AD8605/AD8606/AD8608

Rev. D | Page 16 of 20

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.

50pF

AD8605

PHOTODIODE

+VOS–

10M

Ω

10pF

Figure 51. Equivalent Circuit for Photodiode Preamp

The input bias current of the amplifier contributes an error

The offset voltage causes a dark current induced by the shunt

the output of the amplifier. The error voltage is written as

B

F

D

F

OS

O

I

R

R

R

V

E

+

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

+

=

1

At room temperature, the AD8605 has an input bias current of

in the range of 1 GΩ.

For the circuit shown in Figure 9, the output error voltage is

approximately 100 µV at room temperature, increasing to about

1 mV at 85°C.

maximum achievable signal bandwidth is

T

F

t

MAX

C

R

f

f

π

=

2

AUDIO AND PDA APPLICATIONS

The AD8605’s low distortion and wide dynamic range make it a

great choice for audio and PDA applications, including

microphone amplification and line output buffering.

Figure 52 shows a typical application circuit for headphone/line

out amplification.

R1 and R2 are used to bias the input voltage at half the supply.

This maximizes the signal bandwidth range. C1 and C2 are used

to ac couple the input signal. C1 and R2 form a high-pass filter

whose corner frequency is 1/2πR1C1.

The high output current of the AD8605 allows it to drive heavy

resistive loads.

The circuit of Figure 52 was tested to drive a 16 W 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

1

µF

R1

10k

Ω

R2

10k

Ω

V1

500mV

1/2

AD8606

C3

100

µF

R4

20

Ω

R3

1k

Ω

1/2

AD8606

C4

100

µF

R6

20

Ω

R5

1k

Ω

C2

1

µF

V2

500mV

Figure 52. Single-Supply Headphone/Speaker Amplifier