ADPD4100 datasheet(15/101 Pages) AD

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Data Sheet

ADPD4100/ADPD4101

Rev. 0 | Page 15 of 103

THEORY OF OPERATION

INTRODUCTION

The ADPD4100/ADPD4101 operate as a complete multimodal

sensor front end, stimulating up to eight LEDs and measuring

the return signal on up to eight separate current inputs. Twelve

time slots are available, enabling 12 separate measurements per

sampling period. The analog inputs can be driven single-ended

or in differential pairs. The eight analog inputs are multiplexed

into a single channel or two independent channels, enabling

simultaneous sampling of two sensors.

The AFE consists of a TIA, band-pass filter (BPF), integrator,

and analog-to-digital converter (ADC). The digital block

provides multiple operating modes, programmable timing, four

general-purpose input/output (GPIO) pins, block averaging,

and a selectable second- through fourth-order cascaded

integrator comb (CIC) filter. Eight independent LED drivers are

provided that can each drive up to 200 mA. Four LED drivers can

be enabled in any time slot and can be programmed from

1.5 mA to 200 mA monotonically, with a 7-bit register setting.

The LED drivers enabled in any time slot can provide a total

combined maximum of 400 mA of LED current.

The core circuitry provides stimulus to the sensors connected

to the inputs of the device and measures the response, storing

the results in discrete data locations. The eight inputs can drive

two simultaneous input channels, either in a single-ended or

differential configuration. Data is read directly by a register or

through a FIFO method. This highly integrated system includes

an analog signal processing block, digital signal processing block,

an I2C communication interface on the ADPD4101 or an SPI

port on the ADPD4100, programmable pulsed LED current

sources, and pulsed voltage sources for sensors that require

voltage excitation.

When making optical measurements, the ADPD4100/ADPD4101

provide 60 dB of ac ambient light rejection using a synchronous

modulation scheme with pulses as short as 1 μs combined with

a BPF. Ambient light rejection is automatic without the need of

external control loops, dc current subtraction, or digital

algorithms.

The LED driver is a current sink and is independent from the LED

supply voltage and the LED type. The inputs can be connected

to any sensor that provides currents up to 200 μA. The

ADPD4100/ADPD4101 can also interface with voltage output

sensors with a series resistor placed between the sensor output and

the ADPD4100/ADPD4101 inputs to convert the voltage to a

current. The ADPD4100/ADPD4101 produce a high SNR for

relatively low LED power while greatly reducing the effect of

ambient light on the measured signal.

ANALOG SIGNAL PATH

The ADPD4100/ADPD4101 analog signal path consists of eight

current inputs that can be configured as single-ended or

differential pairs into one of two independent channels. The

two channels can be sampled simultaneously for applications

that require instantaneous sampling of two sensors. Each

channel contains a TIA with programmable gain, a BPF with a

high-pass corner at 100 kHz and a low-pass cutoff frequency of

390 kHz, and an integrator capable of integrating ±7.5 pC per

sample. Each channel is time multiplexed into a 14-bit ADC. In

Figure 16, RF is the TIA feedback resistor, and RINT is the series

resistor to the input of the integrator.

TIA

ADC

BPF

6.3pF

TIA

RINT

6.3pF

IN1

IN3

IN2

IN4

IN5

IN7

IN6

IN8

RINT

BPF

Figure 16. Analog Signal Path Block Diagram

Analog Input Multiplexer

The ADPD4100/ADPD4101 support eight analog input pins.

Each input can be used as a single-ended input or as part of a

differential pair. Figure 17 shows a single representation of the

input switch matrix, which allows programmable connection to

the two AFE channels. Each pair of inputs has an exact duplicate

of this multiplexer: IN1 and IN2, IN3 and IN4, IN5 and IN6, and

IN7 and IN8. The connections are programmable per time slot.

TIA_VREF

IN1

IN2

THERE ARE FOUR

COPIES OF SWITCH MATRIX.

ONE COPY FOR EACH

INPUT PAIR.

TIA

*ALL BIAS CONNECTIONS SHOWN ARE ONLY AVAILABLE DURING

SLEEP AND PRECONDITIONING PERIODS. THE SWITCHES TO

THESE BIAS LEVEL ARE OPEN DURING TIME SLOTS WITH THE

RESPECTIVE INPUTS SELECTED.

Figure 17. Analog Input Multiplexer

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