ADA4530-1ARZ-R7 datasheet(48/52 Pages) AD

48 / 52 page

Data Sheet

ADA4530-1

PHOTODIODE INTERFACE

analog.com

Rev. C | 48 of 52

Table 14. Low Frequency Noise Budget (Continued)

Error Source

25°C

45°C

60°C

VNRSHUNT

9 µV/√Hz

4.7 µV/√Hz

2.8 µV/√Hz

RF/RSHUNT

VNRSHUNT_RTO

18 µV/√Hz

37 µV/√Hz

61 µV/√Hz

IN−

0.07 fA/√Hz

0.15 fA/√Hz

0.24 fA/√Hz

IN−_RTO

700 nV/√Hz

1.5 µV/√Hz

2.4 µV/√Hz

Low Frequency NSD

Total

22 µV/√Hz

39 µV/√Hz

62 µV/√Hz

Low Frequency RMS

Total

194 µV rms

345 µV rms

549 µV rms

The sole high frequency noise contributor is the amplifier voltage

noise, which is multiplied by the high frequency noise gain and

band limited only by the amplifier gain. The −3 dB bandwidth of the

amplifier is 17 kHz (refer to Equation 26, where f3 = fUGC ÷ NG2

= 2 MHz ÷ 118). The equivalent noise bandwidth is 27 kHz. The

high bandwidth is the reason the high frequency noise is significant

even though the noise spectral density is much lower than the low

frequency noise.

Table 15. High Frequency Noise Budget

Error Source

25°C

45°C

60°C

14 nV/√Hz

14.5 nV/√Hz

14.8 nV/√Hz

High Frequency Noise

Gain

118

VN_RTO

1.6 µV/√Hz

1.7 µV/√Hz

High Frequency RMS

Total

271 µV rms

281 µV rms

286 µV rms

At low temperatures, the amplifier voltage noise is more significant

than any other noise source. This is important because the majority

of this noise occurs outside the useful bandwidth of the circuit. For

this reason, it is recommended to add a low-pass filter to the output

of a photodiode TIA circuit. This filter can be active or passive

depending on the needs of the system. A simple resistor capacitor

(RC) filter with a −3 dB cutoff of 500 Hz has an insignificant impact

on the frequency response of the signal path, but it lowers the

integrated noise from 271 µV rms to 45 µV rms (a 6× reduction).

The NSD was measured for this circuit with (blue curve) and

without (red curve) the 300 fF CF capacitor (see Figure 131). At

low frequencies, the NSD is approximately equal to the noise from

the feedback resistor alone (12.8 µV/√Hz). The value of the low

frequency NSD shows that the shunt resistance is much larger than

the specified minimum (which is expected). As frequency increases,

the resistor noise rolls off at the signal bandwidth (50 Hz). The NSD

then plateaus at the amplifier voltage noise level until the bandwidth

limitations of the amplifier roll off the NSD toward zero.

Figure 131. RTO Noise Spectral Density (25°C)

The dashed curves show the integration of the NSD across the

frequency spectrum. These are useful to calculate the rms noise

over a variety of bandwidths. For example, the rms noise over the

entire 100 kHz measurement bandwidth is 400 µV rms, which is

approximately the same as the calculated total noise of 333 µV rms.

If a postfilter is added with a noise bandwidth of 1 kHz, Figure 131

shows that the integrated noise is 200 µV rms (a 2× improvement).

The uncompensated circuit (red curves) shows considerably worse

noise performance. The frequency peaking due to the marginal

loop stability multiplies the noise as well as the signal. In addition,

the high frequency noise gain is larger, which adds much more

noise outside the signal bandwidth. Both of these effects together

generate 1.2 mV rms of total noise. Even if the transient and

frequency response of an undercompensated TIA are acceptable,

the large noise penalty may not be.

Lastly, the NSD was measured for this circuit at 60°C (see Figure

132). As expected, the low frequency noise increased as a result

of the photodiode shunt resistance. The average low frequency

NSD is 22 µV/√Hz. Removing the contribution of RF gives an RTO

contribution of 17 µV/√Hz, which is equivalent to an RTI current

noise of 1.7 fA/√Hz. RSHUNT must be approximately 6.5 GΩ at 60°C

to generate this noise.

Figure 132. RTO Noise Spectral Density (60°C)

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