ADA4530-1ARZ-R7 datasheet(45/52 Pages) AD

45 / 52 page

Data Sheet

ADA4530-1

PHOTODIODE INTERFACE

analog.com

Rev. C | 45 of 52

Figure 126. Transimpedance Noise Gain vs. Frequency

For completeness, the noise gain equations are as follows:

NG f = 1+RFRS 2πff1+12πff2+1

(23)

f1= 1

RFRSHUNT

RF+RSHUNTCF+CSHUNT

(24)

f2= 1RFCF

(25)

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:

f3= fUGC

1+CSHUNTCF

(26)

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:

SignalGain f =RF 12πff2+1

(27)

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)

(28)

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 ther-

mal 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.

Each of these noise contributors is graphed vs. frequency in Figure

127. A summary of the noise sources and their RTO contributions

is shown in Table 12. The total RTO noise adds the contributions of

each noise source in root sum square.

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