Wearables Make Multiparameter Vital Signs Monitoring Tech Easier

The past decade has seen tremendous advancements in mobile phones, wearables and digital health. Especially with the continuous development of Electronic technology and new breakthroughs in technologies such as cloud computing, artificial intelligence (AI), Internet of Things (IoT) and 5G, digital health care has been rapidly expanded and adopted. Some vital signs monitoring (VSM) features are already built into phones, watches, and other smart wearables, making them more widely available.

Introduction

The past decade has seen tremendous advancements in mobile phones, wearables and digital health. Especially with the continuous development of electronic technology and new breakthroughs in technologies such as cloud computing, artificial intelligence (AI), Internet of Things (IoT) and 5G, digital health care has been rapidly expanded and adopted. Some vital signs monitoring (VSM) features are already built into phones, watches, and other smart wearables, making them more widely available. The growing awareness of health has sparked the need for small but highly accurate devices that can measure various vital signs and health indicators such as body temperature, heart rate, respiratory rate, blood oxygen saturation (SpO2), blood pressure and physical Element. The COVID-19 pandemic has led to a surge in demand for multi-parameter vital signs (including body temperature, SpO2 and heart rate) monitoring devices in hospitals and at home. Demand for small and convenient health tracking devices, preferably smart wearables, has reached new heights.

Adding multiple detection capabilities to such a small device is challenging because it requires smaller form factors, lower power consumption, and multiparameter capabilities with significantly improved performance. However, these challenges can now be met with a single analog front end (AFE) solution. This new AFE can be used as a multi-parameter vital signs monitoring center, supporting simultaneous measurements. Its low noise, high signal-to-noise ratio (SNR), small size, and low power consumption can significantly improve medical devices, especially wearable technology. For doctors, patients and consumers, it makes vital signs monitoring easier than ever, and offers higher performance, longer battery life, and greater accuracy without the hassle and discomfort of multiple devices feel. This article discusses some of the breakthrough capabilities and features of this single analog front-end solution.

Overview of the new analog front end

The ADPD4100/ADPD4101 are multimode sensor AFEs with 8 analog inputs supporting up to 12 programmable time slots. These 12 time slots support 12 independent measurements within one sampling period. Eight analog inputs are multiplexed into one channel or two independent channels, enabling simultaneous sampling of two sensors in single-ended or differential configurations. 8 LED drivers can drive up to 4 LEDs simultaneously. These LED drivers are current sinks independent of LED supply voltage and LED type. The chip has two pulsed voltage sources for voltage excitation. The signal path of the new AFE includes transimpedance amplifier (TIA), bandpass filter (BPF), integrator (INT), and analog-to-digital converter (ADC) stages. The digital blocks offer multiple operating modes, programmable timing, general-purpose input/output (GPIO) control, block averaging, and optional second- to fourth-order concatenated-integral-comb (CIC) filters. Data is read directly from the data register, or via a first-in, first-out (FIFO) method.

There are two versions of this new AFE. One has an I2C communication interface and the other has an SPI port. One of the advantages of the ADPD4100/ADPD4101 is related to optical measurements. Its excellent automatic ambient light rejection benefits from the use of pulses as short as 1 µs in a synchronous modulation scheme combined with a BPF, eliminating the need for external control loops, DC current subtraction or digital algorithms. Use a decimation factor higher than 1 to improve the output SNR. It has a subsampling feature that allows selected time slots to operate at a lower sampling rate than the programmed sampling rate, saving power (power consumption is proportional to the sampling rate). It also has a TIA upper limit detection feature that utilizes a voltage comparator on the TIA output pin to set an interrupt bit when the TIA input exceeds the typical operating limit.

The ADPD4100/ADPD4101 are ideal hubs for a variety of electrical and optical sensors in wearable health and fitness devices for heart rate and heart rate variability (HRV) monitoring, blood pressure estimation, stress and sleep tracking, and SpO2 measurement. The multiple operating modes of this new multiparameter VSM AFE can accommodate different sensor measurements in healthcare applications, including but not limited to photoplethysmography (PPG), electrocardiogram (ECG), electrodermal activity (EDA), body composition , respiration, temperature and ambient light measurements.

