ADPD4200 datasheet(13/93 Pages) AD

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Data Sheet

ADPD4200

THEORY OF OPERATION

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Rev. 0 | 13 of 93

INTRODUCTION

The ADPD4200 operates as a complete multimodal sensor front

end, stimulating up to four LEDs and measuring the return signal on

up to four 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 four analog inputs are multiplexed into a single channel or

two independent channels, enabling simultaneous sampling of two

sensors.

The analog front end (AFE) consists of a TIA, band-pass filter

(BPF), integrator, and ADC. The digital block provides multiple

operating modes, programmable timing, two GPIOx pins, block

averaging, and a selectable second-order through fourth-order cas-

caded integrator comb (CIC) filter. Four 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

2 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 four 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 SPI communi-

cation interface, programmable pulsed LED current sources, and

pulsed voltage sources for sensors that require voltage excitation.

When making optical measurements, the ADPD4200 provides

60 dB of 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 ADPD4200

can also interface with voltage output sensors with a series resistor

placed between the sensor output and the ADPD4200 inputs to

convert the voltage to a current. The ADPD4200 produces a high

SNR for relatively low LED power while greatly reducing the effect

of ambient light on the measured signal.

ANALOG SIGNAL PATH

The ADPD4200 analog signal path consists of four 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 cut-

off 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 14, RF is the TIA feedback resistor, and RINT is the

series resistor to the input of the integrator.

Figure 14. Analog Signal Path Block Diagram

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