This paper concerns the design and the implementation of a fully integrated front-end intended to Near-Infrared Spectroscopy System (NIRS) acquisition system. A low-noise transimpedance amplification (TIA) circuit followed by adjustable cut-off frequency and a low-pass filter (LPF) was implemented in order to decrease noise circuit of NIRS detectors. For TIA, a single ended common source, common gate input stage based on a cascode structure is used to get a higher gain-bandwidth closed-loop transimpedance amplifier. To enhance the circuit noise performance, a single feedback transistor technique is used, compared to passive feedback, to achieved high quality data from NIRS acquisition channel. The proposed LPF combines two control methods to adjust the low cut-off frequency. Simulation results show a TIA gain of 104.2 dBΩ, ?3dB bandwidth of 19 MHz and an equivalent input noise current spectral density of 446 fA/√Hz. LPF filter exhibits a relatively constant noise 201nV/√HzQUOTE√Hz from 0 Hz to 700 KHz and linearity performance over its entire tuning range. The proposed front-end of NIRS preamplifier is implemented using 0.18 μm CMOS technology.
NIRS is a new medical device that can be used for monitoring and in several neurological diseases in the human brain. It is considered in many hospitals for brain functional imaging. Among the most used methods in the clinical setting is functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) [
The remainder of this paper is organized as follows: section 2 presents the proposed TIA circuit including the transistor feedback as well as the calculation of the small signal of noise analysis for TIA, followed by the described of the proposed low-pass filter. Layout and simulation results are reported in section 3. Finally, the conclusion is given in section 4.
Integrated analog components within acquisition system play the initial role of the NIRS signal quality. TIA, LPF and analog to digital convertor (ADC) are the components of NIRS acquisition system. TIA is a one from the front-end part of the data acquisition system and its circuit in closed-loop as show in
Block diagram of the proposed NIRS acquisition channels
The proposed transimpedance amplification circuit. (a) Block diagram of the optical receiver front-end; (b) Proposed integrated transimpedance amplification circuit
When the photodiode is connected to the input of the TIA, the current of the photodiode is split between the amplifier and the photodiode capacitor. The trans-impedance is expressed according to
where
Integrated circuit of the transimpedance amplifier is presented in
where
where
In order to reduce the current input noise of the circuit, a small size of feedback transistor is chosen, it is inversely proportional to the static resistor and it can be expressed as following equation:
Non-dominant pole of the circuit represented by the transistor drain and gate capacitors of the common gate amplifier. Consequently, this transistor size is selected to increase the total gain of the circuit and the frequency of the non-dominant pole in order to reduce signal dephasing and stability in closed-loops. The second branch of the input stage is designed to increase the inducing current for getting a high open-loop gain. Second stage is a source follower used as a level up shifter. It maintains a closed common mode DC voltage level at the output and input of the circuit in order to sustain DC stability in the closed-loop system. Finally, the capacitor and resistor load connected at the output of the circuit are used as a load and to compensate the phase margin of the circuit. Notice that the TIA can be computed according to
where
A transistor feedback system is proposed to highly reduce the input current referred noise of the circuit. It is based on a simple transistor used to replace the feedback resistor.
where gds is the trans-conductance of transistor M1.
The linear relationship between the voltages of the transistor and the drain current are simplified as follows:
The resistor
Small signal model of the proposed circuits for noise analysis
The current input noise of transistor M6 can be presented as follows:
The total input referred noise is simplified as shown in equation (12).
where
A minimum transistor size is chosen in order to get high
where
From equation (14), we can express the cut-off frequency as presented in equation (15).
We can determine the cut-off frequency by resistor and capacitor controlling as shown in equation (15).
Schematic of a low-pass filter. (a) Simplified low-pass filter circuit; (b) Proposed integrated low-pass filter based on a tunable resistor and capacitor
flects a low-power circuit topology for μw consumption, which is a main desired for embedded medical devices. It’s necessary in a portable medical system.
All transistors are biased in the saturation region. Transistors M4-M5, M6-M7, M8-M9, M10-M11 and the tail current source represented in three transistors M1, M2 and M3, offer common-mode rejection, gain and frequency response. Transistor M12 represents the variable resistor controlled by bias voltage
Circuits of the trans-impedance amplifier and low-pass filter are implemented in CMOS 0.18 µm. The simulation was done with Spectre using Cadence platform.
. Comparison of the proposed trans-impedance amplifier with conventional topologies
Features | This work | [21] ** | [22] * | [23] ** | [24] ** |
---|---|---|---|---|---|
Input current referred noise | 500fA/√Hz @ 10MHz-100 MHz | 40.8nA/√Hz @ 30 Hz - 5KHz | 9pA/√Hz | 6.3pA/√Hz | 6.4pA/√Hz @ 200MHz |
Technology | 0.18 mm | 0.35 mm | 0.35 mm | 0.6mm | 0.35 mm |
Gain (dBΩ) | 104.1 | 63.5 | 56 - 68 | 58 | 90.4 |
Power | 710 mW | 145 mW | 6.9 mW | 85.0 mW | 30 mW |
Supply Voltage | 1.8 V | 2.5 V | 1.8 V | 5 V | 3 V |
**Measured; *Simulated.
Layout of the proposed circuits with their area. (a) Full layout of integrated circuit; (b) Transimpedance circuit; (c) Low pass-filter. (a) Layout of the proposed TIA and LPF; (b) Transimpedance area is 0.0036 mm2; (c) Low pass-filter
Simulation results of the transimpe- dance amplifier input referred noise
constraints in order to get minimum noise and thus receive accurate medical diagnoses based on: miniaturized NIRS device, reduced power consumption, and minimized electronic component noise. Note that the miniaturization of NIRS device used a low power consumption which provides weak amplitude of NIRS signals however de noising signal can be help to obtain quality NIRS signals [
In this paper, a low-noise front-end dedicated to near infrared spectroscopy applications has been implemented. A topology of transimpedance amplifier is presented, based on a bias input circuit with low-input impedance, which is connected to input stages to isolate the bandwidth dependency from the photodiode parasitic effect. Higher closed- and open-loop gains were obtained by using a common source and common gate topology enhanced by a second active branch. To improve noise performance, we used a feedback transistor in order to en-
Simulation results of the transimpe- dance amplifier gain
Simulation results of Low Pass Filter. (a) Cut-off frequency and curve degrees; (b) Input voltage noise of the opam
hance the input noise performance. We presented a low-pass filter as a wide tunable cut-off frequency to match with input frequency signals from a photodiode. Layout simulation results showed that the TIA exhibit a low- input referred noise, a high open- and closed-loop transimpedance gain and low-power consumption compared to conventional topologies. As well as, the LPF showed a low-input voltage noise, wide tunable cut-off frequen- cy and low-power consumption. Our TIA and LPF circuit can be implemented for most portable biomedical de- vices like EEG and ECG. The electronic analog circuits of TIA and LPF are relatively simple and can be easly connected with ADC to obtain NIRS signal.
Authors would like to acknowledge financial support from Canada Research Chair in Smart Medical Devices, and the design tools from CMC Microsystems.