PPG measurement

PPG measurements detect changes in blood volume in the tissue microvascular bed associated with each cardiac cycle. The total absorption of light correlates with changes in blood volume caused by systolic and diastolic events, resulting in the PPG signal. PPG measurements are performed as follows: LED light pulses are fired into human tissue, then the reflected/transmitted light is collected with a photodiode and converted into photocurrent. The ADPD4100/ADPD4101 process and measure the photocurrent and generate a digital PPG signal. For different PPG measurement situations, the AFE can be flexibly configured into one of four operating modes without any changes to the hardware connection: continuous connection mode, multi-integration mode, floating mode, and digital integration mode.

Wearables Make Multiparameter Vital Signs Monitoring Tech Easier
Figure 1. Typical PPG Circuit

Continuous connection mode

Continuous connection mode is a typical mode for PPG measurements. It provides the best ambient light rejection performance and high SNR. This mode works well at charge transfer ratios (CTR, the ratio of photocurrent to LED current) as low as 5 nA/mA to 10 nA/mA, and provides DC SNR of 95 dB to 100 dB. These performance levels can be improved by increasing the decimation factor. This mode uses the full analog signal path, i.e. TIA + BPF + INT + ADC. The incoming charge is integrated once per ADC conversion. During a single excitation event such as PPG, most of the dynamic range of the integrator is used when integrating the charge from the sensor response. After the preconditioning period, the TIA is continuously connected to the input, so the input signal is not modulated. To reduce noise, the anode of the photodiode is preconditioned to the TIA’s reference voltage (TIA_VREF). Typically TIA_VREF is set to 1.27 V for maximum dynamic range of the TIA. The cathode of the photodiode is connected to the cathode voltage source (VCx) pin, and the device is typically set to supply TIA_VREF + 215 mV to the photodiode cathode to create a reverse bias of 215 mV on the photodiode. This reduces signal path noise and photodiode capacitance. In this mode, the typical LED pulse width is 2µs. Short LED pulses provide the best ambient light rejection. When using multiple LED pulses, every doubling of the number of pulses improves SNR by 3 dB. Since chopping removes the low frequency noise components of the integrator, it is common to enable integrator chopping for the highest SNR. The higher the selected TIA gain, the lower the input-referred noise, but the dynamic range of the TIA is reduced. The dynamic range of the TIA is calculated as follows: Dynamic Range = (TIA_VREF)/(TIA Gain). To increase the ADC saturation level, the TIA gain can be decreased, or the integrator resistance can be increased. Choosing a higher integrator resistance reduces noise, but choosing a lower integrator resistance increases the ambient light margin.

Multiple integration mode

Multi-integration mode is roughly the same as continuous-connect mode, except that the incoming charge is integrated multiple times per ADC conversion. This mode can be used to achieve high SNR in low light situations because it uses only a small amount (sometimes less than 50%) of dynamic range for each stimulus event. It can take advantage of a larger dynamic range of the integrator due to the multiple integrations performed before the ADC conversion. Doubling the number of integrations per ADC conversion improves SNR by 3 dB, which is the same as doubling the number of pulses. This mode is typically used for small inputs, so the highest TIA gain can be selected. This mode is used where CTR is below 5 nA/mA and good ambient light rejection is required.

floating mode

Floating mode is also used in low light conditions for high SNR. Floating mode supports noise-free charge accumulation on the photodiode. The photodiode is disconnected from the AFE (hence the term “floating”) to accumulate photo-induced charge in a noise-free manner. The AFE is then connected back to the photodiode, the charge on the photodiode is flooded into the AFE, and the integration is done in a way that allows the maximum amount of charge to be handled per pulse with a minimal amount of noise added to the signal path. Due to the short modulation pulses, the charge dump occurs quickly. Therefore, the noise increase caused by the signal path is small. Also, the floating time can be increased for higher signal levels, but there is a limit to the amount of charge that the photodiode capacitor can accumulate. In this mode, the bandpass filter (BPF) is bypassed because when the charge in the photodiode is transferred by modulating the TIA connection, the shape of the resulting signal can vary depending on the device and conditions. To reliably align the signal with the integration sequence, the BPF must be bypassed. This mode does not provide good ambient light rejection and is limited by photodiode capacitance, but it provides power efficient and less noisy measurements in very low light conditions.

Floating mode and multiple integration mode selection under low light conditions

in CTR
In PPG measurements, the multiple integration mode is preferred when the photodiode has leakage or when there is a lot of ambient light. Leaky photodiodes cannot be used in floating mode because the charge will leak rather than build up before fast charge transfer occurs. If the ambient light is very strong, the floating mode is not suitable because the ambient light will dominate the amount of charge that can be stored on the photodiode. Multiple integrations inherently provide excellent ambient light rejection due to the use of BPF and short LED pulses.

