ADPD4100 datasheet(28/101 Pages) AD

28 / 101 page

ADPD4100/ADPD4101

Data Sheet

Rev. 0 | Page 28 of 101

APPLICATIONS INFORMATION

OPERATING MODE OVERVIEW

The ADPD4100/ADPD4101 are effectively charge measuring

devices that can interface with many different sensors, enabling

optical and electrical measurements in various healthcare,

industrial and consumer applications, such as PPG, ECG, EDA,

impedance, capacitance, and temperature measurements, and

gas, smoke, and aerosol detection. A selection of operating

modes are built into the device to optimize each of the different

sensor measurements supported.

ANALOG INTEGRATION MODE

Analog integration mode refers to the modes of operation

where the incoming charge from the sensor response to a

stimulus event is integrated by the integrator in the

ADPD4100/ADPD4101. There are several different analog

integration modes, including continuous connect mode, float

mode, pulse connect modulation, modulation of stimulus

source, multiple integration mode, and sleep float mode.

There is also a digital integration mode, where the integrator is

configured as a buffer and the ADC samples are digitally

integrated (see the Digital Integration Mode section for more

information).

Connection Modulation Types

The ADPD4100/ADPD4101 use three different types of modula-

tion connections to a sensor, controlled by the MOD_TYPE_x

bits. Table 17 shows the different functions controlled by MOD_

TYPE_x. The default mode of operation is MOD_TYPE_x = 0,

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

the mode used as described in the Continuous Connect Mode

section.

Table 17. Modulation Connections Based on MOD_TYPE_x

MOD_TYPE_x

(Decimal)

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.

Continuous Connect Mode

Continuous connect mode is used for a single analog integration

of incoming charge per ADC conversion and is the most common

operating mode for the ADPD4100/ADPD4101. In continuous

connect mode, most of the dynamic range of the integrator is

used when integrating the charge from the sensor response to a

single stimulus event, for example, an LED pulse. The TIA is

continuously connected to the inputs after the precondition

period by setting MOD_TYPE_x to 0. Therefore, the input

connection is not modulated in continuous connect mode.

Continuous connect mode is the typical operating mode used

for a PPG measurement, where an LED is pulsed into human

tissue and the resultant charge from the photodiode response is

integrated and subsequently converted by the ADC. Figure 34

shows an example of a typical PPG measurement circuit.

TIA

ADC

BPF

RINT

6.3pF

TIA_VREF

SWITCH

VLED1

CVLED

VCx

INx

LEDx

RINT

Figure 34. Typical PPG Measurement Circuit

Set the PRECON_x bits to 0x5 to set the anode of the photodiode

to the TIA_VREF potential during the preconditioning period.

The VCx pin is connected to the cathode of the photodiode and

is set to TIA_VREF + 215 mV to apply a 215 mV reverse bias

across the photodiode, which reduces the photodiode capacitance

and reduces the noise of the signal path. Set TIA_VREF to

1.27 V using the AFE_TRIM_VREF_x bits for maximum

dynamic range.

The LED pulse is controlled with the LED_OFFSET_x and LED_

WIDTH_x bits. The default LED offset (LED_OFFSET_x =

0x10) is 16 μs from the end of the preconditioning period and is

suitable for most use cases. Recommended LED pulse width is 2

μs when using the BPF. Short LED pulse widths provide the

greatest amount of ambient light rejection and the lowest power

dissipation. The period is automatically calculated by the

ADPD4100/ADPD4101. The automatic calculation is based on

the integration width selected and the number of ADC conver-

sions. To use the automatic calculation, leave the MIN_PERIOD_x

bits at its default value of 0. If a longer period is desired, for

example, if a specific pulse frequency is desired, use the

MIN_PERIOD_x bits to enable a longer period.

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