Datasheet

Data Sheet ADPD1080/ADPD1081
Rev. B | Page 49 of 74
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.
REGISTER 0x59, BITS[12:8]/
REGISTER 0x5E, BITS[12:8]
PRECONDITION TIME
(FLT_RPRECON_x)
REGISTER 0x30, BITS[12:8]/
REGISTER 0x35, BITS[12:8]
CHARGE DUMP TIME
(SLOTx_LED_WIDTH)
REGISTER 0x31, BITS[7:0]/
REGISTER 0x36, BITS[7:0]
FLOAT 2 TIME (SLOTx_PERIOD)
REGISTER 0x39, BITS[15:11]/
REGISTER 0x3B, BITS[15:11]
INTEGRATION TIME (SLOTx_AFE_WIDTH)
REGISTER 0x30 BITS[7:0]/REGISTER 0x35, BITS[7:0]
TIME TO FIRST CHARGE DUMP
(SLOTx_LED OFFSET)
CONNECT/FLOAT
FLOAT 1 TIME
INTEGRATOR SEQUENCE
ACCUMULATED
CHARGE ON PD
16110-052
Figure 54. Example of Float Ambient Mode Timing
Table 31. Float Ambient Mode Registers
Register
Group Register Name 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 0x54, Bit 7 Set to 1 to override Register 0x3C cathode voltage
settings.
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