Data Sheet AD8625/AD8626/AD8627
Rev. F | Page 15 of 20
MINIMIZING INPUT CURRENT
The AD862x is guaranteed to 1 pA maximum input current
with a ±13 V supply voltage at room temperature. Careful
attention to how the amplifier is used maintains or possibly
betters this performance. The amplifier’s operating temperature
should be kept as low as possible. Like other JFET input ampli-
fiers, the AD862x’s input current doubles for every 10°C rise in
junction temperature, as illustrated in Figure 8. On-chip power
dissipation raises the device operating temperature, causing an
increase in input current. Reducing supply voltage to cut power
dissipation reduces the AD862x’s input current. Heavy output
loads can also increase chip temperature; maintaining a
minimum load resistance of 1 kΩ is recommended.
The AD862x is designed for mounting on PC boards. Main-
taining picoampere resolution in those environments requires
a lot of care. Both the board and the amplifier’s package have
finite resistance. Voltage differences between the input pins and
other pins, as well as PC board metal traces may cause parasitic
currents larger than the AD862x’s input current, unless special
precautions are taken. To ensure the best result, refer to the ADI
website for proper board layout seminar materials. Two
common methods of minimizing parasitic leakages that should
be used are guarding of the input lines and maintaining
adequate insulation resistance.
Contaminants, such as solder flux on the board’s surface and
the amplifier’s package, can greatly reduce the insulation
resistance between the input pin and traces with supply or
signal voltages. Both the package and the board must be kept
clean and dry.
PHOTODIODE PREAMPLIFIER APPLICATION
The low input current and offset voltage levels of the AD862x,
together with its low voltage noise, make this amplifier an
excellent choice for preamplifiers used in sensitive photodiode
applications. In a typical photovoltaic preamp circuit, shown in
Figure 45, the output of the amplifier is equal to
f
p
f
OUT
(P)RR)ID(RV −=−=
where:
ID = photodiode signal current (A).
R
p
= photodiode sensitivity (A/W).
R
f
= value of the feedback resistor, in Ω.
P = light power incident to photodiode surface, in W.
The amplifier’s input current, I
B
, contributes an output voltage
error proportional to the value of the feedback resistor. The
offset voltage error, V
OS
, causes a small current error due to the
photodiode’s finite shunt resistance, R
D
.
The resulting output voltage error, V
E
, is equal to
)(IR
V
R
R
V
B
f
OS
D
f
E
+
+
=
1
A shunt resistance on the order of 100 MΩ is typical for a small
photodiode. Resistance R
D
is a junction resistance that typically
drops by a factor of two for every 10°C rise in temperature. In
the AD862x, both the offset voltage and drift are low, which
helps minimize these errors. With I
B
values of 1 pA and V
OS
of
50 mV, V
E
for Figure 45 is very negligible. Also, the circuit in
Figure 45 results in an SNR value of 95 dB for a signal bandwidth
of 30 kHz.
03023-044
R
D
100MΩ
C4
15pF
I
B
I
B
V
OS
C
F
5pF
R
F
1.5MΩ
OUTPUT
AD8627
PHOTODIODE
Figure 45. A Photodiode Model Showing DC Error
