A 1.2-µW 41-dB Ripple Attenuation Chopper Amplifier Using Auto-Zero Offset Cancelation Loop for Area-Efficient Biopotential Sensing

Abstract

In this paper, a low-power and low-noise capacitive-coupled chopper instrumentation amplifier (CCIA) is proposed for biopotential sensing applications. A chopping technique is applied to mitigate the domination of flicker noise at low frequency. A new offset cancellation loop is also used to deal with the intrinsic offset, originating from process variation, to reduce ripple noise at the output of CCIA. Moreover, the optimization of the chip area was resolved by adding a T-network capacitor in the negative feedback loop. The CCIA is designed on 0.18 µm process CMOS technology with a total chip area of 0.09 mm². The post-simulation results show that the proposed architecture can attenuate the output ripple up to 41 dB with a closed-loop gain of 40 dB and up to 800 Hz of bandwidth. The integrated input referred noise (IRN) of the CCIA is 1.8 µV_rms over a bandwidth of 200 Hz. A noise efficiency factor (NEF) of 5.4 is obtained with a total power dissipation of 1.2 µW and a supply voltage of 1 V, corresponding to a power efficiency factor of 9.7 that is comparable with that of state-of-the-art studies.

1. Introduction

Nowadays, more and more portable sensing systems in the form of wearable and implantable devices are being used to monitor biopotential signals from human body. Among those, the two most important are electrocardiograms (ECG) from the heart and electroencephalograms (EEG) from the brain. Electrical tissue impedance (ETI) has been used to detect the tissue composition for physiology and pathology [1]. Local field potentials (LFPs) and action potentials (APs) are important for neuroscience research and therapy. Sensing these signals is vital for diagnosing neurological disorders, brain–machine interfaces, and neuroprosthetic technologies. Biopotential signals often have quite small amplitudes, from 10 to 100 µV for EEG and about 1 mV for ECG. These signals exist at a frequency range from 0.5 to 150 Hz [2,3]. The peak amplitude of LFPs and APs is about 1 mV and 100 µV, respectively. LFPs have a frequency band from 0.5 to 200 Hz, while that of APs runs from 300 Hz to 10 kHz [4].

To read out the inherently low-power biopotential signals, the acquisition system, illustrated in Figure 1, often consists of an instrumentation amplifier (IA). This amplifier is known for its high input impedance and operation at a low frequency of biosignals. However, at this frequency range, flicker noise is the dominant noise source. To suppress this noise, switched-biasing and bulk-switching techniques, also called as chopping techniques, have been previously investigated in [5,6]. However, these techniques cause output ripples as the upmodulated offset is added by an integrator stage. Several approaches have been proposed to reduce these ripples. Passive ripple reduction approaches were introduced by different authors in [7,8]. In [7], a large chip area low-pass filter (LPF) is added at the output stage to filter out the output ripple. The cutoff frequency of the LPF must be much lower than the chopping frequency that affects the bandwidth of the amplifier [9]. In [8], a high-power efficiency DC blocking is inserted at the output of the input stage to prevent the current offset through the output chopper and the integrator output stage. However, these amplifiers all suffer from the noise aliasing issue due to the added capacitor inside the chopper loop [10].

Active approaches were proposed by [9,11,12] to mitigate the drawbacks of the passive approaches. In [9], a combination of passive and active approaches was proposed. This technique uses a larger capacitor at the output of the first stage; therefore, the amplifier still suffers from noise aliasing issue as per [8]. In [11], a switched-capacitor notch filter is added to the input of the output stage to filter out the signals causing output ripple. However due to the phase delay, this architecture suffers from instability around chopping frequency f_CH [13]. In [12], a ripple reduction loop (RRL) using the auto-zero approach has been investigated. This technique allows the instrumentation amplifier to achieve a low output ripple. However, this approach utilizes an AC-coupled capacitor to sense output ripple, which increases the output load [13].

In this paper, a low-power and low-noise CCIA with a higher ripple attenuation factor (RAF) is presented. The proposed architecture employs a modified RRL with a low-pass filter that is equivalent to an auto-zero offset cancellation loop (A-OCL) to remove the inherent intrinsic offset due to process variation. The G_m1’s output offset is sensed and reduced by adjusting the transconductance gain through the A-OCL. In addition, to achieve a highly efficient silicon chip area, a T-network capacitor [14] is also introduced to play the role of the negative feedback capacitor. The rest of this paper is organized as follows: Section 2 introduces the structure of the proposed capacitive-coupled chopper instrumentation amplifier (CCIA), applying a new chopping technique based on the zero offset cancellation loop. Section 3 details the circuit implementation of the feedback loop, including the operational amplifier (op amp) block and its feedback loop. Section 4 discusses the simulation results and benchmarking with recent research results. Section 5 concludes the paper.

