Ultralow Input Bias Current

Operational Amplifier

AD549

Rev. H

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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

CONNECTION DIAGRAM

AD549

OFFSET NULL

OUTPUT

V–

OFFSET

NULL

NONINVERTING

INPUT

V+

GUARD PIN,

CONNECTED

TO CASE

INVERTING

INPUT

–15V

10kΩ

6

1

5

4

7

3

2

8

1

5

4

Figure 1.

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

which results from the bootstrapped input stage, ensures that

the input current is essentially independent of the common-

mode 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

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 guaran-

teed 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%.