Datasheet

ADPD1080/ADPD1081 Data Sheet
Rev. B | Page 50 of 74
Register
Group Register Name Time Slot A Time Slot B Float Mode Description
Float Mode
Timing
FLT_PRECON_x 0x5E, Bits[12:8] 0x59, Bits[12:8] Precondition time (to start of Float 1 time).
SLOTx_PERIOD 0x31, Bits[7:0] 0x36, Bits[7:0] 8 LSBs of float period in µs; Float 2 time = SLOTx_PERIOD
SLOTx_PERIOD 0x37, Bits[1:0] 0x37, Bits[9:8] 2 MSBs of float period.
SLOTx_LED_WIDTH 0x30, Bits[12:8] 0x35, Bits[12:8] Connect time in µs; this is the amount of time given to
dump the accumulated charge from the photodiode
capacitance; typically, this is set to 2 µs.
SLOTx_LED_OFFSET 0x30, Bits[7:0] 0x35, Bits[7:0] Time to first charge dump; Float 1 time =
(SLOTx_LED_OFFSET + SLOTx_LED_WIDTH) −
FLT_PRECONx.
SLOTx_AFE_WIDTH 0x39, Bits[15:11] 0x3B, Bits[15:11] Integration time in µs; set to FLT_CONNx + 1.
SLOTx_AFE_OFFSET 0x39, Bits[10:0] 0x3B, Bits[10:0] Integrator start time in 31.25 ns increments; set to
(SLOTx_LED_OFFSETx − SLOTx_AFE_WIDTH − 9.25) µs.
SLOTx_PULSES 0x31, Bits[15:8] 0x36, Bits[15:8] Number of pulses; set to 2 for float ambient mode.
Float Mode for Synchronous LED Measurements
In float LED mode, photocurrent is generated from ambient
light and pulsed LED light during the float time. Float LED
mode is desirable in low signal conditions where the CTR is
<10 nA/mA. In addition, float mode is a good option in
situations where the user wants to limit the LED drive current
of the green LEDs in a heart rate measurement to keep the
forward voltage drop of the green LED to a level that allows the
elimination of a boost converter for the LED supply. For
example, the LED current can be limited to 10 mA to ensure
that the LED voltage drop is ~3 V so that it can operate directly
from the battery without the need of a boost converter. Float
mode accumulates the received charge during longer LED
pulses without adding noise from the signal path, effectively
yielding the highest SNR and/or photon attainable.
As with float ambient mode, multiple pulses cancel electrical
offsets and drifts; however, in float LED mode, the ambient light
must also be cancelled because only the reflected return from
the LED pulses is desired. To achieve this ambient light rejection,
use an even number of equal length pulses. For every pair of
pulses, the LED flashes in one of the pulses and does not flash
in the other. The return from the LED + ambient + offset is
present in one of the pulses. In the other, only the ambient light
and offset is present. A subtraction of the two pulses is made
that eliminates ambient light as well as any offset and drift. It is
recommended to use groups of four pulses for measurement
where the LED is flashed on Pulse 2 and Pulse 3. The
accumulator adds Pulse 2 and Pulse 3 and then subtract Pulse 1
and Pulse 4. To gain additional SNR, use multiple groups of
four pulses.
The settings of FLT_LED_FIRE_x, Register 0x5A, Bits[15:8]
determine if the LED fires in which pulse position. Which pulse
positions are added or subtracted is configured in the
FLT_MATH12x and FLT_MATH34x bits of Register 0x58.
These sequences are repeated in groups of four pulses. The
value written to the FIFO or data registers is dependent on the
total number of pulses per sample period. For example, if the
device is setup for 32 pulses, the 4 pulse sequence, as defined in
FLT_LED_FIRE_x and FLT_MATHxxx, repeats eight times and
a single register or FIFO write of the final value based on 32
pulses executes. Table 32 details the relevant registers for float
LED mode.
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