Biosignal integrated circuit with simultaneous acquisition of ECG and PPG for wearable healthcare applications

Abstract

BACKGROUND:

Wearable healthcare systems require measurements from electrocardiograms (ECGs) and photoplethysmograms (PPGs), and the blood pressure of the user. The pulse transit time (PTT) can be calculated by measuring the ECG and PPG simultaneously. Continuous-time blood pressure without using an air cuff can be estimated by using the PTT.

OBJECTIVE:

This paper presents a biosignal acquisition integrated circuit (IC) that can simultaneously measure the ECG and PPG for wearable healthcare applications.

METHODS:

Included in this biosignal acquisition circuit are a voltage mode instrumentation amplifier (IA) for ECG acquisition and a current mode transimpedance amplifier for PPG acquisition. The analog outputs from the ECG and PPG channels are muxed and converted to digital signals using 12-bit successive approximation register (SAR) analog-to-digital converter (ADC).

RESULTS:

The proposed IC is fabricated by using a standard 0.18 $\mu$ m CMOS process with an active area of 14.44 mm ${}^{2}$ . The total current consumption for the multichannel IC is 327 $\mu$ A with a 3.3 V supply. The measured input referred noise of ECG readout channel is 1.3 $\mu$ V ${}_{\text{RMS}}$ with a bandwidth of 0.5 Hz to 100 Hz. And the measured input referred current noise of the PPG readout channel is 0.122 nA/ $\surd$ Hz with a bandwidth of 0.5 Hz to 100 Hz.

CONCLUSIONS:

The proposed IC, which is implemented using various circuit techniques, can measure ECG and PPG signals simultaneously to calculate the PTT for wearable healthcare applications.

Keywords

Biosignal acquisition simultaneous biosignal acquisition electrocardiogram (ECG)photoplethysmogram (PPG)pulse transit time (PTT)chopper stabilization

1. Introduction

Recently, healthcare, mobile, and wearable applications for the Internet of Things (IoT) have been gaining attention. Wearable healthcare applications require measuring biosignals such as electroencephalogram (EEG), electrooculogram (EOG), electromyogram (EMG), electrocardiogram (ECG), photoplethysmogram (PPG), and body temperature. In addition, by measuring the ECG and PPG, the pulse transit time (PTT) can be calculated. The continuous-time blood pressure without using an air cuff can be estimated by calculating the PTT [1, 2, 3]. The previous researches reported that the estimated blood pressure using PTT and the types of the blood pressures, including the systolic blood pressure (SBP), the diastolic blood pressure (DBP) and the mean arterial pressure (MAP), have strong relationships with the correlations greater than 87% [2]. One of the important parts of an ECG monitoring IC is the frequency response of the amplifier of the AFE. The capacitive coupled instrumentation amplifier (CCIA) scheme is adopted for the instrumentation amplifier of the ECG monitoring IC. However, a mismatch between the input capacitor and feedback capacitor rejects the common mode rejection ratio (CMRR) and degrades the performance of the biosignal acquisition IC [4, 5, 6, 7]. To overcome this problem, a current balanced instrumentation amplifier (CBIA) scheme has been reported [5, 6, 7]. Because the CBIA scheme is stable without a feedback loop between the input and output, no passive elements are needed. The mismatch between passive elements such as resistors and capacitors causes a degradation of the CMRR. In addition, the CBIA scheme is suitable for low power applications because of its low power consumption and small active area. The PPG monitoring system is composed of photodiodes, light emitting diodes and a transimpedance amplification stage to optically detect the blood volume changes from the skin. In the PPG monitoring system, the large DC offset current exists, and the DC offset currents limit the dynamic range of the PPG ICs, thus, the DC offsets of the PPG signals should be removed. Several researches have been reported for reducing the DC offsets of the PPG monitoring ICs [8, 9, 10]. The previous DC rejection schemes adopt the correlated double sampling (CDS) [9], or on-chip large time-constant circuits including pseudo resistor [8, 10].

2. Proposed biosignal acquisition IC for wearable healthcare applications

A block diagram of the proposed biosignal acquisition IC for wearable healthcare applications is shown in Fig. 1a. The ECG signals are amplified by the IA after filtered by the HPF and modulated by the chopper. The HPF is designed with a pseudo resistor to gain a low cutoff frequency. The implemented C ${}_{\text{HPF}}$ of HPF is 482 pF and R ${}_{\text{HPF}}$ of HPF is controllable through 2 G $\Omega$ to 58 G $\Omega$ by 4-bit control register. Therefore, the cutoff frequency of the HPF is controllable approximately 0.0596 Hz to 0.1 Hz to reject the DC voltage. The amplified output signal is amplified again by the programmable gain amplifier (PGA) using the differential difference amplifier (DDA). On the other hand, the PPG readout channel has the inputs TIA_INP, TIA_INN and Sink_LED. The Sink_LED input is for the LED driver. The LED driver can control the current of the photodiode of the pulse oximeter to monitor peripheral oxygen saturation (SpO ${}_{2}$ ). The TIA amplifies the input signals by a current-to-voltage conversion. The PPG signal is amplified again by the PGA as the ECG readout channel. Each amplified ECG and PPG signal is buffered out after passing through the LPF. The channel selection mux allows for selection of a desired channel by using a digital control pin. The buffered analog signals that are selected by the channel selection mux are converted into 12-bit digital code by the 12-bit SAR ADC. The proposed IC can monitor sensed signals and acquire the data as digital code for the back end DSP.

