ANALOG DEVICES AD549 Service Manual

Ultralow Input Bias Current

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FEATURES

Ultralow input bias current

60 fA maximum (AD549L)
250 fA maximum (AD549J)

Input bias current guaranteed over the common-mode

voltage range

Low offset voltage

0.25 mV maximum (AD549K)

1.00 mV maximum (AD549J)

Low offset drift

5 μV/°C maximum (AD549K)
20 μV/°C maximum (AD549J)

Low power

700 μA maximum supply current

Low input voltage noise
4 μV p-p over 0.1 Hz to 10 Hz
MIL-STD-883B parts available

APPLICATIONS

Electrometer amplifier
Photodiode preamp
pH electrode buffer
Vacuum ion gauge measurement

Operational Amplifier

AD549

CONNECTION DIAGRAM

GUARD PIN,

CONNECTED

TO CASE

OFFSET NULL

INVERTING

INPUT

NONINVERTING

INPUT

1

2

3

1

Figure 1.

8

AD549

4

V–

10kΩ

VOS TRIM

V+

7

5

OFFSET
NULL

5

4

6

OUTPUT

–15V

00511-001

GENERAL DESCRIPTION

The AD5491 is a monolithic electrometer operational amplifier
with very low input bias current. Input offset voltage and input
offset voltage drift are laser trimmed for precision performance.
The ultralow input current of the part is achieved with Topgate™
JFET technology, a process development exclusive to Analog
Devices, Inc. This technology allows fabrication of extremely
low input current JFETs compatible with a standard junction
isolated bipolar process. The 10
which results from the bootstrapped input stage, ensures that
the input current is essentially independent of the common­mode voltage.

The AD549 is suited for applications requiring very low input
current and low input offset voltage. It excels as a preamp for a
wide variety of current output transducers, such as photodiodes,
photomultiplier tubes, or oxygen sensors. The AD549 can also
be used as a precision integrator or low droop sample-and-hold.
The AD549 is pin compatible with standard FET and electrometer
op amps, allowing designers to upgrade the performance of
present systems at little additional cost.

The AD549 is available in a TO-99 hermetic package. The case
is connected to Pin 8, thus, the metal case can be independently

1

Protected by U.S. Patent No. 4,639,683.

Rev. H

Information furnished by Analog Devices is believed to be accurate and reliable. However, no
responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other
rights of third parties that may result from its use. Specifications subject to change without notice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
Trademarks and registered trademarks are the property of their respective owners.

15

Ω common-mode impedance,

connected to a point at the same potential as the input terminals,
minimizing stray leakage to the case. The AD549 is available in
four performance grades. The J, K, and L versions are rated over
the commercial temperature range of 0°C to +70°C. The S grade
is specified over the military temperature range of −55°C to +125°C
and is available processed to MIL-STD-883B, Rev. C. Extended
reliability plus screening is also available. Plus screening includes
168 hour burn-in, as well as other environmental and physical
tests derived from MIL-STD-883B, Rev. C.

PRODUCT HIGHLIGHTS

1. The AD549 input currents are specified, 100% tested, and

guaranteed after the device is warmed up. They are guaran­teed over the entire common-mode input voltage range.

2. The AD549 input offset voltage and drift are laser trimmed

to 0.25 mV and 5 μV/°C (AD549K), and to 1 mV and
20 μV/°C (AD549J).

3. A maximum quiescent supply current of 700 μA minimizes

heating effects on input current and offset voltage.

4. AC specifications include 1 MHz unity-gain bandwidth

and 3 V/μs slew rate. Settling time for a 10 V input step is
5 μs to 0.01%.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: [phone] www.analog.com
Fax: [phone] ©2002–2008 Analog Devices, Inc. All rights reserved.

AD549

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TABLE OF CONTENTS

Features .............................................................................................. 1

Applications ....................................................................................... 1 

Connection Diagram ....................................................................... 1

General Description ......................................................................... 1

Product Highlights ........................................................................... 1

Revision History ............................................................................... 2

Specifications ..................................................................................... 3

Absolute Maximum Ratings ............................................................ 5

ESD Caution .................................................................................. 5

Typical Performance Characteristics ............................................. 6

Functional Description .................................................................. 10

Minimizing Input Current ........................................................ 10

Circuit Board Notes ................................................................... 10

REVISION HISTORY

3/08—Rev. G to Rev. H

Changes to Features .......................................................................... 1

Changes to Figure 1 .......................................................................... 1

Deleted Package Option Parameter ............................................... 4

Inserted ESD Caution ...................................................................... 5

Changes to Figure 2, Figure 3, and Figure 7.................................. 6

Changes to Figure 11 ........................................................................ 7

