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)R

R

)

ID(R

V

−

=

−

=

where:

ID = photodiode signal current (A).

Rp = photodiode sensitivity (A/W).

Rf = value of the feedback resistor, in Ω.

P = light power incident to photodiode surface, in W.

The amplifier’s input current, IB, contributes an output voltage

error proportional to the value of the feedback resistor. The

offset voltage error, VOS, causes a small current error due to the

photodiode’s finite shunt resistance, RD.

The resulting output voltage error, VE, is equal to

)

(I

R

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 RD 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 IB values of 1 pA and VOS of

50 mV, VE 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.

RD

100M

Ω

C4

15pF

IB

IB

VOS

CF

5pF

RF

1.5M

Ω

OUTPUT

AD8627

PHOTODIODE

Figure 45. A Photodiode Model Showing DC Error