AD8014

–8–

Ω, R

Ω and C

20 pF.

APPLICATIONS

CD ROM and DVD Photodiode Preamp

High speed Multi-X CD ROM and DVD drives require high

frequency photodiode preamps for their read channels. To mini-

mize the effects of the photodiode capacitance, the low imped-

ance of the inverting input of a current feedback amplifier is

advantageous. Good group delay characteristics will preserve the

pulse response of these pulses. The AD8014, having many ad-

vantages, can make an excellent low cost, low noise, low power,

and high bandwidth photodiode preamp for these applications.

Figure 21 shows the circuit that was used to imitate a photo-

diode preamp. A photodiode for this application is basically a

high impedance current source that is shunted by a small ca-

pacitance. In this case, a high voltage pulse from a Picosecond

Pulse Labs Generator that is ac-coupled through a 20 k

Ω resis-

tor is used to simulate the high impedance current source of a

photodiode. This circuit will convert the input voltage pulse into

a small charge package that is converted back to a voltage by the

AD8014 and the feedback resistor.

In this case the feedback resistor chosen was 1.74 k

Ω, which is a

compromise between maintaining bandwidth and providing

sufficient gain in the preamp stage. The circuit preserves the

pulse shape very well with very fast rise time and a minimum of

overshoot as shown in Figure 22.

AD8014

1.74k

20k

49.9

49.9

+5V

–5V

OUTPUT

(10 PROBE)

(NO LOAD)

0.1 F

INPUT

Figure 21. AD8014 as a Photodiode Preamp

INPUT

DIV

1

2

OUTPUT

500mV/DIV

CH1 20.0V

CH2 500mV M 25.0ns CH4 380mV

TEK RUN: 2.0GS/s ET AVERAGE

T[

]

Figure 22. Pulse Response

Rev. C

20V/