ADA4817-2ACPZ-R2 datasheet(18/24 Pages) AD

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ADA4817-1/ADA4817-2

Rev. 0 | Page 18 of 24

APPLICATIONS INFORMATION

LOW DISTORTION PINOUT

The ADA4817-1/ADA4817-2 feature a new low distortion

pinout from Analog Devices. The new pinout provides two

advantages over the traditional pinout. The first advantage is

improved second harmonic distortion performance, which is

accomplished by the physical separation of the noninverting

input pin and the negative power supply pin. The second

advantage is the simplification of the layout due to the dedi-

cated feedback pin and easy routing of the gain set resistor

back to the inverting input pin. This allows a compact layout,

which helps to minimize parasitics and increase stability.

The designer does not need to use the dedicated feedback pin to

provide feedback for the ADA4817-1/ADA4817-2. The output

pin of the ADA4817-1/ADA4817-2 can still be used to provide

feedback to the inverting input of the ADA4817-1/ADA4817-2.

WIDEBAND PHOTODIODE PREAMP

The wide bandwidth and low noise of the ADA4817-1/

ADA4817-2 make it an ideal choice for transimpedance

amplifiers, such as those used for signal conditioning with

high speed photodiodes. Figure 44 shows an I/V converter

with an electrical model of a photodiode. The basic transfer

function is

PHOTO

OUT

(13)

where:

IPHOTO is the output current of the photodiode.

The parallel combination of RF and CF sets the signal bandwidth.

RSH = 1011Ω

IPHOTO

VOUT

Figure 44. 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 amplifier’s summing junction, including the

photodiode capacitance (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 zero in the loop transmission that compen-

sates for the pole’s effect and reduces the signal bandwidth. It

can be shown that the signal bandwidth obtained with a 45°

phase margin (f(45)) is defined by

)

(

)

(

(14)

where:

fCR is the amplifier crossover frequency.

RF is the feedback resistor.

CS is the source capacitance including the photodiode and the

board parasitic.

CM is the common-mode capacitance of the amplifier.

CD is the differential capacitance of the amplifier.

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

(15)

The frequency response shows less peaking if bigger CF values

are used.

The preamplifier output noise over frequency is shown in

Figure 45.

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

FREQUENCY (Hz)

NOISE DUE TO AMPLIFIER

VEN

RF NOISE

f1 =

2 RF (CF + CS + CM + CD)

f2 =

2 RFCF

f3 =

fCR

(CF + CS + CM + CD)/CF

Figure 45. Photodiode Voltage Noise Contributions

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