AD8605_15 Datasheet, PDF (19/24 Pages) Analog Devices – Precision, Low Noise, CMOS, Rail-to-Rail

Data Sheet
AD8605/AD8606/AD8608
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.
C_F
10pF
R_F
10MΩ
PHOTODIODE
R_D
I_D
C_D
50pF
V_OS
AD8605
V_OUT
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 RF.
The offset voltage causes a dark current induced by the shunt
resistance of the Diode RD. These error terms are combined at
the output of the amplifier. The error voltage is written as
E_O
= V_OS
^{1 +}
R_F
R_D

 ^{+ RF I B}

Typically, RF is smaller than RD, thus RF/RD 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 RD 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
f _MAX =
f_t
2πR_F C_F
where ft 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
R1
C1 _20kΩ
1µF
R2
V1
20kΩ
500mV
8
3 _1/2
AD8606
2
4
C3
R4
100µF _20Ω
1
R3 HEADPHONES
1kΩ
5V
C2
R7
1µF 20kΩ
V2
R8
500mV ^20kΩ
8
5 _1/2
AD8606
6
4
C4
R6
100µF _20Ω
7
R5
1kΩ
Figure 53. Single-Supply Headphone/Speaker Amplifier
Rev. N | Page 19 of 24