ADPD4200 datasheet(29/93 Pages) AD

29 / 93 page

Data Sheet

ADPD4200

APPLICATIONS INFORMATION

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Rev. 0 | 29 of 93

Connection Modulation

The ADPD4200 uses three different types of modulation connec-

tions to a sensor, controlled by the MOD_TYPE_x bits. Table 18

shows the different functions controlled by this register. The default

mode of operation is MOD_TYPE_x = 0, which is the mode where

there is no modulation of the input connection, and is the mode

used as described in the Using LED as Stimulus section.

Table 18. Modulation Connections Based on MOD_TYPE_x

MOD_TYPE_x Connect Function

TIA is continuously connected to INx after the precondition

period. There is no modulation of the input connection.

Float mode operation. The TIA is connected to INx only during

the modulation pulse and disconnected (floated) between pulses.

Nonfloat mode connection modulation. The TIA is connected

to INx during the modulation pulse and connected to the

precondition value between pulses.

Float Mode Operation

The ADPD4200 has 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 ADPD4200 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 integrat-

ed 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 op-

erate 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 are 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 SUB-

TRACT_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, re-

peats 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 27, 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

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