ADPD7008 datasheet(13/71 Pages) AD

13 / 71 page

Data Sheet

ADPD7008

THEORY OF OPERATION

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Rev. 0 | 13 of 71

OPTICAL SIGNAL CHAIN

The optical signal path stimulates up to eight LEDs and measures

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

optical time slots enable 12 separate optical measurements per

sampling period.

The analog inputs can be driven single-ended or in differential pairs.

The four analog inputs can be multiplexed into a single channel or

two independent channels, enabling simultaneous sampling of two

sensors.

The optical signal chain consists of a TIA, an integrator that can

also be configured as a buffer depending on the register setting,

and an ADC. The digital block provides multiple operating modes,

programmable timing, and block averaging.

Two independent LED drivers are provided that can each drive up

to 200 mA. Two LED drivers can be enabled in any time slot and

can be programmed from 1.57 mA to 200 mA monotonically, with a

7-bit register setting. The LED drivers enabled in any time slot can

provide a combined maximum LED current of 400 mA.

When making optical measurements, 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 optical measurement path

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 analog signal path of the optical signal chain consists of four

current inputs that can be configured as single-ended or differential

pairs into one of four independent channels. The four channels can

be sampled simultaneously for applications that require instantane-

ous sampling of four sensors.

Analog Input Multiplexer

The optical signal chain supports four analog input pins. Each input

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

pair. Figure 15 shows a single representation of the input switch

matrix, which allows a programmable connection to the four optical

channels. Each pair of inputs has a duplicate of this multiplexer: IN1

and IN2, and IN3 and IN4. The connections are programmable per

time slot.

The PAIR12 and PAIR34 bits select whether the matching input

pair is used as two single-ended inputs or as a differential pair.

This selection is valid for all active time slots. The INP12_x and

INP34_x bits specify whether the input pair is enabled during the

corresponding time slot and, if enabled, which input is connected to

which optical channel. Note that Channel 1 and Channel 2 support

single-ended or differential inputs, while Channel 3 and Channel 4

only support a single-ended input.

The sleep conditions are used for any inputs that are not ena-

bled. Sleep conditions are determined by the INP_SLEEP_12 and

INP_SLEEP_34 bits, which specify the state for the input pairs

during sleep and when the inputs are not active. Inputs are only

considered active during the precondition and pulse regions for time

slots where they are enabled.

Preconditioning of the sensor connected to the input is provided

to set the operating point at the input before sampling. There

are several different options for preconditioning determined by the

PRECON_x bits. The PRECON_x bits are provided for each time

slot to specify the precondition for enabled inputs or input pairs

during the corresponding time slot. Preconditioning options include

floating the inputs, VC1, an internal voltage reference signal for

the TIA (TIA_VREF), a TIA input, and shorting the input pair. The

preconditioning time at the start of each time slot is programmable

using the PRE_WIDTH_x bits. The default preconditioning period is

8 µs.

The block diagram in Figure 15 shows the bias levels that can be

switched into the input connections during sleep and precondition-

ing. These connections are not available during the sampling phase

of a time slot in which the input is selected.

Figure 15. Switch Matrix Block Diagram

Ambient Light Cancellation

The ADPD7008 has three modes to perform ambient light cancella-

tion, as follows:

►

Coarse tuning only

►

Coarse and fine tuning loop

►

External microcontroller unit (MCU) control

Coarse tuning only mode and coarse and fine tuning loop mode

are automatically controlled by the ADPD7008 without any software

assistance. External MCU control mode allows tuning the ambient

light rejection through an external algorithm.

Use the AMBIENT_CANCELLATION_x bits to choose the mode.

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