ADPD4100 datasheet(32/101 Pages) AD

32 / 101 page

ADPD4100/ADPD4101

Data Sheet

Rev. 0 | Page 32 of 101

Float Mode Operation

The ADPD4100/ADPD4101 have a unique operating mode, float

mode, that allows high SNR at low power in low light situations.

In float mode, the photodiode is first preconditioned to a known

state. Then, the photodiode anode is disconnected from the

receive path of the device for a preset amount of float time.

During the float time, light falls on the photodiode, either from

ambient light, pulsed LED light, or a combination of the two

depending on the operating mode. Charge from the sensor is

stored directly on the capacitance of the sensor, CPD. At the end

of the float time, the photodiode is switched into the receive

path of the ADPD4100/ADPD4101 and an inrush of the

accumulated charge occurs, which is then integrated, allowing

the maximum amount of charge to be processed per pulse with

the minimum amount of noise added by the signal path. The

charge is integrated externally on the capacitance of the

photodiode for as long as it takes to acquire maximum charge,

independent of the amplifiers of the signal path, effectively

integrating noise free charge. Float mode allows the user the

flexibility to increase the amount of charge per measurement by

either increasing the LED drive current or by increasing the

float time.

In float mode, the signal path bypasses the BPF and uses only

the TIA and integrator. The BPF is bypassed because the shape

of the signal produced when transferring the charge from the

photodiode by modulating the connection to the TIA can differ

across devices and conditions. A filtered signal from the BPF is

not able to be reliably aligned with the integration sequence.

Therefore, the BPF cannot be used. In float mode, the entire

charge transfer is integrated in the negative cycle of the

integrator, and the positive cycle cancels any offsets.

Float LED Mode for Synchronous LED Measurements

Float LED mode is desirable in low signal conditions where the

CTR is below 5 nA/mA. In addition, float mode is an ideal

option when limiting 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 per photon

attainable.

In float LED mode, multiple pulses are used to cancel electrical

offsets, drifts, and ambient light. To achieve this ambient light

rejection, an even number of equal length pulses is used. For

every pair of pulses, the LED flashes in one of the pulses and

does not flash in the other. The return from the combination of

the LED, ambient light, and 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 subtracts Pulse 1 and Pulse 4. To gain additional SNR,

use multiple groups of four pulses.

For each group of four pulses, the settings of LED_DISABLE_x

determine if the LED flashes in a specific pulse position. Which

pulse positions are added or subtracted is configured in the

SUBTRACT_x bits. 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.

With NUM_INT_x set to 1, NUM_REPEAT_x determines the

total number of pulses. For example, if the device is set up for

32 pulses, the four-pulse sequence, as defined in

LED_DISABLE_x and SUBTRACT_x, repeats eight times and a

single register or FIFO write of the final value based on

32 pulses executes.

In float mode, the MIN_PERIOD_x bits must be set to control

the pulse period. The automatic period calculation is not

designed to work with float mode. Set the MIN_PERIOD_x

bits, in 1 μs increments, to accommodate the amount of float

time and connect time required.

Placement of the integration sequence is such that the negative

phase of the integration is centered on the charge transfer

phase. The TIA is an inverting stage. Therefore, placing the

negative phase of the integration during the transferring of the

charge from the photodiode causes the integrator to increase

with the negative going output signal from the TIA.

In the example shown in Figure 37, the LED flashes in the

second and third pulses of the four-pulse sequence.

SUBTRACT_x is set up to add the second and third pulses

while subtracting the first and fourth pulses, effectively

cancelling out the ambient light, electrical offsets, and drift.

Table 20 details the relevant registers for float LED mode.

ADPD4100 Datasheet(PDF) 32 Page - Analog Devices

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