2. Design

Figure 2 illustrates the schematic of the proposed CCIA [15], which consists of two transconductor stages. The first stage employs folded-cascode amplifier topology (G_m1), while the second adopts a single common source amplifier (G_m2) for higher output swing. An auto-zero offset cancellation loop (A-OCL) is applied for the first G_m1 stage. A T-cap loop (TCL) is introduced to the second G_m2 stage and fed back to the input of the first stage. Due to unavoidable process variation during fabrication, the G_m1 is attached with an intrinsic offset V_os1 that creates an output ripple considered as noise and affects the output signal. Therefore, an A-OCL with a new chopping technique is inserted to the CCIA to reduce the output ripple while assuring that loading effect is not added to the amplifier. This technique can remove flicker noise so that low-noise amplifier can be achieved.

The common-mode voltage V_CM is set to 0.5 V for G_m1 through two pseudoresistors R_p1,2. The neural input signal V_in is upmodulated to the chopping frequency band by the chopper CH_in, then downmodulated to the baseband by CH_out. The chopping frequency f_CH is fixed at 10 kHz. To set the DC gain of CCIA to 100 dB, the quiescent currents G_m1 and G_m2 are selected as 980 nA and 180 nA, respectively. The closed-loop gain of the CCIA of 40 dB is defined by the ratio of the input capacitor C_in1,2 and the feedback capacitor C_fb1.2.

Figure 3 shows the schematic of the main path of CCIA with proposed A-OCL in feedback loop with the G_m1. This feedback loop consists of an active RC low-pass filter employing an NMOS pseudoresistor, a capacitor, and a A-G_m3 that contains the transconductor G_m3 and will be detailed in the next paragraph. In a conventional A-OCL approach, as in [4], the cancelation of output offsets V_OS1, caused by the transconductor G_m1, is realized by adding two transconductors G_m3 and G_m4 to create a negative offset compensation. The residual input signal V_in,ω and the intrinsic offset V_OS1 of the transconductor G_m1 result in the two respective currents at its output. They are I_1,ω at the chopping frequency band and the offset current I_1,OS at the signal base-band. To suppress V_1,OS, the third transcoductor G_m3, to form an integrator, and the fourth transconductor G_m4 are integrated to generate a negative output offset voltage to add to the output offset voltage V_1,OS to compensate for each other. This conventional A-OCL is also called a negative A-OCL. The main drawback of this approach is its complex design and high-power consumption. In our design, the G_m4 is not needed; the transconductor G_m3 is connected to a new scheme in a feedback loop to cancel the offset voltage of the G_m1. The A-OCL is connected from the output of the G_m1 to its input voltage instead of from the output of the G_m2 to the output of G_m1 as in the conventional approach. In the proposed approach, the compensation occurs at the input of G_m1 so all the variations associated with mismatches due to PVT (process, voltage, and temperature) are also canceled. The cancellation analysis is detailed as follows.

When chopping is applied for the amplifier, the finite bandwidth of the amplifier creates the output ripple V_OUT,Ripple at the chopping frequency f_CH [12], which can be expressed as follows:

$${\mathrm{V}}_{\text{OUT},\text{Ripple}}=\frac{{\mathrm{V}}_{\mathrm{O}\mathrm{S}1}{\mathrm{G}}_{\mathrm{m}1}}{2f_{\mathrm{C}\mathrm{H}}{\mathrm{C}}_{\mathrm{m}1,2}},$$

(1)

where C_m1,2 is the Miller compensation capacitor. By the feedback operation, V_{OUT, Ripple} is reduced by a factor of the DC loop gain L_G(0) [9]. For the proposed A-OCL, L_G(0) can be written as

$${\mathrm{L}}_{\mathrm{G}}{\left(0\right)=\mathrm{A}}_3{\mathrm{G}}_{\mathrm{mb}1}{\mathrm{R}}_{\mathrm{LP}1,2}=\mathsf{\eta }{\mathrm{A}}_3{\mathrm{G}}_{\mathrm{m}1}{\mathrm{R}}_{\mathrm{LP}1,2},$$

(2)

where A₃ is the finite DC gain of the transconductor G_m3 in A-OCL. The η = G_mb1/G_m1 is around 0.25 [16].

