ADPD1080BCPZ datasheet(26/74 Pages) AD

26 / 74 page

ADPD1080/ADPD1081

Data Sheet

Rev. C | Page 26 of 74

AFE OPERATION

The timing within each pulse burst is important for optimizing

the operation of the ADPD1080/ADPD1081. Figure 30 shows the

timing waveforms for a single time slot as an LED pulse response

propagates through the analog block of the AFE. The first

graph, shown in green, shows the ideal LED pulsed output. The

filtered LED response, shown in blue, shows the output of the

analog integrator. The third graph, shown in orange, shows an

optimally placed integration window. When programmed to the

optimized value, the full signal of the filtered LED response can be

integrated. The AFE integration window is then applied to the

output of the band-pass filter (BPF) and the result is sent to the

ADC and summed for N pulses. If the AFE window is not

correctly sized or located, all of the receive signal is not properly

reported and system performance is not optimal; therefore, it is

important to verify proper AFE position for every new hardware

design or the LED width.

AFE INTEGRATION OFFSET ADJUSTMENT

The AFE integration width must be equal or larger than the LED

width. As AFE width increases, the output noise increases and the

ability to suppress high frequency content from the environment

decreases. It is therefore desirable to keep the AFE integration

width small. However, if the AFE width is too small, the LED

signal is attenuated. With most hardware selections, the AFE

width produces the optimal SNR at 1 µs more than the LED

width. After setting LED width, LED offset, and AFE width, the

ADC offset can then be optimized. The AFE offset must be

manually set such that the falling edge of the first segment of the

integration window matches the zero crossing of the filtered LED

response.

FOR N PULSES

ADC

CONVERSION

LED PULSE

FILTERED LED

RESPONSE

AFE INTEGRATION

WINDOW

TIME (µs)

CONTROLLED BY:

TIA SETTINGS

AFE SETTINGS

LED OFFSET

LED WIDTH

LED PERIOD

9µs + AFE OFFSET

N LED PULSES

LED DRIVE STRENGTH

ADC

CONVERSION

AFE

WIDTH

AFE

WIDTH

Figure 30. AFE Operation Diagram

ADPD1080BCPZ Datasheet(PDF) 26 Page - Analog Devices

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