ADA4817-2ACPZ-R2 datasheet(20/25 Pages) AD

20 / 25 page

ADA4817-1/ADA4817-2

Data Sheet

Rev. C | Page 20 of 25

–5

0.1

100

1000

FREQUENCY (MHz)

G = 63V/V

RL = 100Ω

VS = 10V

VOUT = 6V p-p

Figure 52. 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.

The loop transmission zero introduced by CF limits the

amplification. The noise gain bandwidth extends past the pre-

amp signal bandwidth and is eventually rolled off by the decreasing

loop gain of the amplifier. The current equivalent noise from the

inverting terminal is typically negligible for most applications.

The innovative architecture used in the ADA4817-1/ADA4817-2

makes balancing both inputs unnecessary, as opposed to traditional

FET input amplifiers. Therefore, minimizing the impedance

seen from the noninverting terminal to ground at all frequencies is

critical for optimal noise performance.

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 9

summarizes approximations for the amplifier and feedback and

source resistances. Noise components for an example preamp

with RF = 50 kΩ, CS = 30 pF, and CF = 0.5 pF (bandwidth of

about 6.4 MHz) are also listed.

Table 9. RMS Noise Contributions of Photodiode Preamp

Contributor

Expression

RMS Noise with RF = 50 kΩ, CS = 30 pF, CF = 0.5 pF

2 



94 μV

VEN Amp

3 







VEN

777.5 μV

IEN Amp

2 



IEN

0.4 μV

783 μV (total)

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