AD8065WARTZ-R7 datasheet(25/29 Pages) AD

25 / 29 page

AD8065/AD8066

Rev. J | Page 24 of 28

RSH = 1011Ω

CF +CS

IPHOTO

Figure 58. Wideband Photodiode Preamp

INPUT-TO-OUTPUT COUPLING

To minimize capacitive coupling between the inputs and output,

the output signal traces should not be parallel with the inputs.

WIDEBAND PHOTODIODE PREAMP

Figure 58 shows an I/V converter with an electrical model of a

photodiode. The basic transfer function is

PHOTO

OUT

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 capacitance

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

result in peaking and instability. Adding CF creates a 0 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

()

where fCR is the amplifier crossover frequency, RF is the feedback

resistor, and 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

The frequency response in this case shows about 2 dB of

peaking and 15% overshoot. Doubling CF and cutting the

bandwidth in half results in a flat frequency response with

about 5% transient overshoot.

The preamp’s output noise over frequency is shown in Figure 59.

FREQUENCY (Hz)

πRFCF

πRF (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 59. Photodiode Voltage Noise Contributions

The pole in the loop transmission translates to a 0 in the

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

voltage noise over frequency. The loop transmission 0

introduced by CF limits the amplification. The noise gain

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, which adds to the output noise.

Integrating the square of the output voltage noise spectral

density over frequency and then taking the square root allows

users to obtain the total rms output noise of the preamp. Table 5

summarizes approximations for the amplifier and feedback and

source resistances. Noise components for an example preamp

with RF = 50 kΩ, CS = 15 pF, and CF = 2 pF (bandwidth of about

1.6 MHz) are also listed.

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