ADL5303ACPZ-R7 datasheet(16/24 Pages) AD

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ADL5303

Data Sheet

Rev. 0 | Page 16 of 24

USING THE ADAPTIVE BIAS

For most photodiode applications, the placement of the anode

somewhat above ground is acceptable, as long as the positive

bias on the cathode is adequate to support the peak current for

a particular diode, limited mainly by its series resistance. To

address this matter, the ADL5303 provides for a diode bias that

increases linearly with the current. This bias voltage appears at

the VPDB pin, and varies from 0.6 V (reverse-biasing the diode

by 0.1 V) for IPD = 100 pA and rises to 2.6 V (for a diode bias of

2 V) at IPD = 10 mA. This results in a constant internal junction

bias of 0.1 V when the series resistance of the photodiode is

200 Ω. For optical power measurements over a wide dynamic

range, the adaptive biasing function is valuable in minimizing

dark current while preventing the loss of photodiode bias at

high currents. Use of the adaptive bias feature is shown in

Figure 29.

Capacitor CPB, between the photodiode cathode at the VPDB

pin and ground, is included to lower the impedance at this node

and thereby improve the high frequency accuracy at current

levels where the ADL5303 bandwidth is high. CPB also ensures

a high frequency path for any high frequency modulation on

the optical signal, which might not otherwise be accurately

averaged. CPB is not necessary in all cases, and experimentation

may be required to find an optimum value.

VOUT

500mV/DEC

R18 (RB)

C7 (CFILT)

R15 (RA)

15k

Ω

10k

Ω

NC = NO CONNECT

100nF

750

1nF

CPB

R25

IPD

PDB

BIAS

VREF

VPDB

LOCATION

VSUM

INPT

VSUM

ACOM

VPS2

PWDN

VPS1

VREF

VLOG

BFIN

BFNG

VOUT

0.5V

ADL5303

~10k

Ω

GND

TEMPERATURE

COMPENSATION

Figure 29. Using the Adaptive Biasing

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