Datasheet

Data Sheet ADPD1080/ADPD1081
Rev. B | Page 27 of 74
AFE Integration Offset Starting Point
The starting point of the AFE integration offset, as expressed in
microseconds, is set such that the falling edge of the integration
window aligns with the falling edge of the LED.
LED_FALLING_EDGE = SLOTx_LED_OFFSET +
SLOTx_LED_WIDTH
and,
AFE_INTEGRATION_FALLING_EDGE = 9 +
SLOTx_AFE_OFFSET + SLOTx_AFE_WIDTH
If both falling edges are set equal to each other, solve for
SLOTx_AFE_OFFSET to obtain the following equation:
AFE_OFFSET_STARTING_POINT = SLOTx_LED_
OFFSET + SLOTx_LED_WIDTH − 9 – SLOTx_AFE_
WIDTH
Setting the AFE offset to any point in time earlier than the
starting point is equivalent to setting the integration in the
future; the AFE cannot integrate the result from an LED pulse
that has not yet occurred. As a result, a SLOTx_AFE_OFFSET
value less than the AFE_OFFSET_STARTING_POINT value is an
erroneous setting. Such a result may indicate that current in the
TIA is operating in the reverse direction from intended, where
the LED pulse is causing the current to leave the TIA rather
than enter it.
Because, for most setups, the SLOTx_AFE_WIDTH is 1 µs
wider than the SLOTx_LED_WIDTH, the AFE_OFFSET_
STARTING_POINT value is typically 10 µs less than the
SLOTx_LED_OFFSET value. Any value less than SLOTx_LED_
OFFSET − 10 is erroneous. The optimal AFE offset is some
time after the AFE_OFFSET_STARTING_POINT value. The
BPF response, LED response, and photodiode response each
add some delay. In general, the component choice, board layout,
SLOTx_LED_OFFSET, and SLOTx_LED_WIDTH are the
variables that can change the SLOTx_AFE_OFFSET value.
After a specific design is set, the SLOTx_AFE_OFFSET value
can be locked down and does not need to be optimized further.
Sweeping the AFE Position
The AFE offsets for Time Slot A and Time Slot B are controlled
by Bits[10:0] of Register 0x39 and Register 0x3B, respectively.
Each LSB represents one cycle of the 32 MHz clock, or 31.25 ns.
The register can be thought of as 2
11 − 1
of these 31.25 ns steps,
or it can be broken into an AFE coarse setting using Bits[10:5]
to represent 1 µs steps and Bits[4:0] to represent 31.25 ns steps.
Sweeping the AFE position from the starting point to find a
local maximum is the recommended way to optimize the AFE
offset. The setup for this test is to allow the LED light to fall on
the photodiode in a static way. This test is typically done with a
reflecting surface at a fixed distance. The AFE position can then
be swept to look for changes in the output level. When adjusting
the AFE position, it is important to sweep the position using the
31.25 ns steps. Typically, a local maximum is found within 2 µs
of the starting point for most systems. Figure 31 shows an example
of an AFE sweep, where 0 on the x-axis represents the AFE starting
point defined previously. Each data point in Figure 31 corresponds
to one 31.25 ns step of the SLOTx_AFE_OFFSET. The optimal
location for SLOTx_AFE_OFFSET in this example is 0.687 µs
from the AFE starting point.
0.687
100
95
90
85
80
75
RELATIVE OUTPUT VALUE (%)
AFE OFFSET FROM STARTING POINT (µs)
16110-031
0 0.15 0.30 0.45 0.60 0.75 0.90 1.05 1.20 1.35 1.50
Figure 31. AFE Sweep Example
Table 18 lists some typical LED and AFE values after optimization.
In general, it is not recommended to use the SLOTx_AFE_OFFSET
numbers in Table 18 without first verifying them against the AFE
sweep method. Repeat this method for every new LED width and
with every new set of hardware made with the ADPD1080/
ADPD1081. For maximum accuracy, it is recommended that the
32 MHz clock be calibrated prior to sweeping the AFE.
Table 18. AFE Window Settings
LED Register 0x30 or Register 0x35 AFE Register 0x39 or Register 0x3B Comment
0x0219 0x1A08 2 µs LED pulse, 3 µs AFE width, 25 µs LED delay
0x0319 0x21FE 3 µs LED pulse, 4 µs AFE width, 25 µs LED delay
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