ADA4351-2ACPZ-R7 datasheet(31/36 Pages) AD

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Data Sheet

ADA4351-2

APPLICATIONS INFORMATION

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Rev. 0 | 31 of 36

mate output integrated noise contribution of 23.5 nV/√Hz ×

558 kHz−101 kHz=15.9µVRMS.

c. Then, from P1 to the final single-pole roll-off (fC) shown in

Figure 87 at GBP/(1 + CS/CF) there is a flat spot noise

integrating over a wide span. Here, the high frequency

noise gain (NGHI ) is 1 + 105.5 pF/19 pF = 6.55 to

give a single-pole roll-off at GBP/ NGHI = 8.5 MHz/6.55

= 1.3 MHz. Because the noise up to P1 has already

been integrated, the integration span for this 6.55 × 7.3

nV/√Hz = 47.8 nV/√Hz output spot noise due to this term is

1.57 × 1.3 MHz − 558 kHz,whichgivesa47.8

nV/√Hz × √1.48 MHz = 58.2 μV RMS term, which is by far

the most dominant term. Where possible, a post RC filter at

lower than the self-limited frequency can be used to reduce

this total integrated noise and reduce the spot noise voltage

shown in Figure 50.

Table 9 summarizes each of the separate integrated noise contribu-

tions and the total RMS of these to a combined integrated output

noise. The total output is formed by squaring each term, summing

those, and then taking the square root again. The percent of output

noise power contribution is the ratio of each term squared to the

total RMS noise voltage squared.

Clearly, the dominant term is the output spot noise that is integrated

from P1 to the 1.3 MHz f−3 dB. Note that reducing the integration

span using a lower frequency external RC filter, where possible,

can reduce the total integrated noise rapidly. This approximate

calculation of 62 μV RMS closely matches the example set up in

the Photodiode Circuit Design Wizard, where it reports a 69 μV

RMS total noise.

When the photodiode is reverse biased, there is a DC dark current,

IDARK, in the diode that adds a noise term given by

2q×IDARK.

If present, RMS this term with the input bias current noise term

before computing its RMS contribution to the output. Note that,

sometimes, these is also a relatively low shunt resistance across

the photodiode. To the extent that it is not >> RF, it may add a small

added noise term. If present, get its Johnson noise term and give

it a gain to the output equal to RF/RSH and apply the same noise

power bandwidth as used for the RF noise.

These calculations give an output RMS noise floor before any

signal current is considered. To combine its effects with a noise

source from an input signal, refer this output integrated noise back

to the input as an equivalent spot noise, which can be done by

dividing by the RF gain element, which gives the input referred

integrated noise. Then, divide the input referred RMS noise current

by the square root of the input current noise power bandwidth.

Making the calculation for the 62 μV RMS derived previously gives,

first, an input referred 4.1 nA RMS integrated input noise, and then

dividing by √877 kHz gives an equivalent total input referred spot

current noise of 4.4 pA/√Hz. As the signal current increases from

zero, it also adds its own current noise term to the output using the

same NPBW as the bias current noise. To calculate the total input

RMS noise with an input signal, take the RMS of the input signal

noise and the equivalent total input referred spot current noise as

previously described.

Table 9. Summary Pieces Combining to a Total RMS Noise Voltage at the Output

Separate Output Noise Terms Integrating into the V RMS Pieces Noise Term

Spectral Density

Integrated Noise Term

(μV RMS)

Percent of Output

Noise Power

Input Current Noise of the Op Amp

110 fA/√Hz

1.5

0.06%

RF Noise with G =1 to the Output and 1.57 × P1 Integrated

Bandwidth

RF term

25.8 nV/√Hz

14.7

5.59%

en with G =1 to the Output through Z1

en G =1 term

7.3 nV/√Hz

2.3

0.14%

Rising Integrated Equivalent Spot Noise from Z1 to P1

en Z1 to P1 integrated

7.3 nV/√Hz

15.9

6.54%

Flat from P1 to Final Single-Pole Roll-Off Frequency (f−3 dB)

en × NGHI P1 to f−3 dB

24.8 nV/√Hz

58.2

87.67%

RMS Total Output Noise

Not applicable

62.16

Not applicable

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