ADPD4200 datasheet(25/93 Pages) AD

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Data Sheet

ADPD4200

APPLICATIONS INFORMATION

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Rev. 0 | 25 of 93

OPERATING MODE OVERVIEW

The ADPD4200 is effectively a charge measuring device that

can interface with many different sensors enabling synchronous

measurements of PPG, electrocardiography (ECG), electrodermal

activity (EDA), impedance, capacitance, and temperature measure-

ments. A selection of operating modes are built into the device to

optimize each of the different sensor measurements supported.

SINGLE INTEGRATION MODE

Single integration mode is used for a single integration of incoming

charge per ADC conversion and is the most common operating

mode for the ADPD4200. In single integration mode, most of the

dynamic range of the integrator is used when integrating the charge

from the sensor response to a single stimuli event, for example,

an LED pulse. There is also a multiple integration mode available

for situations with very small sensor responses (see the Multiple

Integration Mode section for more information).

Using LED as Stimulus

Single integration 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 24 shows an

example of a typical PPG measurement circuit.

The MOD_TYPE_x value is left at the default value of 0 so that

the TIA is continuously connected to the input of the TIA. Set the

PRECON_x bit field 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 + 250 mV to apply a 250 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 bit field for maximum dynamic range.

The LED pulse is controlled with the LED_OFFSET_x and LED_

WIDTH_x bit field. 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 widths

are either 2 μs or 3 μs when using the BPF. Shorter LED pulse

widths provide the greatest amount of ambient light rejection and

the lowest power dissipation. The period is automatically calculated

by the ADPD4200. The automatic calculation is based on the

integration width selected and the number of ADC conversions. To

use the automatic calculation, leave the MIN_PERIOD_x bit field 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 bit

field to enable a longer period. In single integration mode for PPG

measurements using 2 μs or 3 μs, the automatic period calculation

Period= 2+2×INTEG_WIDTH+

Number of Cℎannels Enabled×

ADC_COUNT+1

The integration pulses are controlled with the INTEG_OFFSET_x

and INTEG_WIDTH_x bit fields. It is recommended that an integra-

tion width of 1 μs greater than the LED width be used because

the signal spreads due to the response of the BPF. By setting

the integration width 1 μs wider than the LED width, a maximum

amount of charge from the incoming signal is integrated.

The number of ADC conversions defaults to a single ADC con-

version. However, oversampling is available for increased SNR.

The ADC conversions can be set to 1, 2, 3, or 4 based on the

ADC_COUNT_x bit field. If two channels are enabled, Channel 1

occurs first, followed by Channel 2. The total number of pulses

is equal to NUM_INT_x × NUM_REPEAT_x. In single integration

mode, NUM_INT_x = 1 for a single integration sequence per ADC

conversion. Therefore, the total number of pulses is controlled by

NUM_REPEAT_x. Increasing the number of pulses reduces the

noise floor of the measurement by a factor of √n, where n is the

total number of pulses.

Figure 25 shows the timing operation where a single integration

cycle is used per ADC conversion. Table 16 details the relevant

registers using single integration mode for a PPG measurement.

Figure 24. Typical PPG Measurement Circuit

ADPD4200 Datasheet(PDF) 25 Page - Analog Devices

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