REV. B
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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 mV 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
HIGH DC ACCURACY
250 mV max Offset Voltage
1 mV/8C 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
APPLICATIONS
Low Noise Photodiode Preamps
CT Scanners
Precision I-V Converters
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 617/329-4700 Fax: 617/326-8703
PRODUCT DESCRIPTION
The AD645 is a low noise, precision FET input op amp. It of-
fers 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 inher-
ently low noise architecture and advanced manufacturing tech-
niques 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.
1k
100
1.0
10
10k1k1 10 100
VOLTAGE NOISE SPECTRAL DENSITY
nV/ Hz
FREQUENCY – Hz
Figure 1. AD645 Voltage Noise Spectral Density vs.
Frequency
CONNECTION DIAGRAMS
TO-99 (H) Package
8-Pin Plastic Mini-DIP
(N) Package
IMPROVEDDRIFT
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 ultra-
precision 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.
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 V
OS
drift of 1 µV/°C max makes the AD645C an excel-
lent 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.
INPUT OFFSET VOLTAGE DRIFT– m V/ C
–0.5–2.0 1.0 1.5 2.5–2.5 –1.5 –1.0 0.5 2.00.0
NUMBER OF UNITS
0
15
5
10
30
25
20
Figure 2. Typical Distribution of Average Input Offset
Voltage Drift (196 Units)
OUTPUT
V–
NOTE: CASE IS CONNECTED
TO PIN 8
OFFSET
NULL
IN+
IN–
CASE
3
4
5
6
7
8
AD645
1
2
OFFSET
NULL
+V
8
7
6
5TOP VIEW
AD645
1
4
2
3
NC = NO CONNECT
OFFSET
NULL
–IN
NC
OUTPUT
OFFSET
NULL
–V
S
+IN
+V
S