOAD

See Typical Curve

POWER SUPPLY
Specified Voltage Range

Operating Voltage Range

2.5

Quiescent Current (per amplifier)

= 0

3.7

3.8

= 40

C to +85

= 0

4.2

TEMPERATURE RANGE
Specified Range

+85

Operating Range

+125

Storage Range

+150

Thermal Resistance

SO-8 Surface Mount

150

C/W

DIP-8

100

C/W

DIP-14

C/W

SO-14 Surface Mount

100

C/W

Specifications same as OPA228P, U.

SPECIFICATIONS: V

5V to

15V

OPA228 Series

At T

= +25

C, and R

= 10k

, unless otherwise noted.

Boldface limits apply over the specified temperature range, T

= 40

C to +85

OPA227, 2227, 4227
OPA228, 2228, 4228

ABSOLUTE MAXIMUM RATINGS

(1)

Supply Voltage ..................................................................................

18V

Signal Input Terminals, Voltage ........................ (V) 0.7V to (V+) +0.7V

Current ....................................................... 20mA

Output Short-Circuit

(2)

.............................................................. Continuous

Operating Temperature .................................................. 55

C to +125

Storage Temperature ..................................................... 65

C to +150

Junction Temperature ...................................................................... 150

Lead Temperature (soldering, 10s) ................................................. 300

NOTE: (1) Stresses above these ratings may cause permanent damage.
Exposure to absolute maximum conditions for extended periods may degrade
device reliability. (2) Short-circuit to ground, one amplifier per package.

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.

PACKAGE/ORDERING INFORMATION

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.

OFFSET

PACKAGE

VOLTAGE

VOLTAGE DRIFT

DRAWING

TEMPERATURE

ORDERING

TRANSPORT

PRODUCT

max,

PACKAGE

NUMBER

(1)

RANGE

NUMBER

(2)

MEDIA

OPA227 Series

Single
OPA227PA

200

DIP-8

006

C to +85

OPA227PA

Rails

OPA227P

0.6

DIP-8

006

C to +85

OPA227P

Rails

OPA227UA

200

SO-8 Surface Mount

182

C to +85

OPA227UA

Rails

OPA227UA/2K5

Tape and Reel

OPA227U

0.6

SO-8 Surface Mount

182

C to +85

OPA227U

Rails

OPA227U/2K5

Tape and Reel

Dual
OPA2227PA

200

DIP-8

006

C to +85

OPA2227PA

Rails

OPA2227P

0.6

DIP-8

006

C to +85

OPA2227P

Rails

OPA2227UA

200

SO-8 Surface Mount

182

C to +85

OPA2227UA

Rails

OPA2227UA/2K5

Tape and Reel

OPA2227U

0.6

SO-8 Surface Mount

182

C to +85

OPA2227U

Rails

OPA2227U/2K5

Tape and Reel

Quad
OPA4227PA

200

DIP-14

010

C to +85

OPA4227PA

Rails

OPA4227UA

200

SO-14 Surface Mount

235

C to +85

OPA4227UA

Rails

OPA4227UA/2K5

Tape and Reel

OPA228 Series

Single
OPA228PA

200

DIP-8

006

C to +85

OPA228PA

Rails

OPA228P

0.6

DIP-8

006

C to +85

OPA228P

Rails

OPA228UA

200

SO-8 Surface Mount

182

C to +85

OPA228UA

Rails

OPA228UA/2K5

Tape and Reel

OPA228U

0.6

SO-8 Surface Mount

182

C to +85

OPA228U

Rails

OPA228U/2K5

Tape and Reel

Dual
OPA2228PA

200

DIP-8

006

C to +85

OPA2228PA

Rails

OPA2228P

0.6

DIP-8

006

C to +85

OPA2228P

Rails

OPA2228UA

200

SO-8 Surface Mount

182

C to +85

OPA2228UA

Rails

OPA2228UA/2K5

Tape and Reel

OPA2228U

0.6

SO-8 Surface Mount

182

C to +85

OPA2228U

Rails

OPA2228U/2K5

Tape and Reel

Quad
OPA4228PA

200

DIP-14

010

C to +85

OPA4228PA

Rails

OPA4228UA

200

SO-14 Surface Mount

235

C to +85

OPA4228UA

Rails

OPA4228UA/2K5

Tape and Reel

NOTE: (1) For detailed drawing and dimension table, please see end of data sheet, or Appendix C of Burr-Brown IC Data Book. (2) Products followed by a slash
(/) are only available in Tape and Reel in the quantities indicated (e.g. /2K5 indicates 2500 devices per reel). Ordering 2500 pieces of "OPA227UA/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.

