ADPD107 datasheet(26/66 Pages) AD

26 / 66 page

ADPD105/ADPD106/ADPD107

Data Sheet

Rev. A | Page 26 of 66

AFE Integration Offset Starting Point

The starting point of this 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 band-

pass filter 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 28 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 28 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 28. 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 ADPD105/

ADPD107. 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

ADPD107 датащи(PDF) 26 Page - Analog Devices

ADPD107 датащи(HTML) 26 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66

датащи скачать

ссылки URL