ADA4530-1ARZ-R7 datasheet(49/51 Pages) AD

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ADA4530-1

Data Sheet

Rev. A | Page 48 of 50

Table 14. Low Frequency Noise Budget

Error Source

25°C

45°C

60°C

VNRF

12.8 μV/√Hz

13.2 μV/√Hz

13.5 μV/√Hz

RSHUNT

5 GΩ

1.25 GΩ

442 MΩ

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 3,

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 signifi-

cant 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 prudent 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 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.

110

100

1000

0.1

100k

10k

100

FREQUENCY (Hz)

VSY = 10V

VCM = VSY/2

TA = 25°C

NSD, CF = 0fF

NSD, CF = 300fF

RMS, CF = 0fF

RMS, CF = 300fF

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 approxi-

mately 6.5 GΩ at 60°C to generate this noise.

110

100

1000

0.1

100k

10k

100

FREQUENCY (Hz)

VSY = 10V

VCM = VSY/2

TA = 60°C

NSD, CF = 300fF

RMS, CF = 300fF

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

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