Figure 1.

Top level overview of the proposed biosignal acquisition IC.

2.1 Instrumentation amplifier (IA) for ECG readout channel

The ECG IA of the proposed IC is shown in Fig. 1b. The IA input stage is a transconductance amplifier stage, and the IA output stage is a transimpeance amplifier stage. The currents of MP1 and MP3 are fixed by each current source of current $I_{B1}$ and $I_{B2}$ . The input transistor MP1 is a source follower so that the input dependent current flow at $R_{1}$ is determined as

$\displaystyle I_{R1}=\frac{\textit{INP}-\textit{INN}}{R_{1}}$ (1)

The current mirrored to MP5 of the IA output stage can be expressed as (where K is the W/L ratio of the current between MP2 and MP5)

$\displaystyle\frac{\textit{OUTP}-\textit{OUTN}}{R_{2}}=KI_{R1}$ (2)

Therefore, the transfer function of CBIA is given as

$\displaystyle\frac{V_{\textit{OUT}}}{V_{IN}}=\frac{\textit{OUTP}-\textit{OUTN}% }{\textit{INP}-\textit{INN}}=K\frac{R_{2}}{R_{1}}$ (3)

2.2 Transimpedance amplifier (TIA) for PPG readout channel

A schematic of the TIA of the proposed PPG readout channel is shown in Fig. 1c. The current mode TIA amplifies the input current signal into an output voltage signal by a current-to-voltage conversion. The photocurrent $I_{PD}$ of the photodiode flows through the inputs INP and INN of the TIA. The photocurrent is modulated by the chopper of the input stage. The TIA input stage is biased by the two fixed bias currents $I_{B}$ . The two currents $I_{B}+I_{PD}$ and $I_{B}-I_{PD}$ , which flow through MP1, are mirrored by MP3 of the TIA output stage. The currents mirrored from MP1 to MP3 are amplified by the W/L ratio of K between MP1 and MP3. The mirrored currents are converted into output voltage signals after being demodulated by the current flow of resistor R. The currents mirrored by the TIA output stage to MP3 can be expressed as (where K is the ratio of the current between MP1 and MP3)

$\displaystyle\frac{\textit{OUTN}-\textit{OUTP}}{R}=KI_{PD}$ (4)

Therefore, the output voltage of TIA is given as

$\displaystyle V_{\textit{OUT}}=\textit{OUTP}-\textit{OUTN}=-\textit{KRI}_{PD}$ (5)

2.3 Programmable gain amplifier (PGA) and DC servo loop

The schematic of the PGA integrated in the proposed IC is shown in Fig. 1d. The PGA stage is a two stage resistive amplifier implemented using a differential difference amplifier (DDA). The gain of the PGA is controllable by adjusting the 4-bit programmable resistor $R_{F}$ . The programmable resistor can adjust the resistance from 500 k $\Omega$ to 7.5 M $\Omega$ , and the resistance of $R_{\textit{ref}}$ has a fixed value of 1 M $\Omega$ . the output voltage of the PGA can be written as

$\displaystyle V_{\textit{out}}=\left({1+\frac{R_{F}}{R_{\textit{ref}}}}\right)% (V_{\textit{inp}}-V_{\textit{inn}})+V_{\textit{ref}}$ (6)

The DC servo loop of the proposed IC is shown in Fig. 1e. The DC servo loop removes the DC offsets ( $V_{OS}$ ) by negative feedback into the input stage after modulating the integrated output signals OUTP and OUTN of the IA. The output voltage affected by the DC servo loop can be expressed as

$\displaystyle V_{\textit{OUT}}(t)=\textit{OUTP}-\textit{OUTN}=K\frac{R_{2}}{R_% {1}}(V_{\textit{INP}}-V_{\textit{INN}})+\left({V_{OS}-\frac{1}{R_{\textit{DSL}% }C_{\textit{DSL}}}\int{V_{\textit{OUT}}(t)dt}}\right)$ (7)

3. Prototype IC implementation and measurements

3.1 Measurement environment

The measurement environment for the proposed IC is shown in Fig. 2, in which information about the measurement equipment can be seen. The fabricated chip is implemented on the PCB board by the chip on board (COB) process. A wet electrode and clip electrode are used for ECG signal acquisition. SpO ${}_{2}$ is used for PPG acquisition. The output data is stored in an Arduino Due, and data is acquired by a laptop computer. In addition, the direct analog output signal of the proposed IC can be measured. A dynamic signal analyzer is used to measure the input referred noise and the frequency response of the proposed IC.