Digital integration mode

All the modes mentioned above use an integrator to integrate the incoming charge. The ADC samples can also be digitally integrated through the digital integration mode. To achieve digital integration, the integrator is turned into a buffer. The digital integration mode works in two areas. In bright areas, the LEDs send pulses, while in dark areas, the LEDs go out. ADC samples are collected in bright and dark regions at 1 µs intervals and digitally integrated. The signal is calculated by subtracting the integral of the dark samples from the bright samples. This mode can support longer LED pulses. Therefore, this is a typical mode of operation for applications where the photodiode response time is slow and longer pulses are required. BPF is bypassed and turned off. Digital integration mode provides the best power efficiency and achieves the highest SNR levels. However, due to the use of longer LED pulses and bypassing the BPF, its ambient light rejection performance is not as good as the continuous connection mode. Digital integration mode does not support simultaneous sampling of two channels in the same time slot. Digital integration mode supports 100+ dB DC SNR.

Advantages and disadvantages of digital integration mode

As mentioned earlier, the typical operating mode for PPG measurements is continuous connection mode, as it provides high SNR and excellent ambient light rejection at CTRs greater than 5 nA/mA. However, digital integration mode achieves the highest SNR levels and provides the best SNR efficiency per watt. Therefore, if ambient light is not an issue for the application and the target DC SNR is higher than 85 dB, then digital integration mode can be selected to effectively achieve high SNR. If the target DC SNR is lower than 85 dB, the power savings of digital integration mode compared to continuous connection mode are not significant.

In summary, if the photodiode requires longer pulses due to its slow response time, or if it is not necessary to sample both channels simultaneously within a time slot, then digital integration mode can be selected.

Also, if ambient light is not an issue and the target DC SNR is higher than 85 dB, then choosing digital integration mode will enable high power efficiency.

PPG application

Given the COVID-19 pandemic, PPG applications have become more important in vital sign monitoring and health diagnosis. Furthermore, multiple indicators are crucial for detection. For example, some important vital sign measurements include heart rate monitoring (HRM), HRV, and oxygen saturation (SpO2, which can be measured by pulse oximetry and blood pressure).

Optical and non-invasive SpO2 monitoring (also known as pulse oximetry) has become very valuable in the detection of hypoxia in COVID-19 patients. Hypoxia refers to the lack of oxygen supply to body tissues and is one of the main symptoms of COVID-19. Hypoxia may also cause an increased heart rate. Therefore, optical and non-invasive heart rate monitoring is also critical for detection.

For future wearables, the integration of multiple measurement functions is optimal (though not necessarily necessary), and the ADPD4100/ADPD4101 are extremely beneficial for this. The AFE can measure any type of sensor input including temperature, ECG and respiration measurements. Therefore, a complete multiparameter VSM platform can be built using only one sensor AFE.

Pulse oximetry – SpO2 measurement

Pulse oximetry uses red light (usually 660 nm wavelength) and infrared (IR) LEDs (usually 940 nm wavelength). Deoxyhemoglobin mainly absorbs light of 660 nm wavelength, while oxyhemoglobin mainly absorbs light of 940 nm wavelength. Photodiodes sense unabsorbed light and then separate the sensed signal into DC and AC components. The DC component represents light absorption by tissue, venous blood, and non-pulsatile arterial blood. The AC component represents pulsatile arterial blood. Then calculate the percentage of SpO2 according to the following formula:

Any two time slots of the ADPD4100/ADPD4101 can be configured to measure the response to the red and IR LEDs to measure SpO2. The remaining time slots can be configured to measure PPG from LEDs of different wavelengths, and can also support ECG measurements, lead-off detection, respiration measurements, and other sensor measurements.

As an example, Figure 2 shows synchronized red, green, and IR PPG signals, as well as the AC and DC portions of the IR signal.


Figure 2. Red, green, and IR PPG, labeled with the AC and DC portions of the IR PPG signal

Heart rate monitoring

Heart rate monitoring is also critical for detecting COVID-19. As the oxygen supply drops due to lack of oxygen, the heart begins to beat faster to supply enough oxygen to the tissues. Heart rate monitoring is also valuable in detecting heart problems or tracking fitness behaviors.

A green LED with a wavelength of about 540 nm is generally preferred for heart rate monitoring. Its modulation index is higher than that of red or IR LEDs, resulting in the best PPG signal. It also offers decent CTR levels so power consumption isn’t too high.

