ADPD7008 datasheet(21/71 Pages) AD

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Data Sheet

ADPD7008

APPLICATIONS INFORMATION

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Rev. 0 | 21 of 71

Timing Recommendations for Digital

Integration Mode

When setting the timing for digital integration mode, it is important

to place the ADC samples such that the signal being sampled is

given time to settle before the sample being taken. Photodiode

capacitance and the TIA settling time affect the settling time of the

input signal.

If automatic ambient light rejection is on (AMBIENT_CANCELLA-

TION_x is set to either 1 (01) or 2 (10) decimal), time is needed at

the beginning of each time slot to enable the ambient rejection loop.

The start-up time of this loop is 18 μs, and the working time of this

loop is 30 μs.

The TIA_SAT_DET internal block must be turned on to speed up

the TIA settling. Speeding up the TIA settling can help the TIA

enter a normal working state quickly to make the automatic ambient

rejection loop more accurate.

After the ambient loop completes, the first ADC sample of dark data

can be enabled. The DARK1_OFFSET_x setting must be equal or

larger than the ambient loop working time (48 μs).

Figure 24 shows an example of the proper placement of the ADC

sampling edges.

Figure 24. Proper Placement of ADC Sampling Edges in Digital Integration

Mode

The recommended DARK1_OFFSET_x setting after the automatic

ambient loop completes is 48 μs, or 10 μs if automatic ambient

rejection is not turned on.

As shown in Figure 24, different optical devices, including the LED

and photodiode, have different response times. tD_RISE is the rising

time of the photodiode current, and tD_FALL is the falling time of

the photodiode current. tD is either tD_RISE or tD_FALL, depending on

which one is bigger.

See the following equations to calculate the timing:

LED_OFFSET_x=DARK

1_OFFSET_x+

NUM_INT_x+tD−tD_RISE

(4)

LIT_OFFSET_x=LED_OFFSET_x+tD_RISE (5)

DARK

2_OFFSET_x=LED_OFFSET_x

+LED_WIDTH_x+tD

(6)

These values must be characterized in the final application. These

settings only apply to two-region digital integration mode.

Table 14. Empirical Values for Two-Region Digital Integration Mode

Optical Device

Green (μs)

Red (μs)

Infrared (μs)

LED_WIDTH_x

PERIOD_x

138

NUM_INT_x

LED_OFFSET_x

LIT_OFFSET_x

101

DARK1_OFFSET_x

DARK2_OFFSET_x

167

tD_RISE

tD_FALL

Optimizing Sampling Sequence

If the empirical value is not appropriate for the measurement,

optimize the sampling sequence.

See the following reference method for sweeping the curve (this

example is based on TSA Channel 1 in a dark environment):

1. Enable the following settings:

►

One-region digital integration mode

►

1× TIA configuration

►

AFE_TRIM_VREF_A = 3

►

AMBIENT_CANCELLATION_A = 0

►

NUM_INT_A = 1

►

NUM_REPEAT_A = 1

►

DARK1_OFFSET_A = 10

►

LED_OFFSET_A = 20

►

LED_WIDTH_A = 80

►

LIT_OFFSET_A = 130

►

MIN_PERIOD_A = 160

2. Power on the optical devices and enable TSA Channel 1.

3. Collect about 100 lit data values (remove the first 10 data

values) and calculate the mean value.

4. Sweep the LIT_OFFSET_A bit from 130 to 10 and reproduce

the result from Step 3.

5. Plot the mean value of the lit data and LIT_OFFSET_A. The

response time of the optical device (for example, OSRAM

FIREFLY

® CT DBLP31.12) is shown in Figure 25.

When collecting the lit data with the LIT_OFFSET_A bit changing,

the data is lower than 16384 (unsaturated).

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