Figure 2.

Measurement environment for proposed biosignal acquisition IC.

Figure 3.

Measurement results for proposed biosignal acquisition IC.

3.2 Measurement results

The measurement results are shown in Fig. 3. The total current consumption of the proposed IC is 327 $\mu$ A with a 3.3 V power supply. The measured input referred noise of the ECG readout channel is 1.3 $\mu$ V ${}_{\text{RMS}}$ with a bandwidth of 0.5 Hz to 100 Hz. This is shown in Fig. 3a. The frequency response of the ECG readout channel is shown in Fig. 3b. The measured DC gain of the transfer function of the ECG readout channel is 70.4 dB. The input referred current noise of the PPG readout channel is shown in Fig. 3c. The measured result of input referred current noise of the PPG readout channel is 0.122 nA/ $\surd$ Hz with a bandwidth of 0.5 Hz to 100 Hz and a TIA gain of 14.25 M $\Omega$ .

The simultaneous acquisition of the ECG and PPG signals is shown in Fig. 3d. All ECG and PPG signals are clearly visible. In addition, the different waveforms of P and QRS are clearly distinguishable by the high performance of the proposed IC. The proposed biosignal acquisition IC can measure the ECG and PPG signals at the same time. This simultaneous acquisition makes it able to acquire continuous-time blood pressure by calculating the PTT.

Table 1
Performance comparison

	[6]	[8]	[11]	[12]	[13]	This work
Process	0.5 $\mu$ m	0.18 $\mu$ m	0.18 $\mu$ m	0.13 $\mu$ m	0.18 $\mu$ m	0.18 $\mu$ m
Supply voltage	3 V	1.2 V	3.3 V	1.2 V	1 V	3.3 V
Total current consumption	20 $\mu$ A	160 $\mu$ A	3.8 $\mu$ A	22 $\mu$ A	–	327 $\mu$ A
				(without LED		(with ECG $+$ PPG
				power)		readout $+$ LED
						power)
Simultaneous biosignal	NO	NO	NO	NO	NO	YES
acquisition
DC servo loop	YES	YES	YES	YES	YES	YES
ECG readout channel input	0.574 $\mu$ V ${}_{\text{RMS}}$	1.3 $\mu$ V ${}_{\text{RMS}}$	0.94 $\mu$ V ${}_{\text{RMS}}$	–	1.3 $\mu$ V ${}_{\text{RMS}}$	1.3 $\mu$ V ${}_{\text{RMS}}$
referred noise (100 Hz BW)
ECG readout channel transfer	60 dB	50 dB	71.9 dB	–	60 dB	70.4 dB
DC gain (100 Hz BW)
PPG readout channel input	–	–	–	0.026 nA/ $\surd$ Hz	–	0.122 nA/ $\surd$ Hz
referred current noise
Die area	1.95 mm ${}^{2}$	40.00 mm ${}^{2}$	10.50 mm ${}^{2}$	19.84 mm ${}^{2}$	6.25 mm ${}^{2}$	14.44 mm ${}^{2}$

4. Conclusions

This paper presented a biosignal acquisition integrated circuit for wearable healthcare applications. The feature of the proposed IC is that ECG and PPG signals can be acquired simultaneously. The simultaneous measurement of the ECG and the PPG signals can be used to calculate the PTT. The PTT can be used to estimate continuous-time blood pressure. The proposed biosignal acquisition IC has two channels: an ECG readout channel for ECG signal acquisition, and a PPG readout channel for PPG signal acquisition. A performance comparison with previous works is listed in Table 1. The fully integrated biosignal acquisition IC has functions and performance that are suitable to various applications of wearable healthcare systems.

Footnotes

Acknowledgments

This research was supported by the Nano $\cdot$ Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF-2015M3A7B7045527). This research was also partly supported by a grant from Kyung Hee University in 2017 (KHU-20171202) and supported by IC Design Education Center (IDEC).

Conflict of interest

None to report.

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Biosignal integrated circuit with simultaneous acquisition of ECG and PPG for wearable healthcare applications

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSIONS:

Keywords

1. Introduction

2. Proposed biosignal acquisition IC for wearable healthcare applications

3.1 Measurement environment

Table 1 Performance comparison

Footnotes

Acknowledgments

Conflict of interest

References

Table 1
Performance comparison