AD8033AR datasheet(22/25 Pages) AD

22 / 25 page

AD8033/AD8034

Rev. D | Page 21 of 24

When selecting components, the common-mode input capacitance

must be taken into consideration.

Filter cutoff frequencies can be increased beyond 1 MHz using the

AD8033/AD8034 but limited open-loop gain and input impedance

begin to interfere with the higher Q stages. This can cause early

roll-off of the overall response.

Additionally, the stop-band attenuation decreases with decreasing

open-loop gain.

Keeping these limitations in mind, a 2-pole Sallen-Key Butterworth

filter with fC = 4 MHz can be constructed that has a relatively

low Q of 0.707 while still maintaining 15 dB of attenuation an

octave above fC and 35 dB of stop-band attenuation. The filter

and response are shown in Figure 60 and Figure 61, respectively.

–VS

+VS

VIN

2.49kΩ

22pF

VOUT

AD8033

2.49kΩ

49.9Ω

10pF

Figure 60. 2-Pole Butterworth Active Filter

100M

100k

FREQUENCY (Hz)

–45

–40

–35

–30

–25

–20

–15

–10

–5

10M

Figure 61. 2-Pole Butterworth Active Filter Response

WIDEBAND PHOTODIODE PREAMP

Figure 62 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.

RSH = 1011Ω

IPHOTO

VOUT

CF + CS

Figure 62. Wideband Photodiode Preamp

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 summing junction of the amplifier, including

CS and the amplifier input capacitance. RF and the total capacitance

produce a pole in the loop transmission of the amplifier that

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

in the loop transmission that compensates for the effect of the

pole 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

)

(

where:

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) is

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.

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