Figure 4 shows the schematic of A-G_m3, including the proposed auto-zero (AZ) offset technique, by controlling the switches S_1,2, the transconductor G_m3, and the timing diagram. Because of the process variation, G_m3 is also associated with an intrinsic offset V_OS3, which contributes to the output ripple as well. Therefore, the auto-zero (AZ) technique is applied to reduce the offset voltage V_OS3. The operation of switches S_1,2 is controlled by the control signals f_S1,2, as shown in the timing diagram of Figure 4, which are chosen at 50% of chopping frequency f_CH. The auto-zero loop, independent of f_CH, does not affect the operated rippled reduction. During φ1, the V_OS3 is charged to the stored capacitor C_AZ. During φ2, the charged voltage in the capacitor C_AZ is charged to the opposite input of G_m3. Hence, the DC voltage at the differential input of G_m3 is balanced so that the offset voltage of G_m3 can be suppressed.

In the CCIA, the mid-band gain is defined by C_in1,2/C_fb,eq. Increasing C_in1,2 for a higher gain results in reduction of the input impedance Z_in as well as an increase in the chip area. The minimum capacitance C_fb,eq that can be designed is limited by the technology. To reduce the chip area without increasing the input capacitor C_in1,2, a T- network capacitor [14] is employed. Considering differential operations, C_fb,eq, realized using the T-network capacitor, can be expressed as follows:

$${\mathrm{C}}_{\mathrm{fb},\mathrm{eq}}=\frac{{\mathrm{C}}_{\mathrm{U}}}{2(N+1)},$$

(3)

where N is the number of unit capacitors C_U = 200 fF used to implement the shunt capacitor N × C_U (see Figure 4). The T-network capacitor in the feedback path, as illustrated in Figure 5, increases the closed loop gain by shunting some of the feedback signals. As a result, C_fb,eq is reduced for a given gain. It is noticed that the overall area saving is achieved by the decrease in the C_in1,2 value enabled by the T-network capacitor. Increasing N can further reduce C_fb,eq and the chip area, resulting in increased noise [14]. Considering this tradeoff, the value of N is set to 4. This approach allows us to reduce the area of C_in1,2 and C_fb,eq by 45.6%. A gain of 40 dB was achieved by using C_in1,2 of only 2 pF. In previous works, the values of C_in1,2 are much higher, such as 12 pF [12], 10 pF [17], and 21 pF [18].

The T-network capacitor will increase the asymmetry between the nodes connected to the feedback network. To tackle this drawback, the configuration of the T-network capacitor is modified. N × C_U is divided into two (N/2) × C_U pieces, connected symmetrically to balance the parasitic behavior at two nodes TN1 and TN2. Then the top side of C_U is connected to reduce the parasitic capacitance.

3. Circuit Implementation

Figure 6 and

Table 1. Dimension of MOSFETs in a folded cascode op amp.

MOSFETs	W/L (µm/µm)	MOSFETs	W/L (µm/µm)
M0	60/0.7	M5,6	15/0.7
M1,2	30/0.7	M7,8	0.7/6
M3,4	15/0.7	M9,10	0.7/6

show the folded cascode op amp for G_m1 using the body control technique and the dimension of the MOSFETs adopted in the schematic of the op amp, respectively. All the transistors are set to work in the subthreshold region for the sake of power efficiency. To isolate the body-control terminals from the noise coupled through the substrate, a deep n-well is used for the input differential pair. The bias current of G_m1 is 840 nA. The CMFB circuit (not shown) generates the output V_CMFB2 using a 140 nA bias current.

Figure 7 shows a schematic of the two-stage op amp for G_m3. In order to achieve high output swing, the output stage of G_m3 utilizes a class-A amplifier. The biased current for G_m3 is shown in Figure 7.

Using the noise equivalent circuit shown in Figure 6, the input referred noise of G_m1, $\overline{{\mathrm{V}}_{\mathrm{n},\mathrm{in},\mathrm{Gm}1}^2}$, can be expressed as follows:

