Datasheet

ADPD1080/ADPD1081 Data Sheet
Rev. B | Page 42 of 74
Table 26. Channel Power-Down Settings
Number of Channels Channels Enabled
Register 0x3C,
Bits[8:6]
Register 0x3C,
Bits[5:3]
Register 0x37,
Bits[15:13]
Register 0x37,
Bits[12:10]
1 Channel 1 0x7 0x0 Not applicable Not applicable
1 Channel 2 0x0 0x7 0x7 0x0
2 Channel 1, Channel 2 0x0 0x0 0x7 0x0
2 Channel 3, Channel 4 0x0 0x7 0x0 0x7
3 Channel 2, Channel 3, Channel 4 0x0 0x7 0x0 0x0
4 All channels 0x0 0x0 0x0 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 (R
F
) and the
input resistor to the integrator (R
INT
). When R
F
is reduced to
provide additional dynamic range at the input to the TIA to
accommodate additional ambient light, R
INT
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
Register 0x44 (Time Slot B) to choose the resistor setting of R
INT
as shown in Table 27.
TIA_VREF
16110-146
TIA
6.3pF
R
INT
R
F
R
F
R
INT
6.3pF
ADC
BPF
Figure 48. Receive Path Block Diagram
Table 27. Values of R
INT
Register 0x42, Bits[9:8], Register 0x44, Bits[9:8] R
INT
(kΩ)
00 (default) 400
01 200
10 100
Table 28 shows an example of how the SNR can be optimized as
a function of the R
F
and R
INT
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. R
F
and R
INT
Combinations
R
F
(kΩ) R
INT
(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
50 100 3.3 1.85 18.8 22.1 65
100 400 6.8 1.38 4.3 11.1 73.9
50 200 6.8 2.1 15.3 22.1 70.2
50 400 13.2 2.7 8.9 22.1 73.8
Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.Downloaded from Arrow.com.