AD8605ARTZ datasheet(19/24 Pages) AD

19 / 24 page

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.

50pF

AD8605

VOUT

PHOTODIODE

VOS

10MΩ

10pF

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













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

MAX

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.

HEADPHONES

1µF

500mV

1/2

AD8606

100µF

1/2

AD8606

100µF

1µF

500mV

20kΩ

20Ω

1kΩ

20Ω

1kΩ

Figure 53. Single-Supply Headphone/Speaker Amplifier

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