OPA227, 2227, 4227
OPA228, 2228, 4228

TYPICAL PERFORMANCE CURVES

At T

= +25

C, R

= 10k

, and V

15V, unless otherwise noted.

0.01 0.10

100

10k 100k

1M 10M 100M

180

160

140

120

100

(dB)

100

120

140

160

180

200

Phase (

)

Frequency (Hz)

OPEN-LOOP GAIN/PHASE vs FREQUENCY

OPA228

100

10k

20k

0.01

0.001

0.0001

0.00001

THD+Noise (%)

Frequency (Hz)

TOTAL HARMONIC DISTORTION + NOISE

vs FREQUENCY

G = 1, R

= 10k

OUT

= 3.5Vrms

OPA227

100

10k

50k

0.01

0.001

0.0001

0.00001

THD+Noise (%)

Frequency (Hz)

TOTAL HARMONIC DISTORTION + NOISE

vs FREQUENCY

G = 1, R

= 10k

OUT

= 3.5Vrms

OPA228

0.01 0.10

100

10k 100k

1M 10M 100M

180

160

140

120

100

(dB)

100

120

140

160

180

200

Phase (

)

Frequency (Hz)

OPEN-LOOP GAIN/PHASE vs FREQUENCY

OPA227

0.1

100

10k

100k

140

120

100

-20

PSRR, CMRR (dB)

Frequency (Hz)

POWER SUPPLY AND COMMON-MODE

REJECTION RATIO vs FREQUENCY

+CMRR

+PSRR

PSRR

0.1

100

10k

100k

10k

100

Voltage Noise (nV/

Hz)

Current Noise (fA/

Hz)

Frequency (Hz)

INPUT VOLTAGE AND CURRENT NOISE

SPECTRAL DENSITY vs FREQUENCY

Current Noise

Voltage Noise

OPA227, 2227, 4227
OPA228, 2228, 4228

TYPICAL PERFORMANCE CURVES

(CONT)

At T

= +25

C, R

=10k

, and V

15V, unless otherwise noted.

OFFSET VOLTAGE PRODUCTION DISTRIBUTION

Percent of Amplifiers (%)

Offset Voltage (

150

135

120

105

120

135

150

17.5

15.0

12.5

10.0

5.5

5.0

2.5

Typical distribution
of packaged units.

OFFSET VOLTAGE DRIFT PRODUCTION DISTRIBUTION

Percent of Amplifiers (%)

Offset Voltage Drift (

V)/

Typical distribution
of packaged units.

0.5

1.0

1.5

Offset Voltage Change (

100

150

300

Time from Power Supply Turn-On (s)

WARM-UP OFFSET VOLTAGE DRIFT

200

250

100

10k

100k

140

120

100

Channel Separation (dB)

Frequency (Hz)

CHANNEL SEPARATION vs FREQUENCY

Dual and quad devices. G = 1, all channels.
Quad measured Channel A to D, or B to C;
other combinations yield similiar or improved
rejection.

INPUT NOISE VOLTAGE vs TIME

1s/div

50nV/div

VOLTAGE NOISE DISTRIBUTION (10Hz)

Percent of Units (%)

Noise (nV/

Hz)

3.16

3.25

3.34 3.43

3.51 3.60

3.69 3.78

OPA227, 2227, 4227
OPA228, 2228, 4228

TYPICAL PERFORMANCE CURVES

(CONT)

At T

= +25

C, R

= 10k

, and V

15V, unless otherwise noted.

100

120 140

2.0

1.5

1.0

0.5

1.0

1.5

2.0

Input Bias Current (nA)

Temperature (

INPUT BIAS CURRENT vs TEMPERATURE

100

125

Short-Circuit Current (mA)

Temperature (

SHORT-CIRCUIT CURRENT vs TEMPERATURE

QUIESCENT CURRENT vs TEMPERATURE

100

120 140

Temperature (

5.0

4.5

4.0

3.5

3.0

2.5

Quiescent Current (mA)

10V

2.5V

18V

15V

12V

QUIESCENT CURRENT vs SUPPLY VOLTAGE

Supply Voltage (

3.8

3.6

3.4

3.2

3.0

2.8

Quiescent Current (mA)

100

125

160

150

140

130

120

110

100

, CMRR, PSRR (dB)

Temperature (

, CMRR, PSRR vs TEMPERATURE

CMRR

PSRR

OPA227

100

125

160

150

140

130

120

110

100

, CMRR, PSRR (dB)

Temperature (

, CMRR, PSRR vs TEMPERATURE

CMRR

PSRR

OPA228

OPA227, 2227, 4227
OPA228, 2228, 4228

TYPICAL PERFORMANCE CURVES

(CONT)

At T

= +25

C, R

= 10k

, and V

15V, unless otherwise noted.

