ADA4817-2ACPZ-R2 datasheet(19/25 Pages) AD

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Data Sheet

ADA4817-1/ADA4817-2

Rev. C | Page 19 of 25

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 dedicated

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 photo-

diodes. Figure 50 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 50. 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 the

photodiode capacitance (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

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 CF value that produces f(45) is 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 51.

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 51. Photodiode Voltage Noise Contributions

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