ADPD4200 datasheet(34/93 Pages) AD

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Data Sheet

ADPD4200

APPLICATIONS INFORMATION

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Rev. 0 | 34 of 93

Table 20. Modulate Stimulus Settings

Group

Time Slot A Register Address1 Bit Field Name

Description

0x010A, Bits[4:0]

INTEG_WIDTH_x

Integration time in µs. Set to MOD_WIDTH_x + 1.

0x010B, Bits[12:0]

INTEG_OFFSET_x

Integration sequence start time. Set to MOD_OFFSET_x – 1 and then sweep IN-

TEG_OFFSET_x[4:0] in 31.25 ns steps to find optimal operating point.

0x0107, Bits[15:8]

NUM_INT_x

Set to 1 for a single integration per ADC conversion.

0x0107, Bits[7:0]

NUM_REPEAT_x

Number of sequence repeats. SNR increases as √n, where n = NUM_REPEAT_x ×

NUM_INT_x.

1 This is the Time Slot A register address. Add 0x020 for the identical register address for each subsequent time slot. For example, Register 0x0100 is the location

for SAMPLE_TYPE_A. For Time Slot B, this register is at Address 0x0120. For Time Slot C, this register is at Address 0x0140. For Time Slot D, this register is at

Address 0x0160, and so on.

MULTIPLE INTEGRATION MODE

Multiple integration mode provides multiple analog integrations of

incoming charge per ADC conversion. This mode is most useful

when there is a small response that uses a small amount of

the available dynamic range per stimuli event. Multiple integration

mode allows multiple integrations of charge prior to an ADC conver-

sion so that a larger amount of the available dynamic range of the

integrator is utilized.

Figure 32 shows multiple integration mode using the LED as the

stimulus. The number of LED pulses and subsequent integrations

of charge from the photodiode response is determined by the

setting of the NUM_INT_x bits. Following the final integration, there

is a single ADC conversion. This process is repeated NUM_RE-

PEAT_x times.

Prior to setting the number of integrations using the NUM_INT_x

bits, determine the optimal TIA gain and LED current setting. When

the TIA gain and LED current are set, measure how much of the

integrator dynamic range is used to integrate the charge created by

a single LED pulse. If the amount of integrator dynamic range used

for a single pulse is less than half the available dynamic range,

it may be desirable to use multiple integrations prior to an ADC

conversion. For example, if the amount of integrator dynamic range

used for a single pulse is 1/8 of the available dynamic range, set

NUM_INT_x to 0x6 to use six pulses and integrations, using most

of the available dynamic range (75%) per ADC conversion while

leaving 25% of headroom for margin so that the integrator does

not saturate as the input level varies. As each pulse is applied to

the LED, the charge from the response is integrated and held. The

charge from the response to each subsequent pulse is added to

the previous total integrated charge, as shown in Figure 32, until

NUM_INT_x integrations are reached.

In multiple integration mode, the minimum period is automatically

calculated. In the example shown, the minimum period is calculated

at 2 × INTEG_WIDTH_x so that subsequent pulses occur immedi-

ately following the completion of the previous integration. Extra time

is automatically added to accommodate the ADC conversions at the

end of NUM_INT_x integrations.

Use NUM_REPEAT_x to increase the iterations to improve the

overall SNR. The entire multiple integration per ADC conversion

process repeats NUM_REPEAT_x number of times. Increasing

NUM_REPEAT_x serves the same purpose as multiple pulses in

continuous connect mode, where n pulses improve the SNR by √n.

In multiple integration mode, the SNR increases by √n, where n =

NUM_REPEAT_x. The total number of LED pulses in this mode is

equal to NUM_INT_x × NUM_REPEAT_x.

ADPD4200 Datasheet(PDF) 34 Page - Analog Devices

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