Changes to Figure 17 ........................................................................ 8

Changes to Figure 41 ...................................................................... 14

7/07—Rev. F to Rev. G

Changes to Figure 45 ...................................................................... 16

Changes to Temperature Compensated pH Probe

Amplifier Section ............................................................................ 17

Changes to Figure 46 ...................................................................... 17

Changes to Ordering Guide .......................................................... 18

Offset Nulling ............................................................................. 11

AC Response with High Value Source and Feedback

Resistance .................................................................................... 12

Common-Mode Input Voltage Overload ............................... 12

Differential Input Voltage Overload ........................................ 13

Input Protection ......................................................................... 13

Sample and Difference Circuit to Measure Electrometer

Leakage Currents ........................................................................ 13

Photodiode Interface ................................................................. 14

Log Ratio Amplifier ................................................................... 15

Temperature Compensated pH Probe Amplifier ................... 16

Outline Dimensions ....................................................................... 18

Ordering Guide .......................................................................... 18

5/06—Rev. E to Rev. F

Removed ESD Caution ..................................................................... 5

8/05—Rev. D to Rev. E

Change to Figure 22 .......................................................................... 9

5/04—Rev. C to Rev. D

Updated Format .................................................................. Universal

Changes to Features .......................................................................... 1

Updated Outline Dimensions ....................................................... 18

Added Ordering Guide .................................................................. 18

10/02—Rev. B to Rev. C

Deleted Product Highlights #5 ........................................................ 1

Edits to Specifications ....................................................................... 3

Deleted Metallization Photograph .................................................. 3

Updated Outline Dimensions ....................................................... 13





7/02—Rev. A to Rev. B

Edits to Specifications ....................................................................... 2

Rev. H | Page 2 of 20

AD549

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SPECIFICATIONS

@ 25°C and VS = ±15 V dc, unless otherwise noted; all minimum and maximum specifications are guaranteed; specifications in boldface
are tested on all production units at final electrical test, and results from those tests are used to calculate outgoing quality levels.

Table 1.

AD549J AD549K AD549L AD549S
Parameter Min Typ Max Min Typ Max Min Typ Max Min Typ Max Unit

INPUT BIAS CURRENT

Either Input, VCM = 0 V 150

Either Input, VCM = ±10 V 150 250 75 100 40 60 75 100 fA

Either Input at T

= 0 V

V

CM

Offset Current 50 30 20 30 fA

Offset Current at T

INPUT OFFSET VOLTAGE

Initial Offset 0.5

Offset at T

MAX

vs. Temperature 10

vs. Supply 32

vs. Supply, T

Long-Term Offset Stability 15 15 15 15 μV/month

INPUT VOLTAGE NOISE

f = 0.1 Hz to 10 Hz 4 4

f = 10 Hz 90 90 90 90 nV/√Hz

f = 100 Hz 60 60 60 60 nV/√Hz

f = 1 kHz 35 35 35 35 nV/√Hz

f = 10 kHz 35 35 35 35 nV/√Hz

INPUT CURRENT NOISE

f = 0.1 Hz to 10 Hz 0.7 0.5 0.36 0.5 fA rms

f = 1 kHz 0.22 0.16 0.11 0.16 fA/√Hz

INPUT IMPEDANCE

Differential

V

= ±1 1013||1 1013||1 1013||1 1013||1 Ω||pF

DIFF

Common Mode

VCM = ±10 V 1015||0.8 1015||0.8 1015||0.8 1015||0.8 Ω||pF

OPEN-LOOP GAIN

V

@ ±10 V, RL = 10 kΩ

OUT

V

@ ±10 V, RL = 10 kΩ,

OUT

to T

T

MIN

V

= ±10 V, RL = 2 kΩ

OUT

V

= ±10 V, RL = 2 kΩ,

OUT

to T

T

MIN

INPUT VOLTAGE RANGE

Differential

Common-Mode Voltage

Common-Mode Rejection

Ratio

−10 V ≤ VCM ≤ +10 V
T

to T

MIN

1

75

250

,

MAX

MAX

to T

MIN

MAX

MAX

MAX

3

±20 ±20 ±20 ±20 V

11 4.2 2.8 420 pA

2.2 1.3 0.85 125 pA

2

0.15

1.0

1.9

2

20

10

100

32

1000

300

800

300

250

100

200

80

−10

10

100

300
300

100
80

+10 −10

1000
800

250
200

40

100

0.3

0.25

0.4

5

5

10

32

10

32

4 4 μV p-p

6

1000

300

800

300

250

100

200

80

+10 −10

75

60

0.3

0.5

0.9

10

10

10

32

32

32

300
300

100
25

+10 −10

100

0.5

2.0
15
32
50

1000 V/mV
800 V/mV

250 V/mV
150 V/mV

+10

MAX

90

80

80

76

100

90

90

80

100

90

90

80

100 dB

90

90 dB

80

fA

mV
mV
μV/°C
μV/V
μV/V

V

Rev. H | Page 3 of 20

AD549

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AD549J AD549K AD549L AD549S
Parameter Min Typ Max Min Typ Max Min Typ Max Min Typ Max Unit

