It‟s necessary about a front-end biopotential amplifier for the readout system due to the slight amplitude biopotential signal. The amplifier must have two characteristics: to amplify the signal and property of low noise. Considering the portable systems and implantation of human security, the power dissipation of the circuit must be minimized for long-term biopotential signal monitoring. High common-mode rejection ratio (CMRR) and high input impedance design to reduce the effect of interference from human body at 50/60Hz. Another important issue is the differential DC offset, which is generated by electrodes, results in the saturation of the amplifier. In conclusion, design the biopotential amplifier has a lot of challenges to overcome. Recently, there are four structures of circuit apply to biopotential amplifier, and are described below.
A type of biopotential amplifiers is chopper stabilized amplifier. It uses chopper technique to cancel the dynamic offset and low frequency noise of the amplifier. The low frequency noise especially means flicker noise (i.e. 1/f noise), power spectral density of which is inversely proportional to the frequency. The operation principle of
the chopper technique is that the input signal is modulated to the chopping frequency, amplified and demodulated to the baseband. The offset and noise is modulated only once, and its frequency is shifted to the chopping frequency and its odd harmonics. These modulated offset and noise can be filtered by low pass filter.
must be larger than two time than the bandwidth of the input signal to prevent aliasing.
And that is limited by noise issue, too. To remove effectively the 1 noise, it should be higher than the 1 noise corner frequency (at which means the frequency thermal noise and flicker noise of a circuit contributes the same spectral density).
Fig. 1 The architecture of rail to rail instrumentation amplifier [10]
Fig. 1 shows a rail to rail instrumentation amplifier for portable EEG/ECG monitoring applications. The parallel input structure utilized PMOS and NMOS replica structures provides large input common-mode range. Furthermore, it use chopper stabilized technique to reduce noise and low frequency disturbance. To reject DC offset occurred at electrode-tissue interface, R1, R2, and C1 are used to generate a
very large time constants. For achieving the goal, the capacitors are made of off-chip device. The size of off-chip devices are usually large than integrated chip. Therefore, external component is NOT suitable for implantation. Architecture of [10] consumes large power due to replica input stage. Meanwhile, this ICs is designed for conventional EEG/ECG monitoring, them possess a narrow bandwidth (150 Hz).
Fig. 2 Structure of [11]
Another chopper stabilized technique biopotential amplifier is shown in Fig. 2.
The architecture consists of a micro-power chopper amplifier and multi-loop feedbacks. Gain of whole circuit is determined by the ratio of Ci and Cfb and output buffer, gain of which is generated by ratio of resistor. It improves the issue of using external component. The high pass corner is generated by Chp and switch integrator.
The overall gain is associated with low cut-off frequency. It results in the complication of the design. Although it has very low power consumption, the bandwidth of [11] is merely 180 Hz. It possesses moderated noise performance and not well enough noise efficiency factor (NEF).
Fig. 3 Architecture of [12]
Fig. 3 shows the structure of [12]. That architectures are composed of a chopper stabilized amplifier and negative feedback loop is very similar to this thesis. That case has low input referred noise and very excellent noise efficiency factor performance.
Switch capacitor resistors are used to generate a 150GΩ resistance. Therefore, to let high pass corner closed to DC and provide appropriate gain, the external components are still used. Although it has very low power consumption, the bandwidth of [12] is only 100 Hz and the CMRR performance is NOT good enough.
Fig. 4 The schematic of biopotential amplifier rejects DC offset [13].
The bandwidths of biopotential signals are from the millihertz range to few kilohertz range. Therefore, to rejects DC electrode offset without influence biopotential signal is difficult challenge. Fig. 4 shows the schematic of one of biopotential amplifier design [13], it claim that can generate a tens millihertz low frequency cutoff given by 1 𝐶 . 𝐶 is set to 200fP, and is a MOS-bipolar pseudoresistor consist of transistor Ma-Md. The resistance of the pseudoresistor can reach more than 1012 ohm when the voltage across this device is between 0.2 V. the mid-band gain of this architecture is set by 𝐶 𝐶 . And -3dB corner is approximately 𝐶 as 𝐶 , 𝐶 𝐶 . Where is the transconductance of the operational transconductance amplifier.
There are a lot of interferences when measure the biopotential signals.
Environmental noise (introduce in chapter 3) is one of them. Device electronic noise is the other. It is the important issue in the topic of biopotential amplifier design, since it limits the minimum signal level that the amplifier can process. Thermal and flicker noise is common type of device electronic noise. Since electrons in a conductor move
randomly by heat, the voltage across the conductor is fluctuant. The noise is called as thermal noise. The power spectral density is
(1) Where 1 1 is the Boltzmann constant. is absolute temperature. is the resistance of the conductor. According to eq. (1) the power spectral density of thermal noise does not change by frequency. It‟s a typical white noise. The flicker noise spectral density, however, depends on the frequency. Due to many dangling bonds are formed at the interface between the gate oxide and the silicon substrate in a MOSFET, the interface rises to extra energy states. The movements of the change carriers are influenced by such energy states. The flicker noise can be modeled as a voltage source with the gate terminal, and the noise spectral density is expressed as performance and bandwidth of different circuits and can be expressed as:
√
Where is the total input referred voltage noise of the amplifier, is the total amplifier supply current, and BW is the -3dB bandwidth of the amplifier in Hertz. The NEF of a single bipolar transistor without 1/f noise is 1. Therefore, NEF is
a factor which implies the performance of the biopotential amplifier in comparison with a single bipolar transistor. In [13], the amplifier reduces the noise by using large MOS transistor size and the NEF of the circuit is down to 4.