A Current-Mode TransImpedance Amplifier for Capacitive Sensors ^†

Abstract

A Current-Mode (CM) TransImpedance Amplifier (TIA) based on Second Generation Current Conveyors (CCIIs) for capacitive microsensor measurements is presented. The designed electronic interface performs a capacitance-to-voltage conversion using 3 CCIIs and 3 resistors exploiting a synchronous-demodulation technique to improve the overall detection sensitivity and resolution of the system. A CM-TIA solution designed at transistor level in AMS0.35 µm integrated CMOS technology with a power consumption lower than 900 µW is proposed. Experimental results obtained with a board-level prototype show linear behavior of the proposed interface circuit with a resolution up to 34.5 fF and a sensitivity up to 223 mV/nF, confirming the theoretical expectations.

1. Introduction

Capacitive sensors can be used in a wide range of applications for the detection of mechanical [1,2], chemical [3,4] and physical [5,6] quantities by measuring capacitance variations. Capacitive sensing has gained a crucial role particularly in Micro Electro-Mechanical Systems (MEMS) where quantities such as acceleration, displacement or pressure have to be measured without generating excessive electrostatic forces on the capacitive sensing element [7,8,9,10]. A TransImpedance Amplifier (TIA) based on Second Generation Current Conveyors (CCII) with good performances in terms of linearity, sensitivity, resolution and capability to reveal large dynamic ranges of capacitance sensor variations is proposed. In particular, with respect to voltage-mode architectures based on Operational Amplifiers (OAs), CCII-based Current-Mode (CM) solutions are mainly suitable for their integration in a standard CMOS technology through simple architectures with a reduced number of transistors, within a small silicon area and having low-power characteristics [11,12,13]. The use of CCII promotes the design of integrated circuits for portable sensor applications, overcoming constraints typically provided by circuits based on OAs, ensuring a larger frequency bandwidth and a good capability to work at low supply voltage levels with a wide input-output dynamic range [14,15]. The proposed CM-TIA description and operating principle are illustrated in Section 2, experimental results are presented in Section 3, and conclusions are given in Section 4.

2. Current-Mode Transimpedance Amplifier

The block diagram of the CM-TIA is shown in Figure 1a while Figure 1b reports the schematic circuit of the proposed CCII designed at transistor level in AMS0.35 µm CMOS integrated technology with a power consumption lower than 900 µW at ±1 V dual supply voltage. Time and frequency domain simulation results of the integrated CM-TIA are reported in Figure 2. The current i_s(t) produced by the excitation sinusoidal signal v_s(t) applied to the capacitance under test C_x is applied to the CCII₁ low-impedance input X. The three resistors R₁ = 100 kΩ, R₂ = 51 kΩ and R₃ = 100 kΩ set the total gain of the TIA:

G = v_t(t)/i_s(t) = (2R₁R₃)/R₂

(1)

Thanks to the virtual ground at the CCII₁ X node, the current i_s(t) is given by:

i_s(t) = 2πf_sC_x v_s(t)

(2)

The output voltage of the TIA v_t(t) is then equal to:

v_t(t) = G 2πf_sC_x v_s(t)

(3)

3. Experimental Results

A board-level prototype of the TIA has been implemented with three commercial CCII (AD844) for preliminary measurements as shown in Figure 3. The experimental frequency responses of the board-level CM-TIA obtained using different reference capacitances within the range of 1–56 pF are reported in Figure 4.

A good agreement with the theoretical behavior is obtained, showing a linear trend for frequencies up to 80 kHz, exhibiting a lower bandwidth with respect to the proposed optimized integrated version, as reported in Figure 4a. A synchronous demodulation has been implemented to provide a DC output voltage and improve the signal-to-noise ratio by using an analog multiplier (AD835) and a low-pass filter as reported in the inset of Figure 4b. The sinusoidal reference voltage v_ref(t) has a peak amplitude A_ref = 0.5 V and a nominal phase shift ϕ = −90 deg with respect to the excitation signal v_s(t) whose peak amplitude is A_s = 10 mV. The DC output signal is equal to:

V_out = 0.5 k⁻¹ G A_s A_ref 2πf_sC_x cos(ϕ + 90)

(4)

where k = 1.05 V is the multiplier gain factor. Figure 4b shows the measured output voltage V_out as a function of the reference capacitance C_x obtained at different frequency values f_s of 10, 25, 35, 50 kHz. Experimental results are in good agreement with the expected values, showing a good linearity within the measurement range. The obtained maximum sensitivity is about 223 mV/nF for f_s = 50 kHz with an estimated resolution calculated at 3${\mathsf{\sigma }}_{V_{\mathrm{out}}}$ = 7.68 µV of about 34.5 fF.

4. Conclusions

A current-mode transimpedance amplifier based on second generation current conveyors able to measure capacitive variation with low sensing signal amplitude has been presented.

The integration of CCII promotes the design of low-power integrated circuits, overcoming restraints typically provided by circuits based on operational amplifiers, ensuring a larger frequency bandwidth and a good capability to work at a low supply voltage levels with a wide dynamic range. The proposed electronic interface performs a capacitance-to-voltage conversion using 3 CCIIs and 3 resistors exploiting a synchronous-demodulation technique to improve the overall sensitivity and resolution of the system. Experimental results obtained with a board-level prototype have led to a sensitivity of about 223 mV/nF for f_s = 50 kHz with an estimated resolution calculated at 3${\mathsf{\sigma }}_{V_{\mathrm{out}}}$ = 7.68 µV of about 34.5 fF. The obtained values of sensitivity and resolution show that the proposed CM-TIA is suitable for capacitive measurement of comb-finger parallel-plate MEMS sensors.