2.0

1.5

1.0

0.5

1.0

1.5

2.0

(nA)

Supply Voltage (V)

CHANGE IN INPUT BIAS CURRENT

vs POWER SUPPLY VOLTAGE

Curve shows normalized change in bias current
with respect to V

10V. Typical I

may range

from 2nA to +2nA at V

10V.

CHANGE IN INPUT BIAS CURRENT

vs COMMON-MODE VOLTAGE

Common-Mode Voltage (V)

1.5

1.0

0.5

1.0

1.5

(nA)

15V

Curve shows normalized change in bias current
with respect to V

= 0V. Typical I

may range

from 2nA to +2nA at V

= 0V.

OUTPUT VOLTAGE SWING vs OUTPUT CURRENT

V+

(V+) 1V

(V+) 2V

(V+) 3V

(V) +3V

(V) +2V

(V) +1V

Output Current (mA)

Output Voltage Swing (V)

125

100

Settling Time (

100

Gain (V/V)

SETTLING TIME vs CLOSED-LOOP GAIN

0.01%

OPA227

0.1%

15V, 10V Step

= 1500pF

= 2k

0.01%

OPA228

0.1%

SLEW RATE vs TEMPERATURE

125

Temperature (

100

3.0

2.5

2.0

1.5

1.0

0.5

Slew Rate (

V/V)

Negative Slew Rate

LOAD

= 2k

LOAD

= 100pF

Positive Slew Rate

OPA227

SLEW RATE vs TEMPERATURE

125

Temperature (

100

Slew Rate (

V/V)

LOAD

= 2k

LOAD

= 100pF

OPA228

OPA227, 2227, 4227
OPA228, 2228, 4228

FIGURE 1. OPA227 Offset Voltage Trim Circuit.

APPLICATIONS INFORMATION

The OPA227 and OPA228 series are precision op amps with
very low noise. The OPA227 series is unity-gain stable with
a slew rate of 2.3V/

s and 8MHz bandwidth. The OPA228

series is optimized for higher-speed applications with gains
of 5 or greater, featuring a slew rate of 10V/

s and 33MHz

bandwidth. Applications with noisy or high impedance
power supplies may require decoupling capacitors close to
the device pins. In most cases, 0.1

F capacitors are ad-

equate.

OFFSET VOLTAGE AND DRIFT

The OPA227 and OPA228 series have very low offset
voltage and drift. To achieve highest dc precision, circuit
layout and mechanical conditions should be optimized.
Connections of dissimilar metals can generate thermal po-
tentials at the op amp inputs which can degrade the offset
voltage and drift. These thermocouple effects can exceed
the inherent drift of the amplifier and ultimately degrade its
performance. The thermal potentials can be made to cancel
by assuring that they are equal at both input terminals. In
addition:

· Keep thermal mass of the connections made to the two

input terminals similar.

· Locate heat sources as far as possible from the critical

input circuitry.

· Shield op amp and input circuitry from air currents such

as those created by cooling fans.

OPERATING VOLTAGE

OPA227 and OPA228 series op amps operate from

2.5V to

18V supplies with excellent performance. Unlike most op

amps which are specified at only one supply voltage, the
OPA227 series is specified for real-world applications; a
single set of specifications applies over the

5V to

15V

supply range. Specifications are guaranteed for applications
between

5V and

15V power supplies. Some applications

do not require equal positive and negative output voltage
swing. Power supply voltages do not need to be equal. The
OPA227 and OPA228 series can operate with as little as 5V
between the supplies and with up to 36V between the
supplies. For example, the positive supply could be set to
25V with the negative supply at 5V or vice-versa. In
addition, key parameters are guaranteed over the specified
temperature range, 40

C to +85

C. Parameters which vary

significantly with operating voltage or temperature are shown
in the Typical Performance Curves.

OFFSET VOLTAGE ADJUSTMENT

The OPA227 and OPA228 series are laser-trimmed for
very low offset and drift so most applications will not
require external adjustment. However, the OPA227 and
OPA228 (single versions) provide offset voltage trim con-
nections on pins 1 and 8. Offset voltage can be adjusted by
connecting a potentiometer as shown in Figure 1. This
adjustment should be used only to null the offset of the op

amp. This adjustment should not be used to compensate for
offsets created elsewhere in the system since this can
introduce additional temperature drift.