OUTPUT CHARACTERISTICS

V

@ RL = 10 kΩ, T

OUT

T

MAX

V

@ RL = 2 kΩ, T

OUT

Short-Circuit Current

T

to T

MIN

MAX

Load Capacitance Stability,

G = +1

FREQUENCY RESPONSE

Unity Gain, Small Signal 0.7 1.0 0.7 1.0 0.7 1.0 0.7 1.0 MHz
Full Power Response 50 50 50 50 kHz
Slew Rate 2 3 2 3 2 3 2 3 V/μs
Settling Time, 0.1% 4.5 4.5 4.5 4.5 μs
Settling Time, 0.01% 5 5 5 5 μs
Overload Recovery, 50%

Overdrive, G = −1

POWER SUPPLY

Rated Performance ±15 ±15 ±15 ±15 V
Operating
Quiescent Current 0.60

TEMPERATURE RANGE

Operating, Rated

Performance

Storage −65 +150 −65 +150 −65 +150 −65 +150 °C

1

Bias current specifications are guaranteed after five minutes of operation at TA = 25°C. Bias current increases by a factor of 2.3 for every 10°C rise in temperature.

2

Input offset voltage specifications are guaranteed after five minutes of operation at TA = 25°C.

3

Defined as maximum continuous voltage between the inputs, such that neither input exceeds ±10 V from ground.

to

MIN

to T

MAX

MIN

−12

−10

20

15

9 9 6 mA

9

4000 4000 4000 4000 pF

2 2 2 2 μs

±5

0 70 0 70 0 70 −55 +125 °C

+12 −12

+10 −10
35 15

±18 ±5

0.70

20

0.60

+12 −12

+10 −10
35 15

±18 ±5

0.60

0.70

20

+12 −12

+10 −10
35 15

±18 ±5

0.60

0.70

20

+12

+10
35

±18

0.70

V

V
mA

V
mA

Rev. H | Page 4 of 20

AD549

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ABSOLUTE MAXIMUM RATINGS

Table 2.

Parameter Rating

Supply Voltage ±18 V
Internal Power Dissipation 500 mW
Input Voltage
Output Short-Circuit Duration Indefinite
Differential Input Voltage +VS and −VS
Storage Temperature Range −65°C to +125°C
Operating Temperature Range

AD549J, AD549K, AD549L 0°C to +70°C

AD549S −55°C to +125°C

Lead Temperature (Soldering, 60 sec) 300°C

1

For supply voltages less than ±18 V, the absolute maximum input voltage is

equal to the supply voltage.

1

±18 V

Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.

ESD CAUTION

Rev. H | Page 5 of 20

AD549

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TYPICAL PERFORMANCE CHARACTERISTICS

20

800

15

V

IN+

10

V

INPUT VOLTAGE (V)

5

0

0 5 10 15 20

SUPPLY VOLTAGE (±V)

IN–

Figure 2. Input Voltage Range vs. Supply Voltage

20

25°C
R

= 10kΩ

L

15

10

5

OUTPUT VO LTAGE SW ING (V)

0

0 5 10 15 20

SUPPLY VOLTAGE (±V)

+V

Figure 3. Output Voltage Swing vs. Supply Voltage

OUT

–V

OUT

700

600

500

AMPLIFI ER QUIESCENT CURRENT (µA)

00511-002

400

0 5 10 15 20

SUPPLY VOLTAGE (±V)

00511-005

Figure 5. Quiescent Current vs. Supply Voltage

120

110

100

90

80

COMMON-MODE REJECTION RATIO (dB)

00511-003

70

–20 –10 0 10 20

INPUT COMMON-MODE VOLTAGE (V)

00511-006

Figure 6. CMRR vs. Input Common-Mode Voltage

30

25

20

15

10

5

OUTPUT VOLTAGE SWING (V p-p)

0

10 100 1k 10k 100k

LOAD RESIST ANCE (Ω)

VS = ±15V

Figure 4. Output Voltage Swing vs. Load Resistance

00511-004

3000

1000

300

OPEN-LOOP GAIN (V/mV)

100

0 5 10 15 20

SUPPLY VOLTAGE (±V)

Figure 7. Open-Loop Gain vs. Supply Voltage

Rev. H | Page 6 of 20

00511-007