ADPD1081 datasheet(49/74 Pages) AD

49 / 74 page

Data Sheet

ADPD1080/ADPD1081

Rev. C | Page 49 of 74

given amount of time. Ultimately, the type of measurement

being made (ambient or pulsed LED), the photodiode capacitance,

and the CTR of the system determine the float times.

Float Mode for Ambient Light Measurements

Float mode is used for ambient light measurements where the

background light is sufficiently small. Use TIA ADC mode for

ambient light measurements of higher intensities. Small amounts

of light can be measured with adequate float times, allowing the

incoming charge to accumulate to levels large enough to be

measured above the noise floor of the system. The source of

this light can be any combination of synchronous light (for

example, from a pulsed LED) and asynchronous light (that is,

background). If there is no system generated light source, the

measurement is simply a measure of the background light.

Use a two pulse differential measurement technique to cancel

out electrical drifts and offsets. Take two measurements, each

of a different float time. The first float time is considerably

shorter than the second pulse. After the two measurements are

taken, Measurement 1 is subtracted from Measurement 2,

which effectively cancels out any offset and drift common to

both measurements. What is left is an ambient light

measurement based on an amount of charge that is integrated

over a time that is the difference of the first and second float

times. For example, if Float Time 1 is 6 µs and Float Time 2 is

26 µs, the ambient light measurement is based on 20 µs of

charge integrated on the photodiode capacitance with any

offset and drift removed. In float mode for ambient light, the

number of pulses must be set to two to cancel drifts and offsets

because only the first pulse can be short. More than two pulses

can be used; however, pulses two through n are always the same

length. If drift cancellation is not required, any number of

pulses can be used and added together. Figure 54 shows an

example of float ambient mode timing, and Table 31 details the

relevant registers that must be configured.

PRECONDITION TIME

(FLT_RPRECON_x)

CHARGE DUMP TIME

(SLOTx_LED_WIDTH)

FLOAT 2 TIME (SLOTx_PERIOD)

INTEGRATION TIME (SLOTx_AFE_WIDTH)

TIME TO FIRST CHARGE DUMP

(SLOTx_LED OFFSET)

CONNECT/FLOAT

FLOAT 1 TIME

INTEGRATOR SEQUENCE

ACCUMULATED

CHARGE ON PD

Figure 54. Example of Float Ambient Mode Timing

Table 31. Float Ambient Mode Registers

Group

Time Slot A

Time Slot B

Float Mode Description

Float Mode

Operation

SLOTx_LED_SEL

0x14, Bits[1:0]

0x14, Bits[3:2]

Set to 0 to enable float mode.

FLT_EN_x

0x5E, Bits[14:13]

0x59, Bits[14:13]

Set to 3 to enable float between connect pulses.

FLT_MATH12_x

0x58, Bits[2:1]

0x58, Bits[6:5]

Set to 2 to subtract first pulse and add second pulse.

SLOTx_AFE_CFG

0x43, Bits[15:0]

0x45, Bits[15:0]

Set to 0xAE65 for TIA and integrator, bypass BPF.

SLOTx_TIA_VREF

0x42, Bits[5:4]

0x44, Bits[5:4]

Set to 2 for TIA_VREF = 0.9 V.

SLOTx_V_CATHODE

0x54, Bits[9:8]

0x54, Bits[11:10]

Set to 2 for 250 mV reverse bias on the photodiode at

the precondition.

REG54_VCAT_ENABLE

0x54, Bit 7

Set to 1 to override Register 0x3C cathode voltage

settings.

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