AD8033 datasheet(18/20 Pages) AD

18 / 20 page

REV. B

–18–

AD8033/AD8034

Wideband Photodiode Preamp

Figure 12 shows an I/V converter with an electrical model of a

photodiode.

The basic transfer function is

sC R

OUT

PHOTO

where IPHOTO is the output current of the photodiode, and the

parallel combination of RF and CF sets the signal bandwidth.

The stable bandwidth attainable with this preamp is a function

of RF, the gain bandwidth product of the amplifier, and the total

capacitance at the amplifier’s summing junction, including CS

and the amplifier input capacitance. RF and the total capaci-

tance produce a pole in the amplifier’s loop transmission that can

result in peaking and instability. Adding CF creates a zero in the

loop transmission that compensates for the pole’s effect and

reduces the signal bandwidth. It can be shown that the signal

bandwidth resulting in a 45

° phase margin (f(45)) is defined by the

expression

()

××

fCR is the amplifier crossover frequency.

RF is the feedback resistor.

CS is the total capacitance at the amplifier summing junction

(amplifier + photodiode + board parasitics).

The value of CF that produces f(45) can be shown to be

FCR

××

The frequency response in this case will show about 2 dB of

peaking and 15% overshoot. Doubling CF and cutting the

bandwidth in half will result in a flat frequency response, with

about 5% transient overshoot.

The preamp’s output noise over frequency is shown in Figure 13.

The pole in the loop transmission translates to a zero in the

amplifier’s noise gain, leading to an amplification of the input

voltage noise over frequency. The loop transmission zero

introduced by CF limits the amplification. The noise gain’s

bandwidth extends past the preamp signal bandwidth and

is eventually rolled off by the decreasing loop gain of the

amplifier. Keeping the input terminal impedances matched is

recommended to eliminate common-mode noise peaking

effects that will add to the output noise.

Integrating the square of the output voltage noise spectral density

over frequency and then taking the square root results in the

total rms output noise of the preamp.

RSH = 10

IPHOTO

CF + CS

Figure 12. Wideband Photodiode Preamp

FREQUENCY – Hz

2 RFCF

2 RF (CF + CS + CM + 2CD)

(CS + CM + 2CD + CF) /CF

RF NOISE

VEN (CF + CS + CM + 2CD) /CF

f3 =

VEN

f2 =

f1 =

fCR

NOISE DUE TO AMPLIFIER

Figure 13. Photodiode Voltage Noise Contributions

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