INPUT PROTECTION

Back-to-back diodes (see Figure 2) are used for input protec-
tion on the OPA227 and OPA228. Exceeding the turn-on
threshold of these diodes, as in a pulse condition, can cause
current to flow through the input protection diodes due to the
amplifier's finite slew rate. Without external current-limiting
resistors, the input devices can be destroyed. Sources of high
input current can cause subtle damage to the amplifier.
Although the unit may still be functional, important param-
eters such as input offset voltage, drift, and noise may shift.

FIGURE 2. Pulsed Operation.

When using the OPA227 as a unity-gain buffer (follower), the
input current should be limited to 20mA. This can be accom-
plished by inserting a feedback resistor or a resistor in series
with the source. Sufficient resistor size can be calculated:

= V

/20mA R

SOURCE

where R

is either in series with the source or inserted in

the feedback path. For example, for a 10V pulse (V

10V), total loop resistance must be 500

. If the source

impedance is large enough to sufficiently limit the current
on its own, no additional resistors are needed. The size of
any external resistors must be carefully chosen since they
will increase noise. See the Noise Performance section of
this data sheet for further information on noise calcula-
tion. Figure 2 shows an example implementing a current-
limiting feedback resistor.

OPA227

20k

0.1

V+

Trim range exceeds

offset voltage specification

OPA227 and OPA228 single op amps only.

Use offset adjust pins only to
null offset voltage of op amp.

See text.

OPA227

Output

500

Input

OPA227, 2227, 4227
OPA228, 2228, 4228

INPUT BIAS CURRENT CANCELLATION

The input bias current of the OPA227 and OPA228 series is
internally compensated with an equal and opposite cancella-
tion current. The resulting input bias current is the difference
between with input bias current and the cancellation current.
The residual input bias current can be positive or negative.

When the bias current is cancelled in this manner, the input
bias current and input offset current are approximately equal.
A resistor added to cancel the effect of the input bias current
(as shown in Figure 3) may actually increase offset and noise
and is therefore not recommended.

Design of low noise op amp circuits requires careful
consideration of a variety of possible noise contributors:
noise from the signal source, noise generated in the op
amp, and noise from the feedback network resistors. The
total noise of the circuit is the root-sum-square combina-
tion of all noise components.

The resistive portion of the source impedance produces
thermal noise proportional to the square root of the
resistance. This function is shown plotted in Figure 4.
Since the source impedance is usually fixed, select the op
amp and the feedback resistors to minimize their contri-
bution to the total noise.

Figure 4 shows total noise for varying source imped-
ances with the op amp in a unity-gain configuration (no
feedback resistor network and therefore no additional
noise contributions). The operational amplifier itself con-
tributes both a voltage noise component and a current

FIGURE 3. Input Bias Current Cancellation.

FIGURE 4. Noise Performance of the OPA227 in Unity-

Gain Buffer Configuration.

NOISE PERFORMANCE

Figure 4 shows total circuit noise for varying source imped-
ances with the op amp in a unity-gain configuration (no
feedback resistor network, therefore no additional noise con-
tributions). Two different op amps are shown with total circuit
noise calculated. The OPA227 has very low voltage noise,
making it ideal for low source impedances (less than 20k

A similar precision op amp, the OPA277, has somewhat higher
voltage noise but lower current noise. It provides excellent
noise performance at moderate source impedance (10k

100k

). Above 100k

, a FET-input op amp such as the

OPA132 (very low current noise) may provide improved
performance. The equation is shown for the calculation of the
total circuit noise. Note that e

= voltage noise, i

= current

noise, R

= source impedance, k = Boltzmann's constant =

1.38 · 10

J/K and T is temperature in K. For more details on

calculating noise, see the insert titled "Basic Noise Calcula-
tions."

noise component. The voltage noise is commonly mod-
eled as a time-varying component of the offset voltage.
The current noise is modeled as the time-varying compo-
nent of the input bias current and reacts with the source
resistance to create a voltage component of noise. Conse-
quently, the lowest noise op amp for a given application
depends on the source impedance. For low source imped-
ance, current noise is negligible and voltage noise gener-
ally dominates. For high source impedance, current noise
may dominate.

Figure 5 shows both inverting and noninverting op amp
circuit configurations with gain. In circuit configurations
with gain, the feedback network resistors also contribute
noise. The current noise of the op amp reacts with the
feedback resistors to create additional noise components.
The feedback resistor values can generally be chosen to
make these noise sources negligible. The equations for
total noise are shown for both configurations.

