Datasheet

ADPD1080/ADPD1081 Data Sheet
Rev. B | 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 environ-
ment 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
FOR N PULSES
FOR N PULSES
ADC
CONVERSION
LED PULSE
FILTERED LED
RESPONSE
AFE INTEGRATION
WINDOW
TIME (µs)
CONTROLLED BY:
TIA SETTINGS
AFE SETTINGS
REGISTER 0x42,
REGISTER 0x43,
REGISTER 0x44,
REGISTER 0x45
LED OFFSET
REGISTER 0x30, BITS[7:0]
REGISTER 0x35, BITS[7:0]
LED WIDTH
REGISTER 0x30, BITS[12:8]
REGISTER 0x35, BITS[12:8]
LED PERIOD
REGISTER 0x31, BITS[7:0]
REGISTER 0x36, BITS[7:0]
REGISTER 0x39, BITS[15:11]
REGISTER 0x3B, BITS[15:11]
9µs + AFE OFFSET
REGISTER 0x39, BITS[10:0]
REGISTER 0x3B, BITS[10:0]
N LED PULSES
REGISTER 0x31, BITS [15:8]
REGISTER 0x36, BITS [15:8]
LED DRIVE STRENGTH
REGISTER 0x22, REGISTER 0x23,
REGISTER 0x24, REGISTER 0x25
ADC
CONVERSION
16110-030
AFE
WIDTH
AFE
WIDTH
Figure 30. AFE Operation Diagram
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