$$\begin{array}{l}\overline{{\mathrm{V}}_{\mathrm{n},\mathrm{i}\mathrm{n},\mathrm{G}\mathrm{m}1}^2}=\frac{4\mathrm{k}\mathrm{T}\mathrm{n}}{{\mathrm{g}}_{\mathrm{m}1,2}}\times \left(\frac{{\mathrm{g}}_{\mathrm{m}1,2}+{\mathrm{g}}_{\mathrm{m}3,4}+{\mathrm{g}}_{\mathrm{m}9,10}}{{\mathrm{g}}_{\mathrm{m}1,2}}\right)+2\overline{{\mathrm{V}}_{\mathrm{n},\mathrm{o}\mathrm{u}\mathrm{t},\mathrm{OCL}}^2}\times {\left(\frac{{\mathrm{g}}_{\mathrm{mb}1,2}}{{\mathrm{g}}_{\mathrm{m}1,2}}\right)}^2\\ \cong \frac{4\mathrm{k}\mathrm{T}\mathrm{n}}{{\mathrm{g}}_{\mathrm{m}1,2}}\times \left(\frac{{\mathrm{g}}_{\mathrm{m}1,2}+{\mathrm{g}}_{\mathrm{m}3,4}+{\mathrm{g}}_{\mathrm{m}9,10}}{{\mathrm{g}}_{\mathrm{m}1,2}}\right)\end{array}$$

(4)

where $\overline{{\mathrm{V}}_{\mathrm{n},\mathrm{out},\mathrm{OCL}}^2}$ is the output referred noise of the A-OCL and n = 1.5 is the subthreshold slope factor [17]. As mentioned above, the flicker noise of A-OCL is also modulated by the chopper technique at the output of G_m1, so the contribution of noise from A-OCL can be negligible. It is also confirmed by simulation results that the output noise of A-OCL is about $\overline{{\mathrm{V}}_{\mathrm{n},\mathrm{out},\mathrm{OCL}}^2}$$\cong$ 10⁻¹⁵ V_rms. So the input referred noise of the CCIA can be calculated as follows:

$$\overline{{\mathrm{V}}_{\mathrm{n},\mathrm{in}}^2}={\left(\frac{{\mathrm{C}}_{\mathrm{tot}}}{{\mathrm{C}}_{\mathrm{in}1,2}}\right)}^2\overline{{\mathrm{V}}_{\mathrm{n},\mathrm{in},\mathrm{Gm}1}^2}\cong {\left(\frac{{\mathrm{C}}_{\mathrm{tot}}}{{\mathrm{C}}_{\mathrm{in}1,2}}\right)}^2\frac{4\mathrm{k}\mathrm{T}\mathrm{n}}{{\mathrm{g}}_{\mathrm{m}1,2}}\times \left(\frac{{\mathrm{g}}_{\mathrm{m}1,2}+{\mathrm{g}}_{\mathrm{m}3,4}+{\mathrm{g}}_{\mathrm{m}9,10}}{{\mathrm{g}}_{\mathrm{m}1,2}}\right),$$

(5)

where C_tot = C_in1,2 + C_fb1,2 + C_p, C_p is the parasitic capacitance, $\overline{{\mathrm{V}}_{\mathrm{n},\mathrm{in},\mathrm{Gm}1}^2}$ are the input-referred noise of G_m1, and g_mi represents the transconductance of the transistors in G_m1.

4. Simulation Results

Figure 8 shows the CCIA layout using 180-nm CMOS technology, in which the core occupies an area of 0.09 mm². From this section onward, all post-simulation results, in a standard 0.18-µm CMOS technology, are carried out with a full extraction of parasite by choosing RCC option. Open-loop simulations are run to observe the gain bandwidth and the phase margin of the main path amplifier with active A-OCL. As shown in Figure 9, the gain bandwidth is achieved at 300 kHz, corresponding to a phase margin of about 60 degrees. The frequency response of the CCIA in a closed loop is presented in Figure 10. A closed-loop gain of 40 dB is observed with a low-pass cutoff frequency at 800 Hz (Figure 10a). In addition, the integrated input referred noise (IRN) reaches 1.8 µV_rms over a bandwidth of 200 Hz at a thermal noise of 121 nV/√Hz and a 1/f corner of 10 Hz (Figure 10b).

In order to verify the impact of variations in the fabrication process and power supply on the proposed CCIA, a Monte Carlo simulation was carried out by considering the local and global mismatches due to the process corner. Figure 11 shows the Monte Carlo simulation results of the mid-band gain of the CCIA obtained using 200 samples. At V_DD = 1 V, as shown in Figure 11a, the average mid-band gain is 39.7 dB, with a standard deviation of 60.6 mdB. V_DD varies by about 10% and the mid-band gain changes from 39.6 to 40 dB with corresponding standard deviations of 56.1 and 62.5 mdB, respectively.

Monte Carlo simulations, with random device mismatches to investigate the effect of process corners on the noise, were realized. Figure 12a shows that the input-referred noise of the proposed amplifier varied from 1.78 µV_rms to 1.96 µV_rms across different process corners. Figure 12b shows that the average input-referred noise was 1.81 µV_rms, with a standard deviation of 62.2 nV_rms.

