Datasheet

AD549
Rev. H | Page 14 of 20
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
ERR1
and V
ERR2
are measured. These
voltages are the errors caused by the offset voltages and leakage
currents of the I-to-V converters.
V
ERR1
= 10 (V
OS
A – I
B
A × RSa)
V
ERR2
= 10 (V
OS
B – I
B
B × RSb)
00511-040
V
OS
+
–
2
3
6
8
A
AD549
R2
9.01kΩ
R1
1kΩ
RSa
10
10
Ω
+
V
ERR1
/V
A
V
ERR2
/V
B
V
OUT
–
CAL/TEST
C
C
20pF
C
F
0.1µF
+
–
3
2
6
8
B
AD549
R2
9.01kΩ
R1
1kΩ
RSb
10
10
Ω
C
C
20pF
C
F
0.1µF
C
F
0.1µF
R1
1kΩ
R2
9.01kΩ
DEVICE
UNDER
TEST
I (+)
I (–)
GUARD
Figure 40. Sample and Difference Circuit for Measuring
Electrometer Leakage Currents
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 I-to-V 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
A
– V
ERR1
= 10[RSa × I
B
(+)]
V
X
– V
ERR2
= 10[RSb × I
B
(–)]
Although a series of devices can be tested after only one calibra-
tion measurement, calibration should be updated periodically
to compensate for any thermal drift of the I-to-V 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 low input current and low input offset voltage of the AD549
make it an excellent choice for very sensitive photodiode preamps
(see Figure 41). The photodiode develops a signal current, I
S
,
equal to
I
S
= R × P
where P is light power incident on the diode surface, in watts,
and R is the photodiode responsivity in amps/watt. R
F
converts
the signal current to an output voltage
V
OUT
= R
F
× I
S
00511-041
2
3
6
5
1
4
AD549
–V
S
C
F
10pF
I
S
R
F
10
9
Ω
10kΩ
1µF
V
OU
T
+
–
Figure 41. Photodiode Preamp
The dc error sources and an equivalent circuit for a small area
(0.2 mm square) photodiode are indicated in Figure 42.
00511-042
A
+
V
OUT
–
V
OS
+–
I
S
–I
S
R
S
10
9
Ω
C
S
20pF
R
F
10
9
Ω
C
F
10pF
Figure 42. Photodiode Preamp DC Error Sources