ADL5309ACBZ-R7 datasheet(15/25 Pages) AD

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Data Sheet

ADL5309

THEORY OF OPERATION

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Rev. A | 15 of 25

In a reverse biased photodiode, however, the photon-generated IPD

itself is directly proportional to the optical power (POPT) absorbed in

the detector, as shown in the following equation:

IPD=ρ×POPT

(7)

where:

ρ is the responsivity (that is, the conversion gain from the optical

power to the electrical current.

POPT is the absorbed optical power.

The proportionality constant ρ, representing the conversion gain

from optical power to electrical current, is called the responsivity

of the photodiode. Using the same responsivity, the logarithmic

intercept current (IZ) of the TIA can be related to an optical intercept

power level (PZ) for which the ideal log-linear transfer produces an

output voltage equal to zero. The transfer from measured optical

power to amplifier output voltage can therefore be expressed as the

following equation:

VOUT=SLOPE×log10 POPTPZ

(8)

For incident optical power expressed in dB, Equation 8 becomes

Equation 9.

POPT, dB=10×log10POPT

(9)

VOUT=SLOPE10× POPT, dB−PZ, dB

(10)

Thus, the logarithmic slope in mV/dB optical power equals twice the

logarithmic slope in mV/dB of input current IPD (see Equation 3).

Similarly, the optical dynamic range of the TIA in dB equals half the

electrical dynamic range in dB, that is, 90 dB optical vs. 180 dB

electrical.

PHOTODIODE BIAS

The photodiode bias function maximizes the dynamic range of

optical power measurements by minimizing the impact of dark

current and series resistance on the measurement accuracy.

Dark current is a small leakage current through the diode that

does not change proportionally to the incident optical power, and

therefore, limits the sensitivity of an optical power measurement.

Because dark current generally increases with the reverse bias

voltage, a low reverse bias voltage minimizes the dark current and

maximizes the sensitivity of the optical power measurement.

The series resistance introduces measurement errors at high cur-

rent levels through the photodiode. The voltage drop across this

resistance reduces the reverse bias voltage across the photodiode

junction itself. A sufficiently high reverse bias voltage, preferably

proportional to the diode current to maintain constant reverse bias

across the junction, is needed to minimize the impact of the photo-

diode series resistance.

The photodiode bias function of the ADL5309 adjusts the reverse

bias across the photodiode as a function of the current through the

photodiode, as shown in Figure 42. At low photodiode currents, the

reverse bias is kept at a specified low-level offset voltage (VOS) to

minimize the impact of the dark current. As the photodiode current

increases, the reverse bias increases accordingly to minimize the

impact of the series resistance. To use this function, the cathode of

the photodiode should be connected to the PDB pin.

The ADL5309 photodiode bias can be optimized for specific photo-

diodes through the I2C interface. The reverse bias level at low input

currents VOS, the RT (that is, the change in bias voltage for a given

change in bias current), and the ITH (RT takes effect when a current

is greater than the ITH) can all be adjusted through the I2C interface.

For input currents greater than the ITH, ringing could be observed

on the PDB pin due to the positive feedback of the adaptive

photodiode bias via the photodiode capacitance. Typically, the

frequency of the ringing is around 65 MHz and does not propagate

noticeably to the VOUT, because it is attenuated strongly because

its bandwidth is much lower. For photodiodes with capacitance less

than 8 pF, no sustained ringing (oscillation) is observed even at cold

temperatures. It is possible, however, that sustained ringing may

occur for photodiodes with larger capacitance. In that case, it is

recommended to connect a snubber network consisting of a 10 Ω

resistor in series with a 220 pF capacitor between the PDB pin and

ground to reduce ringing. The ringing depends on the capacitance

of the photodiode vs. the reverse voltage of the photodiode, series

resistance, and PCB layout, it is good practice to measure the

ringing on the PDB pin using a pulsed optical source that generates

an input current greater than ITH to prevent excessive ringing. If

needed, the 220 pF capacitor can be scaled up, or the PDBG

Figure 42. Adaptive Photo Diode Bias Principle of Operation

The RT can be enabled or disabled (that is, effectively set to zero)

through the PDBG_FIX flag in SREG_07 (see Table 9). When

disabled (PDBG_FIX = 1), the reverse bias voltage across the

diode does not change with the diode current but remains constant

over the entire input current range. When enabled (PDBG_FIX =

0), the PDBG bit field adjusts the RT value in 15.625 Ω steps. An

expression for RT in terms of the SREG_07 register bit fields is

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