ADPD1081 datasheet(19/74 Pages) AD

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Data Sheet

ADPD1080/ADPD1081

Rev. C | 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 I2C 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:

tA (µs) = SLOTA_LED_OFFSET + nA × SLOTA_PERIOD

where nA is the number of pulses for Time Slot A (Register 0x31,

Bits[15:8]).

tB (µs) = SLOTB_LED_OFFSET + nB × SLOTB_PERIOD

where nB 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

t1 and t2 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

SLEEP

TIME SLOT A

TIME SLOT B

ACTIVE

1/fSAMPLE

nA PULSES

nB PULSES

Figure 17. Time Slot Timing Diagram (fSAMPLE is the sampling frequency (Register 0x12, Bits[15:0]).)

Table 15. LED Timing and Sample Timing Parameters

Parameter

Bits

Test Conditions/Comments

Min

Typ

Max

Unit

SLOTA_LED_OFFSET1

0x30

[7:0]

Delay from power-up to LEDA rising edge

µs

SLOTB_LED_OFFSET1

0x35

[7:0]

Delay from power-up to LEDB rising edge

µs

SLOTA_PERIOD2

0x31

[7:0]

Time between LED pulses in Time Slot A; SLOTx_AFE_WIDTH = 4 µs

µs

SLOTB_PERIOD2

0x36

[7:0]

Time between LED pulses in Time Slot B; SLOTx_AFE_WIDTH = 4 µs

µs

N/A

Compute time for Time Slot A

µs

N/A

Compute time for Time Slot B

µs

tSLEEP

N/A

Sleep time between sample periods

222

µs

Setting the SLOTx_LED_OFFSET less than the specified minimum value can cause failure of ambient light rejection for large photodiodes.

Setting the SLOTx_LED_PERIOD less than the specified minimum value can cause invalid data captures.

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