BASIC NOISE CALCULATIONS

Op Amp

= R

|| R

External Cancellation Resistor

Not recommended

for OPA227

Conventional Op Amp Configuration

Recommended OPA227 Configuration

OPA227

No cancellation resistor.

See text.

VOLTAGE NOISE SPECTRAL DENSITY

vs SOURCE RESISTANCE

100k

10M

Source Resistance, R

(

)

100

10k

1.00+03

1.00E+02

1.00E+01

1.00E+00

Votlage Noise Spectral Density, E

Typical at 1k (V/

Hz)

OPA227

OPA277

Resistor Noise

OPA277

OPA227

= e

+ (i

)

+ 4kTR

OPA227, 2227, 4227
OPA228, 2228, 4228

Figure 6 shows the 0.1Hz 10Hz bandpass filter used to test
the noise of the OPA227 and OPA228. The filter circuit was
designed using Burr-Brown's FilterPro software (available
at www.burr-brown.com). Figure 7 shows the configura-
tion of the OPA227 and OPA228 for noise testing.

FIGURE 6. 0.1Hz to 10Hz Bandpass Filter Used to Test Wideband Noise of the OPA227 and OPA228 Series.

FIGURE 7. Noise Test Circuit.

USING THE OPA228 IN LOW GAINS

The OPA228 family is intended for applications with signal
gains of 5 or greater, but it is possible to take advantage of
their high speed in lower gains. Without external compen-
sation, the OPA228 has sufficient phase margin to maintain
stability in unity gain with purely resistive loads. However,
the addition of load capacitance can reduce the phase
margin and destabilize the op amp.

A variety of compensation techniques have been evaluated
specifically for use with the OPA228. The recommended
configuration consists of an additional capacitor (C

) in

parallel with the feedback resistance, as shown in Figures
8 and 11. This feedback capacitor serves two purposes in
compensating the circuit. The op amp's input capacitance
and the feedback resistors interact to cause phase shift that
can result in instability. C

compensates the input capaci-

tance, minimizing peaking. Additionally, at high frequen-
cies, the closed-loop gain of the amplifier is strongly
influenced by the ratio of the input capacitance and the
feedback capacitor. Thus, C

can be selected to yield good

stability while maintaining high speed.

9.09k

97.6k

40.2k

0.47

22nF

402k

634k

Input from

Device

Under

Test

(OPA227)

226k

178k

0.47

10nF

178k

(OPA227)

OUT

100k

OUT

OPA227

22pF

Device

Under

Test

OPA227, 2227, 4227
OPA228, 2228, 4228

Without external compensation, the noise specification of
the OPA228 is the same as that for the OPA227 in gains of
5 or greater. With the additional external compensation, the
output noise of the of the OPA228 will be higher. The
amount of noise increase is directly related to the increase
in high frequency closed-loop gain established by the C

ratio.

Figures 8 and 11 show the recommended circuit for gains
of +2 and 2, respectively. The figures suggest approximate

FIGURE 8. Compensation of the OPA228 for G =+2.

FIGURE 9. Large-Signal Step Response, G = +2, C

LOAD

100pF, Input Signal = 5Vp-p.

FIGURE 10. Small-Signal Step Response, G = +2, C

LOAD

100pF, Input Signal = 50mVp-p.

400ns/div

25mV/div

values for C

. Because compensation is highly dependent

on circuit design, board layout, and load conditions, C

should be optimized experimentally for best results. Fig-
ures 9 and 10 show the large- and small-signal step re-
sponses for the G = +2 configuration with 100pF load
capacitance. Figures 12 and 13 show the large- and small-
signal step responses for the G = 2 configuration with
100pF load capacitance.

200ns/div

25mV/div

FIGURE 11. Compensation for OPA228 for G = 2.

FIGURE 12. Large-Signal Step Response, G = 2, C

LOAD

100pF, Input Signal = 5Vp-p.

400ns/div

25mV/div

200ns/div

25mV/div

FIGURE 13. Small-Signal Step Response, G = 2, C

LOAD

100pF, Input Signal = 50mVp-p.

OPA228

22pF

100pF

15pF

OPA228

100pF

OPA228

Ð­Ð»ÐµÐºÑ‚Ñ€Ð¾Ð½Ð½Ñ‹Ð¹ ÐºÐ¾Ð¼Ð¿Ð¾Ð½ÐµÐ½Ñ‚: OPA228

ÐÐ»ÐµÐºÑ‚Ñ€Ð¾Ð½Ð½Ñ‹Ð¹ ÐºÐ¾Ð¼Ð¿Ð¾Ð½ÐµÐ½Ñ‚: OPA228