ad549sh-883b Analog Devices, Inc., ad549sh-883b Datasheet - Page 14

• Manufacturer Part Number: ad549sh-883b
• Description: Ultralow Input Bias Current Operational Amplifier
• Manufacturer: Analog Devices, Inc.

AD549
The test apparatus is calibrated without a device under test
present. After power is turned on, a 5 minute stabilization
period is required. First, V
voltages are the errors caused by the offset voltages and leakage
currents of the current-to-voltage converters.
Once measured, these errors are subtracted from the readings
taken with a device under test present. Amplifier B closes the
feedback loop to the device under testing in addition to pro-
viding the current-to-voltage conversion. The offset error of the
device under testing appears as a common-mode signal and
does not affect the test measurement. As a result, only the
leakage current of the device under testing is measured.
V
V
V
V
V
ERR1
ERR2
A
X
OS
– V
– V
–
+
I (+)
I (–)
Figure 40. Sample and Difference Circuit for Measuring
= 10 (V
= 10 (V
ERR1
ERR2
DEVICE
= 10[RSa × I
= 10[RSb × I
UNDER
TEST
OS
OS
Electrometer Leakage Currents
A – I
B – I
9.01kΩ
B
B
1kΩ
B × RSb)
B
A × RSa)
B
R2
R1
ERR1
B
B
(+)]
B
(–)]
B
and V
10
10
20pF
20pF
RSa
RSb
C
2
3
3
2
C
10
10
C
C
AD549L
AD549L
Ω
Ω
ERR2
A
B
8
8
are measured. These
R1
1kΩ
R1
1kΩ
C
0.1µF
GUARD
9.01kΩ
9.01kΩ
F
0.1µF
0.1µF
R2
R2
C
C
F
F
6
6
VERR1/V
VERR2/V
V
OUT
CAL/TEST
–
+
+
–
A
B
Rev. G | Page 14 of 20
Although a series of devices can be tested after only one
calibration measurement, calibration should be updated
periodically to compensate for any thermal drift of the current-
to-voltage converters or changes in the ambient environment.
Laboratory results have shown that repeatable measurements
within 10 fA can be realized when this apparatus is properly
implemented. These results are achieved in part by the design of
the circuit, which eliminates relays and other parasitic leakage
paths in the high impedance signal lines, and in part by the
inherent cancellation of errors through the calibration and
measurement procedure.
PHOTODIODE INTERFACE
The AD549’s low input current and low input offset voltage
make it an excellent choice for very sensitive photodiode
preamps (see Figure 41). The photodiode develops a signal
current, I
where P is light power incident on the diode surface, in watts,
and R is the photodiode responsivity in amps/watt. R
the signal current to an output voltage
The dc error sources and an equivalent circuit for a small area
(0.2 mm square) photodiode are indicated in Figure 42.
I
S
I
V
S
OUT
= R × P
10
S
, equal to
= R
R
9
Ω
S
Figure 42. Photodiode Preamp DC Error Sources
F
× I
20pF
C
S
S
Figure 41. Photodiode Preamp
2
3
AD549
–V
4
I
S
S
–
1
10
10pF
R
C
9
F
F
–
Ω
5
V
6
OS
10
R
+
9
F
Ω
10kΩ
1µF
A
10pF
C
F
+
–
V
OUT
F
converts
V
OUT
+
–