ADPD1081 datasheet(27/74 Pages) AD

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Data Sheet

ADPD1080/ADPD1081

Rev. C | 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 211 − 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

AFE OFFSET FROM STARTING POINT (µs)

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

ADPD1081 Datasheet(PDF) 27 Page - Analog Devices

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