AC SNR is a parameter related to signal quality and can be calculated by multiplying the DC SNR by the modulation index. For example, at a modulation index of 1%, 95 dB DC SNR is equivalent to 55 dB AC SNR.

ECG measurement

ECG measurements have been incorporated into wearable devices such as watches for spot checks and chest patches for continuous monitoring. Such devices typically use electrodes made of metals and other conductive materials, which are polarized electrodes known as dry electrodes. The main challenges of using dry electrodes for ECG measurements are the high electrode-skin contact impedance and relatively high overpotentials.

Conventional instrumentation amplifier-based ECG solutions use buffers to mitigate the effects of high electrode-skin contact impedance associated with signal attenuation. The right leg drive (RLD) technique, which requires a third electrode and drives the reference voltage back to the body, works in ECG systems that measure voltage to suppress common-mode voltages to which the body, electrodes, and cables are exposed.

When applied to ECG measurements, the ADPD4100/ADPD4101 employ a novel approach that uses a passive resistor-capacitor (RC) circuit to track the differential voltage across a pair of electrodes. Passive RC circuits can be as simple as three components, two resistors RS and one capacitor CS, as shown in Figure 3a. Each sampling process of ECG data is divided into two steps.

During the charging step, two input pins (IN7 and IN8) are left floating. If the charging time is >3τ, the charge on the capacitor CS is proportional to the differential voltage on the two electrodes, where τ is the time constant defined by RS and CS, and τ=2RSCS. In the charge transfer step, the capacitor is connected to the TIA and the charge is transferred to the AFE for measurement. This charge-measurement-based ECG solution offers several advantages, including the need for a buffer and a third electrode for RLDs, reduced system size due to fewer external components, and power savings.


Figure 3. ECG measurement configuration. (a) RC sampling circuit and lead-off detection circuit. (b) Illustration of the charging and charge transfer process for each ECG data sample.

With the design flexibility of the ADPD4100/ADPD4101, lead-off detection can be easily added to this ECG solution using a bioimpedance-based approach. Figure 3a shows the lead-off detection circuit, which drives pulses to one electrode and receives current at the other. If one or both electrodes are detached from the skin, the path is broken and no current is received. Figure 4 shows ECG traces and received current for lead-off detection, where ECG is measured in slot A and lead-off detection is performed in slot B.

Lead-off detection in conventional ECG solutions uses a pull-up resistor circuit that affects the input impedance of the ECG circuit; in contrast, this bioimpedance-based lead-off detection in a separate time slot has no effect on the ECG. Measurements make a difference. With this DC-coupled circuit, the ECG signal is captured once the electrode-skin contact is re-established.


Figure 4. ECG measurement and lead-off detection. Instant recovery of ECG via DC coupling.

Impedance-based respiration measurement

When using the ADPD4100/ADPD4101 for respiration measurements, the changes in the bioimpedance of the lung during the inspiratory and expiratory cycles are detected. In the intensive care unit (ICU), as well as during sleep, respiration measurements of patients are beneficial for patient management and provide timely alarms to save lives. This is critical for patients with respiratory problems and sleep apnea. Sleep apnea alone is a serious public health and safety threat, affecting more than 25 million adults in the United States. 1

As the patient breathes, the volume of the lungs expands and contracts, causing changes in the impedance of the chest. This impedance change can be measured by injecting current into the chest path and measuring the voltage drop. Figure 5a shows a reference design employing two electrodes for ECG measurement and respiration monitoring. Figure 5b shows the synchronously recorded ECG, respiration-related impedance wave, and PPG. ECG and respiration were measured using stainless steel dry electrodes on the left and right wrists, and PPG was measured using green LEDs.


Figure 5. ECG and respiration measurements. (a) External circuit for sleep-floating ECG and respiration measurements using the Kelvin detection method. (b) Example of simultaneous ECG, respiration, and PPG measurements.

Summarize

Vital signs monitoring has expanded its presence in the consumer market in the form of smart wearables. Health information generated by wearable devices can play an important role in health and disease management. To meet demand and make these devices available to a wider population, designers must consider common requirements such as cost, size, and power consumption. This breakthrough AFE ADPD4100/ADPD4101 from Analog Devices demonstrates its enormous advantages as a hub for multiparameter vital signs monitoring. A single AFE design can reduce the IC count of a multiparameter VSM system, resulting in significant cost and size savings. In addition, multi-parameter systems designed with ADPD4100/ADPD4101 can generate synchronized data, eliminating the burden of data synchronization.

The Links:   NL8060BC21-11F LP104V1