Datasheet

Data Sheet AD8605/AD8606/AD8608
Rev. N | Page 19 of 24
I-V CONVERSION APPLICATIONS
PHOTODIODE PREAMPLIFIER APPLICATIONS
The low offset voltage and input current of the AD8605 make
it an excellent choice for photodiode applications. In addition,
the low voltage and current noise make the amplifier ideal for
application circuits with high sensitivity.
R
D
I
D
C
D
50p
F
AD8605
V
OUT
PHOTODIODE
V
OS
R
F
10MΩ
C
F
10pF
02731-051
Figure 52. Equivalent Circuit for Photodiode Preamp
The input bias current of the amplifier contributes an error
term that is proportional to the value of R
F
.
The offset voltage causes a dark current induced by the shunt
resistance of the Diode R
D
. These error terms are combined at
the output of the amplifier. The error voltage is written as
BF
D
F
OSO
IR
R
R
VE +








+=
1
Typically, R
F
is smaller than R
D
, thus R
F
/R
D
can be ignored.
At room temperature, the AD8605 has an input bias current of
0.2 pA and an offset voltage of 100 µV. Typical values of R
D
are
in the range of 1 GΩ.
For the circuit shown in Figure 52, the output error voltage is
approximately 100 µV at room temperature, increasing to about
1 mV at 85°C.
The maximum achievable signal bandwidth is
FF
t
MAX
CR
f
f
π 2
=
where f
t
is the unity gain frequency of the amplifier.
AUDIO AND PDA APPLICATIONS
The low distortion and wide dynamic range of the AD860x
make it a great choice for audio and PDA applications,
including microphone amplification and line output buffering.
Figure 53 shows a typical application circuit for headphone/
line-out amplification.
R1 and R2 are used to bias the input voltage at half the supply,
which maximizes the signal bandwidth range. C1 and C2 are
used to ac couple the input signal. C1, R1, and R2 form a high-
pass filter whose corner frequency is 1/[2π(R1||R2)C1].
The high output current of the AD8606 allows it to drive heavy
resistive loads.
The circuit in Figure 53 is tested to drive a 16 Ω headphone. The
THD + N is maintained at approximately −60 dB throughout the
audio range.
5V
4
2
3
8
1
HEADPHONES
5V
4
6
5
8
7
C1
1µF
V1
500mV
1/2
AD8606
C3
100µF
1/2
AD8606
C4
100µF
C2
1µF
V2
500mV
02731-052
R1
20kΩ
R2
20kΩ
R7
20kΩ
R8
20kΩ
R4
20Ω
R3
1kΩ
R6
20Ω
R5
1kΩ
Figure 53. Single-Supply Headphone/Speaker Amplifier