Figure 13 shows the Monte Carlo simulation results of the proposed CCIA, referring to the common mode rejection ratio (CMRR) and power supply rejection ratio (PSRR). By running 200 samples, the CCIA achieved a mean value of CMRR of 108.9 dB and PSRR of 87 dB with standard deviations of 39.5 and 24.7 dB, respectively. The CCIA’s input was set to be short-circuited during the simulation to measure the output spectrum. Both VOS1 and VOS3 were set to 5 mV. Figure 14 illustrates the output spectrum voltage and Monte Carlo simulation of the ripple attenuation factor (RAF). When A-OCL was disabled, the output spectrum at chopping frequency was around 7 mV. This spike in the CCIA was reduced to 61 µV when A-OCL was enabled, which allowed RAF to achieve a high value of 41 dB. The RAF was also double-checked by running 200 samples in the Monte Carlo simulation, considering local and global process variations, which obtained a mean value of RAF of 41.7 dB with a standard deviation of 3.37 dB. A significant reduction in the output ripple voltage was observed, which confirmed that the mismatches due to the PVT-generated offset voltage (VOS1, VOS3) can be compensated for by the proposed feedback loop.

To verify the linearity of the amplifier, an FFT of output voltage is analyzed with a differential input of 2 mV and an input frequency of 100 Hz (Figure 15). The total harmonic distortion was about −56.2 dB, which is determined by the ratio between the output amplitude at the input frequency and its third-order harmonic of 300 Hz, given that the fifth- and seventh-order harmonics are small enough to be ignored.

The power breakdown of the proposed CCIA is given in

Table 2. Power breakdown.

Block	Components	Current (nA)	Voltage (V)
Gm1 (Folded cascode)	Input pair Cascode branches + CMFB	700 280	1
Gm2 (Common-source)	Input pair	200
Gm3 (Two-stage op amp)	Input pair Common source + CMFB	5 25
Total power	1210 (nW)

. The power efficiency factor (PEF) is used to evaluate the tradeoff between noise and power efficiency for biopotential amplifiers. As in [18], PEF is calculated as follows:

$$\mathrm{PEF}={\mathrm{V}}_{\mathrm{ni},\mathrm{rms}}^2\frac{2{\mathrm{P}}_{\mathrm{DC}}}{\pi {\mathrm{U}}_{\mathrm{th}}4\mathrm{k}\mathrm{T}\cdot \mathrm{B}\mathrm{W}}={\mathrm{NEF}}^2\cdot {\mathrm{V}}_{\mathrm{DD}},$$

(6)

where V_ni,rms is the input-referred noise voltage, P_DC is the power consumption, and BW is the amplifier bandwidth. The performance of the proposed design, in comparison with state-of-art studies, is shown in

Table 3. Performance summary and comparison.

	[8]	[13]	[19]	[20]	This Work
Power (µW)	2.0	3.96	19.8	2	1.21
Supply (V)	1.2	1.2	1.8	1.2	1.0
Current (µA)	1.2	3.3	11	1.7	1.21
Gain (dB)	34	40	40	26	40
CMRR (dB)	>94	N/A	82	N/A	>108
IRN (µVrms)	N/A	N/A	0.8	9	1.8
Noise floor (nV/√Hz)	37	43	40	100	121
Bandwidth (Hz)	11K	N/A	100	5000	800
RAF	78	14.8	26	N/A	>41
NEF/PEF **	1.8/3.3	3.8/17.3	12.3/272	7/49	5.4/29.7
Tech. (nm)	65	130	180	40	180
Area (mm2)	N/A	0.3	1.3	0.07	0.09

** Calculated using (6).

.

5. Conclusions

In this paper, a sub-μW capacitively coupled chopper instrumentation amplifier (CCIA), using an auto-zero offset cancellation loop A-OCL and T-capacitor network, is introduced. The proposed A-OCL not only reduces the output ripple but also avoids increasing the output loading effect. Applying the proposed approach, the ripple attenuation factor achieves a value higher than 41 dB. For area efficiency, the T-network capacitor is added to the negative feedback loop, enabling the usage of a low-input capacitor. The simulation results show an IRN of 1.8 μV_rms with the A-OCL enabled. The noise density is 121 nV/√Hz at a 40 dB gain with a power consumption of 1.21 µW. The NEF/PEF is 5.4/29.7, which compares favorably with the state-of-the art studies. The obtained results suggest another potential application of this amplifier in autonomous fire-rescue robots for the detection and biosignal recognition of human beings.