AD8605/AD8606/AD8608
I-V CONVERSION APPLICATIONS
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 Gꢀ.
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.
For the circuit shown in Figure 51, the output error voltage is
approximately 100 μV at room temperature, increasing to about
1 mV at 85°C.
C
F
10pF
The maximum achievable signal bandwidth is
R
F
ft
10MΩ
f MAX
=
2πRF CF
PHOTODIODE
V
OS
where ft is the unity gain frequency of the amplifier.
C
50pF
D
AD8605
R
I
AUDIO AND PDA APPLICATIONS
D
D
V
OUT
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 51. Equivalent Circuit for Photodiode Preamp
Figure 52 shows a typical application circuit for headphone/
line-out amplification.
The input bias current of the amplifier contributes an error
term that is proportional to the value of RF.
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 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
The high output current of the AD8606 allows it to drive heavy
resistive loads.
⎛
⎜
⎝
⎞
⎟
⎟
⎠
RF
RD
⎜
EO =VOS 1+
+ RF IB
The circuit in Figure 52 is tested to drive a 16 ꢀ headphone. The
THD + N is maintained at approximately −60 dB throughout the
audio range.
Typically, RF is smaller than RD, thus RF/RD can be ignored.
5V
R1
20kΩ
C1
1µF
8
C3
R4
100µF
20Ω
R2
20kΩ
3
2
1/2
V1
500mV
AD8606
1
R3 HEADPHONES
1kΩ
4
5V
R7
20kΩ
C2
1µF
8
C4
R6
100µF
5
6
20Ω
1/2
R8
20kΩ
V2
500mV
AD8606
7
R5
1kΩ
4
Figure 52. Single-Supply Headphone/Speaker Amplifier
Rev. H | Page 18 of 24
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