ADPD1081 datasheet(42/74 Pages) AD

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ADPD1080/ADPD1081

Data Sheet

Rev. C | Page 42 of 74

Table 26. Channel Power-Down Settings

Number of Channels

Channels Enabled

Bits[8:6]

Bits[5:3]

Bits[15:13]

Bits[12:10]

Channel 1

0x7

0x0

Not applicable

Channel 2

0x0

0x7

0x0

Channel 1, Channel 2

0x0

0x7

0x0

Channel 3, Channel 4

0x0

0x7

0x0

0x7

Channel 2, Channel 3, Channel 4

0x0

0x7

0x0

All channels

0x0

OPTIMIZING DYNAMIC RANGE FOR HIGH

AMBIENT LIGHT CONDITIONS

Large amounts of ambient light use large amounts of the

available dynamic range of the TIA. The band-pass filter

rejects the ambient light prior to the charge being integrated

by the integrator; therefore, the ambient light is not a primary

concern for the integrator. However, to accommodate

increased levels of ambient light, it may be necessary to use

lower TIA gain to avoid saturation of the TIA. When the TIA

gain is reduced, the referred to input (RTI) noise of the desired

signal increases. The impact of this increase can be reversed by

increasing the gain of the integrator so that the LED signal gain

in mA per LSB remains the same.

For example, start with an amount of pulsed signal (desired)

where the TIA gain has been optimized such that the pulsed

signal is using the desired amount of ADC dynamic range

available, typically ~70% full scale. If the ambient light level

increases and the gain of the TIA must be decreased to

accommodate for the increase in ambient light without saturating

the TIA, then the amount of pulsed signal presented to the ADC is

attenuated by the factor that the TIA gain is reduced, which

results in the SNR of the desired signal reducing.

To increase the SNR of the desired signal in this situation, use

one of the following two methods. The first method simply

increases the LED current by the amount required to bring the

level of the pulsed signal at the ADC back to the desired amount

of full scale. However, this is at the expense of increasing the

overall power of the system. The second method is to increase

the gain of the integrator to achieve a similar result. Figure 48

shows a block diagram of the receive path. The gain of the signal

path is determined by the TIA feedback resistor (RF) and the

input resistor to the integrator (RINT). When RF is reduced to

provide additional dynamic range at the input to the TIA to

accommodate additional ambient light, RINT can be reduced to

provide more gain through the integrator such that the same

amount of pulsed signal at the input to the TIA utilizes the same

amount of dynamic range of the ADC before the TIA gain is

reduced. Use Bits[9:8] of Register 0x42 (Time Slot A) and

as shown in Table 27.

TIA_VREF

TIA

6.3pF

RINT

6.3pF

ADC

BPF

Figure 48. Receive Path Block Diagram

Table 27. Values of RINT

RINT (kΩ)

00 (default)

400

200

100

Table 28 shows an example of how the SNR can be optimized

as a function of the RF and RINT vs. the amount of ambient light

that must be accommodated. The values shown in Table 28 are

for a 2 µs LED pulse and a photodiode capacitance of 30 pF.

Table 28. Examples of SNR vs. RF and RINT Combinations

RF (kΩ)

RINT (kΩ)

Pulsed Current at

70% Full Scale (µA)

Noise (nA rms)

Maximum Ambient Current (µA)

TIA Linear Range (µA)

SNR (dB)

200

400

3.3

0.82

2.2

5.5

72.1

100

200

3.3

1.26

7.8

11.1

68.4

100

3.3

1.85

18.8

22.1

100

400

6.8

1.38

4.3

11.1

73.9

200

6.8

2.1

15.3

22.1

70.2

400

13.2

2.7

8.9

22.1

73.8

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