ADA4817-1ARDZ-R7 datasheet(24/29 Pages) AD

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Data Sheet

ADA4817-1/ADA4817-2

APPLICATIONS INFORMATION

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Rev. H | 24 of 29

LOW DISTORTION PINOUT

The ADA4817-1/ADA4817-2 feature a 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 sep-

aration 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 pinout 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 63 shows a current to voltage converter with an electrical

model of a photodiode. The basic transfer function is

VOUT=IPHOTO×RF

1+sCFRF

(12)

where:

IPHOTO is the output current of the photodiode.

RF and CF are the parallel combination that sets the signal band-

width.

Figure 63. 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 transmis-

sion 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

f45 = fCR

2π×RF× CS+CM+CD

(13)

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

CF= CS+CM+CD

2π×RF×fCR

(14)

The frequency response shows less peaking if larger CF values are

used.

Figure 64 shows the preamplifier output noise over frequency.

Figure 64. Photodiode Voltage Noise Contributions

Figure 65. Photodiode Preamp Frequency Response

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

gain of the amplifier, leading to an amplification of the input voltage

noise over frequency.

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