a
Low Noise, Low Drift
FET Op Amp
AD645
FEATURES
Improved Replacement for Burr-Brown
OPA-111 and OPA-121 Op Amp
LOW NOISE
2 V p-p max, 0.1 Hz to 10 Hz
10 nV/√Hz max at 10 kHz
11 fA p-p Current Noise 0.1 Hz to 10 Hz
CONNECTION DIAGRAMS
TO-99 (H) Package
8-Pin Plastic Mini-DIP
(N) Package
CASE
OFFSET
NULL
ED
V
O T
PR IF
M DR
I
+IN
PRODUCT DESCRIPTION
– IN
–VS
4
+V
7 +VS
3
2
6
OUTPUT
6 OUTPUT
TOP VIEW
3
5 OFFSET
NULL
+ IN
NC = NO CONNECT
AD645
5
4
OFFSET
NULL
–V
NOTE: CASE IS CONNECTED
TO PIN 8
The AD645 is available in six performance grades. The AD645J
and AD645K are rated over the commercial temperature range
of 0°C to +70°C. The AD645A, AD645B, and the ultraprecision AD645C are rated over the industrial temperature
range of –40°C to +85°C. The AD645S is rated over the military
temperature range of –55°C to +125°C and is available
processed to MIL-STD-883B.
The AD645 is available in an 8-pin plastic mini-DIP, 8-pin
header, or in die form.
B
SO
The AD645 is a low noise, precision FET input op amp. It offers the pico amp level input currents of a FET input device
coupled with offset drift and input voltage noise comparable to a
high performance bipolar input amplifier.
The AD645 has been improved to offer the lowest offset drift in
a FET op amp, 1 µV/°C. Offset voltage drift is measured and
trimmed at wafer level for the lowest cost possible. An inherently low noise architecture and advanced manufacturing techniques result in a device with a guaranteed low input voltage
noise of 2 µV p-p, 0.1 Hz to 10 Hz. This level of dc performance
along with low input currents make the AD645 an excellent
choice for high impedance applications where stability is of
prime concern.
PRODUCT HIGHLIGHTS
1. Guaranteed and tested low frequency noise of 2 µV p-p max
and 20 nV/√Hz at 100 Hz makes the AD645C ideal for low
noise applications where a FET input op amp is needed.
2. Low VOS drift of 1 µV/°C max makes the AD645C an excellent choice for applications requiring ultimate stability.
3. Low input bias current and current noise (11 fA p-p 0.1 Hz to
10 Hz) allow the AD645 to be used as a high precision
preamp for current output sensors such as photodiodes, or as
a buffer for high source impedance voltage output sensors.
30
O
1k
25
NUMBER OF UNITS
VOLTAGE NOISE SPECTRAL DENSITY
nV/ Hz
7
LE
APPLICATIONS
Low Noise Photodiode Preamps
CT Scanners
Precision I-V Converters
2
AD645
8
1
TE
HIGH DC ACCURACY
250 V max Offset Voltage
1 V/؇C max Drift
1.5 pA max Input Bias Current
114 dB Open-Loop Gain
Available in Plastic Mini-DIP, 8-Pin Header Packages, or
Chip Form
–IN
OFFSET
NULL
8 NC
1
100
10
20
15
10
5
0
1.0
1
10
100
FREQUENCY – Hz
1k
10k
Figure 1. AD645 Voltage Noise Spectral Density vs.
Frequency
–2.5
–2.0
–1.5
–1.0
–0.5
0.0
0.5
1.0
1.5
2.0
2.5
INPUT OFFSET VOLTAGE DRIFT– µV/ °C
Figure 2. Typical Distribution of Average Input Offset
Voltage Drift (196 Units)
REV. B
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
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 617/329-4700
Fax: 617/326-8703
