ADA4530-1ARZ-R7 datasheet(46/51 Pages) AD

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Data Sheet

ADA4530-1

Rev. A | Page 45 of 50

GAIN (dB)

FREQUENCY (Hz)

CLOSED-LOOP BANDWIDTH

SIGNAL BANDWIDTH

IF CF =0fF

NG2

NG1

NOISE GAIN

OPEN-LOOP GAIN

fUGC

Figure 126. Transimpedance Noise Gain vs. Frequency

For completeness, the noise gain equations are as follows:































)

(





SHUNT





For simplicity, bandwidth limitations are ignored in the noise

gain equations. The noise gain starts to roll-off when it

intersects with the open-loop gain of the amplifier. This pole

frequency (f3) is determined by the unity gain crossover

frequency (fUGC) of the amplifier and the high frequency noise

gain, NG2, as follows:

















SHUNT

UGC

(3)

The addition of CF has an impact on the signal frequency

response. At low frequencies, the transimpedance gain is equal

to RF. As the frequency increases, the impedance of CF drops

below RF and starts to reduce this transimpedance gain. This

signal gain equation is as follows:

















)

(

Gain

Signal



NOISE ANALYSIS

Photodiode TIA circuits have four noise sources that must be

considered:



The thermal noise of the feedback resistor (RF)



The saturation current noise of the photodiode



The current noise of the amplifier



The voltage noise of the amplifier

The noise contributions of these sources are typically referred to

output for analysis. The thermal noise of RF appears directly at

the output. This noise is filtered by the feedback capacitance so

that its −3 dB bandwidth is the same as the signal bandwidth (f2).

The photocurrent of a photodiode, IPD, produces shot noise equal to

INPD = √(2qIPD)

It is a mistake to assume that the noise goes to zero as the diode

current goes to zero. Zero net current out of the diode simply

means that the saturation current flowing in one direction is at

thermal equilibrium with the saturation current flowing in the

opposite direction. These currents are uncorrelated and add in a

root sum square fashion. This net current noise is equivalent to

the thermal noise of a physical resistor with a value of RSHUNT.

This convenient fact allows the photodiode to be accurately

modeled with a simple resistor, RSHUNT. The thermal noise of

RSHUNT is amplified by the ratio of the feedback resistance to the

shunt resistance. This noise is also filtered to the signal bandwidth.

The current noise of the amplifier flows through the feedback

resistor to become a noise voltage at the output. It is subject to

the same bandwidth limitations as the previous noise contributors.

The voltage noise of the amplifier is multiplied by the noise gain

of the circuit to the output. This noise source is significant for

two reasons. First, the high frequency noise gain can be high

due to the large ratio between the shunt capacitance and the

feedback capacitance. Second, the voltage noise bandwidth is

much higher than the other contributors. The noise bandwidth

is limited only by bandwidth of the amplifier.

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