ADPD1081 datasheet(46/74 Pages) AD

46 / 74 page

ADPD1080/ADPD1081

Data Sheet

Rev. C | Page 46 of 74

PULSE CONNECT MODE

In pulse connect mode, the photodiode input connections are

pulsed according to the timing set up in the LED pulse timing

registers. In this mode, if the LED pulse timing is set up to provide

a 2 µs LED pulse, the device pulses the connection to the photo-

diode input for 2 µs instead of providing a 2 µs LED pulse. This

mode is an alternate to TIA ADC mode, allowing the entire

signal path, including the BPF and integrator, to measure ambient

light as well as other types of measurements with different types

of sensors (for example, electrocardiogram (ECG)).

To enable pulse connect mode, the device is configured identically

to normal mode, except that Register 0x14, Bits[3:2] = 0 for

Time Slot B, and Register 0x14, Bits[1:0] = 0 for Time Slot A.

SYNCHRONOUS ECG AND PPG MEASUREMENT

USING TIA ADC MODE

In wearable devices developed for monitoring the health care of

patients, it is often necessary to have synchronized measurements

of biomedical signals. For example, a synchronous measurement

of patient ECG and PPG can determine the pulse wave transit

time (PWTT), which can then estimate blood pressure.

The circuit shown in Figure 51 shows a synchronous ECG and

PPG measurement using the AD8233 and the ADPD1080. The

AD8233 implements a two-pole high-pass filter (HPF) with a

cutoff frequency at 0.3 Hz, and a two-pole low-pass filter (LPF)

with a cutoff frequency of 37 Hz. The output of the AD8233 is

fed to one of the current inputs of the ADPD1080 through a

200 kΩ resistor to convert the voltage output of the AD8233

into a current.

The ADPD1080 is configured to alternately measure the

photodiode signal and the ECG signal from the AD8233 on

consecutive time slots to provide fully synchronized PPG and

ECG measurements. Data can be read out of the on-chip FIFO

or straight from data registers. The ADPD1080 channel used to

process the ECG signal is set up in TIA ADC mode, and the input

bias voltage must be set to the 0.90 V setting using Bits[5:4] of

on Time Slot B. The TIA gain setting can be set to optimize the

dynamic range of the signal path. The channel used to process

the PPG signal is configured in its normal operating mode.

Figure 50 shows a plot of a synchronized ECG and PPG

measurement using the AD8233 with the ADPD1080.

6000

6500

7000

7500

8000

8500

9000

9500

10000

50000

50500

51000

51500

52000

PPG

ECG

SAMPLE RATE (ms)

Figure 50. Plot of Synchronized ECG and PPG Waveforms

1.8V

+IN

–IN

HPDRIVE

+VS

HPSENSE

IAOUT

REFIN

GND

AC/DC

SDN

LOD

RLD

RLDFB

OUT

OPAMP+

OPAMP–

REFOUT

AD8233

6.8nF

1MΩ

250kΩ

2.7nF

100kΩ

180kΩ

1MΩ

10MΩ

0.1µF

10MΩ

1nF

10MΩ

0.1µF

360kΩ

4.7µF

TO DIGITAL

INTERFACE

4.7µF

RLD SDN

1.8V

50kΩ

PD5

PD1

PDC

LEDX1

VLED

VREF

1µF

AGND

DGND

LGND

0.1µF

DVDD

AVDD

1.8V

10kΩ

1.8V

SCL

SDA

GPIO0

GPIO1

TO DIGITAL

INTERFACE

ADPD1080

Figure 51. Synchronized PPG and ECG Measurement Using the ADPD1080 with the AD8233

ADPD1081 Datasheet(PDF) 46 Page - Analog Devices

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