ADL5304ACPZ-R2 datasheet(25/32 Pages) AD

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Data Sheet

ADL5304

Rev. 0 | Page 25 of 32

APPLICATIONS INFORMATION

USING THE ADL5304

The basic connections for single-supply operation are shown in

Figure 55. Supply decoupling is not critical and the suggested values

are conservative; however, it is recommended that a ferrite bead

be placed in the supply lines together with a 0.1 μF decoupling

capacitor. Ferrite beads are preferable to resistors because they do

not produce a dc voltage drop that can affect reference levels. In

Figure 55, the slope is 10 mV/dB or 0.2 V/decade, and the intercept

is 3.162 fA. For the full dynamic range of 200 dB (100 dB optical),

VLOG varies from 0.5 V to 2.5 V (see small diagram at the output

in Figure 55) with VLOG = VOFS = 1.5 V, when INUM = IDEN. Because

the IDEN pin is connected to the IREF pin, IDEN = 100 nA.

Figure 55 also shows the setup for the adaptive photodiode bias.

If this is not desired, ground the IMON pin, remove RMNTR, and

provide the desired bias voltage greater than 1.5 V to the cathode of

the PD. As noted in the Photodiode Bias section, the on-chip 2 V

reference can be used for this purpose and provides an exact 0.5 V

reverse bias together with the 1.5 V that is forced by the FET

amp to the anode via the INUM pin.

Using the Adaptive Bias

The positive bias on the photodiode cathode must be adequate

to support the peak current, which is limited by its internal series

resistance, RS. Typical values of RS are 5 Ω. A model of a repre-

sentative photodiode (JDSU EPM 605) is shown in Figure 54.

CASE

1nH

1.5nH

0.55pF

0.5pF

0.13pF

5nH

Figure 54. Photodiode Model

It is desirable to use a small bias at very low levels of

illumination to minimize the error due to current leakage

across the diode terminals. The adaptive bias achieves this

automatically even for larger currents through the addition

of the external resistor, RMNTR, that is 10 times RS. In case of

uncertainty in RS, an RMNTR that is slightly greater than 10 times

RS is recommended. In the limit, when RMNTR is not present at

all, the voltage at the IMON pin increases until the current

source saturates and absorbs the excess 10% of current that the

IMON output generates. However, this defeats the purpose of

the adaptive bias; therefore, users must ensure that RMNTR is

present when using the adaptive bias.

ADL5304

MONITOR AND

PD BIAS

(1.1× INUM)

TEMPERATURE

COMPENSATION

IMON

VNUM INNM

VDEN

ACOM

COMM

INDN

1P5V

2VLT

DCBI

VPOS

BSDC

VSM1

1.5V

VSM2

1.5V

IREF

IDEN

INUM

VSM3

1.5V

VSM4

1.5V

100nA

1.5V

BIAS AND

VREF

INPS

INMS

VLOG

5kΩ

7.5kΩ

SCL1

SCL2

SCL3

ACOM

RMNTR

SHIELD

0.1µF

HFCP

0.1µF

1µF

4.02Ω

VNUM

VDEN

IMPORTANT: ~1mA

BIAS CURRENT FLOWS OUT

OF DCBI. NEEDS TO BE

CONNECTED TO 1P5V.

VPOS

ILOG

RLOAD

2.5V

1.5V

0.5V

1p 100n 10m

NMFS VNEG DNFS

RNMFS

RDNFS

Figure 55. Basic Connections for Single-Supply Operation

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