ADA4530-1ARZ-R7 datasheet(46/52 Pages) AD

46 / 52 page

Data Sheet

ADA4530-1

PHOTODIODE INTERFACE

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Rev. C | 46 of 52

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 signal-to-noise ratio (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 follow-

ing:

►

Amplifier output swing. The maximum photocurrent multiplied by

RF must be less than amplifier swing limitations.

►

Signal bandwidth (or settling time). Signal bandwidth is depend-

ent on RF × CF. Achieving high signal bandwidths with large

feedback resistors can require vanishingly small feedback capac-

itors 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. For example, 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 dominated 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 opera-

tion. 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 photo-

metry grade photodiode (Hamamatsu S1226-18BQ). This medium

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

(UV) through visible frequency range. The minimum shunt resist-

ance (RSHUNT) is specified at 5 GΩ at 25°C. The shunt capacitance

(CSHUNT) is specified at 35 pF. The quartz window limits the maxi-

mum operating temperature to 60°C.

Based on the specified minimum shunt resistance and the recom-

mendations 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

The total RTI error over the entire temperature range is less than

150 fA, which is equal to 300 ppm of the 500 pA full-scale range.

The low input bias current of the ADA4530-1 is not a significant

contributor to the total error over temperature. The interaction of

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