ADPD4100 datasheet(38/101 Pages) AD

38 / 101 page

ADPD4100/ADPD4101

Data Sheet

Rev. 0 | Page 38 of 101

Group

Time Slot A Register Address1

Bit Field Name

Description

Self Capacitance-Based

Proximity Timing

0x010C, Bits[7:0]

MOD_OFFSET_x

Sets start time of first modulation pulse in 1 μs

increments. Typical value of 16 μs.

0x010C, Bits[15:8]

MOD_WIDTH_x

Sets width of modulation pulse in 1 μs

increments. Typical value of 6 μs.

0x010A, Bits[4:0]

INTEG_WIDTH_x

Integration time in µs. Typical value of 10 μs .

0x010A, Bits[10:8], Bits[14:12]

CHx_AMP_DISABLE_x

Set 0x010A, Bit 9 to 1 to power down BPF for

Channel 1, Bit 13 to 1 to power down BPF for

Channel 2 if Channel 2 is enabled.

0x010B, Bits[12:0]

INTEG_OFFSET_x

Integration sequence start time. Set to typical

value of 9 μs and then sweep INTEG_OFFSET_x,

Bits[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.

0x0108, Bits[13:12]

MOD_TYPE_x

Set to 0x0 for continuous TIA connection.

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 conversion so that a larger amount of

the available dynamic range of the integrator is utilized.

Figure 44 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_REPEAT_x times.

Prior to setting the number of integrations using the NUM_INT_x

bits, set the TIA gain to 200 kΩ and determine the optimal LED

current setting, which is close to the maximum current. 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 44, until NUM_INT_x

integrations is 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 immediately 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.

Integrator chop mode is recommended for multiple integration

mode for optimal SNR performance.

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