Datasheet

Data Sheet ADPD1080/ADPD1081
Rev. B | Page 19 of 74
THEORY OF OPERATION
INTRODUCTION
The ADPD1080/ADPD1081 operate as a complete optical
transceiver stimulating up to three LEDs and measuring the
return signal on up to two separate current inputs. The core
consists of a photometric front end coupled with an ADC,
digital block, and three independent LED drivers. The core
circuitry stimulates the LEDs and measures the return in the
analog block through one to eight photodiode inputs, storing
the results in discrete data locations. The two inputs can drive
four simultaneous input channels. Data can be read directly by
a register or through a first in, first out (FIFO) method. This
highly integrated system includes an analog signal processing
block, digital signal processing block, an I
2
C communication
interface on the ADPD1080 or an SPI port on the ADPD1081,
and programmable pulsed LED current sources.
The LED driver is a current sink and is agnostic to the LED supply
voltage and the LED type. The photodiode (PDx) inputs can
accommodate any photodiode with an input capacitance of
less than 100 pF. The ADPD1080/ADPD1081 produces a high
SNR for relatively low LED power while greatly reducing the effect
of ambient light on the measured signal.
DUAL TIME SLOT OPERATION
The ADPD1080/ADPD1081 operate in two independent time
slots, Time Slot A and Time Slot B, that operate sequentially.
The entire signal path from LED stimulation to data capture and
processing executes during each time slot. Each time slot has a
separate datapath that uses independent settings for the LED
driver, AFE setup, and the resulting data. Time Slot A and
Time Slot B operate in sequence for every sampling period, as
shown in Figure 17.
The timing parameters for Time Slot A and Time Slot B are
defined as follows:
t
A
(µs) = SLOTA_LED_OFFSET + n
A
× SLOTA_PERIOD
where n
A
is the number of pulses for Time Slot A (Register 0x31,
Bits[15:8]).
t
B
(µs) = SLOTB_LED_OFFSET + n
B
× SLOTB_PERIOD
where n
B
is the number of pulses for Time Slot B (Register 0x36,
Bits[15:8]).
Calculate the LED period using the following equation:
LED_PERIOD, minimum = 2 × SLOTx_AFE_WIDTH + 11
t
1
and t
2
are fixed and based on the computation time for each
slot. If a slot is not in use, these times do not add to the total
active time. Table 15 defines the values for these LED and
sampling time parameters.
ACTIVE
t
A
t
1
t
B
t
2
SLEEP
TIME SLOT A TIME SLOT B
ACTIVE
1/f
SAMPLE
n
A
PULSES n
B
PULSES
16110-017
Figure 17. Time Slot Timing Diagram (f
SAMPLE
is the sampling frequency (Register 0x12, Bits[15:0]).)
Table 15. LED Timing and Sample Timing Parameters
Parameter Register Bits Test Conditions/Comments Min Typ Max Unit
SLOTA_LED_OFFSET
1
0x30 [7:0] Delay from power-up to LEDA rising edge 23 63 µs
SLOTB_LED_OFFSET
1
0x35 [7:0] Delay from power-up to LEDB rising edge 23 63 µs
SLOTA_PERIOD
2
0x31 [7:0] Time between LED pulses in Time Slot A; SLOTx_AFE_WIDTH = 4 µs 19 63 µs
SLOTB_PERIOD
2
0x36 [7:0] Time between LED pulses in Time Slot B; SLOTx_AFE_WIDTH = 4 µs 19 63 µs
t
1
N/A N/A Compute time for Time Slot A 68 µs
t
2
N/A N/A Compute time for Time Slot B 20 µs
t
SLEEP
N/A N/A Sleep time between sample periods 222 µs
1
Setting the SLOTx_LED_OFFSET less than the specified minimum value can cause failure of ambient light rejection for large photodiodes.
2
Setting the SLOTx_LED_PERIOD less than the specified minimum value can cause invalid data captures.
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