Datasheet AD8014 (Analog Devices) - 9

AD8014
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 Figure 19. Large Signal Step Response; V overshoot as shown in Figure 22. S = ±5 V, VO = 4 V Step
1.74k
V
+5V 0.1
m
F 20k
V
INPUT 49.9 49.9
V
OUTPUT
V
AD8014 (10
3
PROBE) (NO LOAD) –5V
Figure 21. AD8014 as a Photodiode Preamp
TEK RUN: 2.0GS/s ET AVERAGE T[ ] INPUT 1
20V/
DIV
Figure 20. Large Signal Step Response; VS = +5 V, VO = 2 V Step Note: On Figures 19 and 20 RF = 500 Ω, RS = 50 Ω and CL =
OUTPUT 2
20 pF.
500mV/DIV CH1 20.0V CH2 500mV M 25.0ns CH4 380mV APPLICATIONS CD ROM and DVD Photodiode Preamp
Figure 22. Pulse Response 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. –8– Rev. C