ADA4530-1ARZ-R7 datasheet(47/51 Pages) AD

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ADA4530-1

Data Sheet

Rev. A | Page 46 of 50

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.

FREQUENCY

100

0.1

110

100

RSHUNT

IN–

Figure 127. Photodiode TIA RTO Noise Spectral Density

Table 12. Photodiode Interface Noise Sources

Noise Source

RTO Noise

Noise Bandwidth

√(4kTRF)

π/2 × f2

Photodiode

(RF/RSHUNT)√(4kTRSHUNT)

π/2 × f2

IN− Amplifier

RF × IN−

π/2 × f2

VN Amplifier

VN × noise gain

π/2 × f3

DESIGN RECOMMENDATIONS

The design goal for a large area photodiode TIA circuit is

usually to maximize SNR and minimize dc errors. Increasing

the feedback resistor size accomplishes both goals. The signal

gain increases directly with RF, whereas the noise increases in a

square root fashion. High gains also make the output signal

large relative to output voltage errors (such as VOS).

The upper limit for RF is typically determined by one of the

following:



Amplifier output swing. The maximum photocurrent

multiplied by RF must be less than amplifier swing

limitations.



Signal bandwidth (or settling time). Signal bandwidth is

dependent on RF × CF. Achieving high signal bandwidths

with large feedback resistors can require vanishingly small

feedback capacitors to implement. The ultimate limitation

is due to the parasitic feedback capacitance from the

fringing electric fields in the circuit. Parasitic capacitances

in the 50 fF to 100 fF range are possible. To put this in

perspective, a 100 fF parasitic capacitance limits the signal

bandwidth of a 100 GΩ TIA to 16 Hz.



The thermal noise of the photodiode (RSHUNT). When RF is

significantly larger than RSHUNT, the total noise is domi-

nated by the photodiode and the SNR stops improving.



The current noise of the amplifier. When the current noise

of the amplifier is larger than the noise of RF, the SNR stops

improving. The photodiode noise is higher than the

amplifier current noise in nearly all practical photodiodes.



The low frequency noise gain due to RSHUNT. When RF is

larger than RSHUNT, the noise gain multiplies VOS and TCVOS

errors and the signal to error ratio stops improving.

The signal bandwidth increases as the feedback capacitance (CF)

decreases. The lower limit for CF is typically limited by one of

the following:



Parasitic feedback capacitances limit the minimum value of

CF to 50 fF to 100 fF.



Available component values. Physical components can be

found in surface mount packages for values from 0.1 pF to

1 pF in 100 fF increments.



Feedback loop stability. CF must be large enough to recover

enough phase shift prior to the loop crossover for stable

operation. This capacitance value can be a significant

consideration for smaller values of RF. Large values

(>1 GΩ) tend to be self compensating through the parasitic

feedback capacitance.



High frequency noise gain. The high frequency noise gain

is set by the ratio of CSHUNT to CF. For very large noise gains,

it is possible for the amplifier voltage noise to be greater

than the feedback resistor noise.

DESIGN EXAMPLE

In this section, an example TIA circuit is designed using a

photometry grade photodiode (Hamamatsu S1226-18BQ). This

medium area (1.2 mm2) silicon photodiode is responsive in the

ultraviolet (UV) through visible frequency range. The mini-

mum shunt resistance (RSHUNT) is specified at 5 GΩ at 25°C. The

shunt capacitance (CSHUNT) is specified at 35 pF. The quartz

window limits the maximum operating temperature to 60°C.

Based on the specified minimum shunt resistance and the

recommendations in the Design Recommendations section, a

value of 10 GΩ is chosen for RF. This example circuit is powered

from ±5 V with the input common-mode voltage set at 0 V, which

allows a maximum photocurrent of approximately 500 pA.

An error budget is constructed based on the DC Error Analysis

section (see Table 13). The amplifier offset voltage applies the

maximum temperature drift limit to the maximum room

temperature offset limit. The photo diode shunt resistance limit

is reduced by half for every 10°C.

Table 13. Photodiode Interface DC Error Budget

Error Source

25°C

45°C

60°C

VOS

40 μV

40 μV + 10 μV

40 μV + 18 μV

RSHUNT

5 GΩ

1.25 GΩ

442 MΩ

Noise Gain

VOS Error RTO

120 μV

450 μV

1.3 mV

20 fA

IB Error RTO

200 μV

Total Error RTO

320 μV

650 μV

1.5 mV

Total Error RTI

32 fA

65 fA

150 fA

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