Datasheet

AD8014
–8–
Note: On Figures 19 and 20 R
F
= 500 Ω, R
S
= 50 Ω and C
L
=
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 kΩ 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 kΩ, 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
1.74kV
20kV
49.9V
49.9V
+5V
–5V
OUTPUT
(103 PROBE)
(NO LOAD)
0.1mF
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
Figure 19. Large Signal Step Response; V
S
= ± 5 V,
V
O
= 4 V Step
Figure 20. Large Signal Step Response; V
S
= +5 V,
V
O
= 2 V Step
Rev. C
20V/