AD8233ACBZ-R7 datasheet(29/30 Pages) AD

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AD8233

Data Sheet

Rev. 0 | Page 28 of 29

SYNCHRONIZED ECG AND PPG MEASUREMENT

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

a ECG and photoplethysmograph (PPG) can be used to determine

the pulse wave transit time (PWTT), which can then be used to

estimate blood pressure.

The circuit shown in Figure 77 shows a synchronous ECG and

PPG measurement using the AD8233 and the ADPD105

photometric front end. The AD8233 implements a two-pole,

high-pass filter with a cutoff frequency of 0.3 Hz, and a two-pole,

low-pass filter with a cutoff frequency of 37 Hz. The output of

the AD8233 is fed to one of the current inputs of the ADPD105

through a 50 kΩ resistor to convert the voltage output of the

AD8233 into a current. The PPG signal is acquired by the

ADPD105, which is a complete optical transceiver with

integrated LED drivers, multiple photodiode current inputs, an

integrated, 14-bit, successive approximation (SAR) ADC, and a

FIFO. In the circuit shown, the chip scale ADPD105 is used; the

ADPD105 is a two input device. The ADPD105 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 the data registers. The

ADPD105 channel that processes the ECG signal must be set up

in either pulse connect mode or transimpedance amplifier (TIA)

ADC mode, and the input bias voltage must be set to the 0.9 V

setting. 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 76

shows a plot of a synchronized ECG and PPG measurement

using the AD8233 with the ADPD105.

0.5

1.0

1.5

2.0

2.5

3.0

3.5

TIME (Seconds)

SHOWN WITH

fSAMPLE = 100Hz

PPG

ECG

Figure 76. Synchronous ECG and PPG Measurement Using the AD8233 with

the ADPD105

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

PD3-4

PD1-2

PDC

LEDX1

VLED

VREF

1µF

AGND

DGND

LGND

0.1µF

DVDD

AVDD

1.8V

10kΩ 10kΩ

1.8V

SCL

SDA

GPIO0

GPIO1

TO DIGITAL

INTERFACE

ADPD105

Figure 77. Synchronous ECG and PPG Measurement Circuit

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