ADA4817-1ACPZ-R7 datasheet(19/24 Pages) AD

19 / 24 page

ADA4817-1/ADA4817-2

Rev. 0 | Page 19 of 24

–5

0.1

100

1000

FREQUENCY (MHz)

G = 63V/V

RL = 100Ω

VS = 10V

VOUT = 6V p-p

Figure 46. Photodiode Preamp Frequency Response

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

amplifier’s noise gain, 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 preamp 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 8

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 8. 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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