ADA4817-2ACPZ-R7 datasheet(25/29 Pages) AD

25 / 29 page

Data Sheet

ADA4817-1/ADA4817-2

APPLICATIONS INFORMATION

analog.com

Rev. H | 25 of 29

The loop transmission zero introduced by CF limits the amplifica-

tion. 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 termi-

nal is typically negligible for most applications. The innovative archi-

tecture 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 the user

to obtain the total rms output noise of the preamp. Table 9 summa-

rizes 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. VEN is the equivalent voltage noise and IEN is the

equivalent current noise.

Table 9. RMS Noise Contributions of Photodiode Preamp

Contributor

Expression

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

4kT×RF×f2×1.57

94 µV

VEN Amp

VEN× CS+CM+CD+CF

CF × f3 × 1.57 777.5µV

IEN Amp

IEN×RF× f2×1.57

0.4 µV

Total

783 µV

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