Low Frequency Continuous-time Filters Research Articles

Applying the Difference Term Approach for Low Frequency Biomedical Filter

Low frequency continuous time filters are essential analog blocks for biomedical applications. Integrating such filters having large time constants is difficult as it requires large component values. A novel approach to scale down the pole frequency is presented. A 5-bit reduction in the cut off frequency is achieved. This is made possible through adding a passive resistor in the forward path of the op-amp based integrator introducing a difference term of the pole frequency. Also, the filter topology is modified to avoid changing the quality factor. As an example, a 2nd order low pass filter is designed and simulated. Simulation results show that the pole frequency is scaled down from 1.43 MHz to 4.97 kHz while maintaining tuning of 30% around the nominal value by controlling only one resistor.

A 0.5 V bulk-input OTA with improved common-mode feedback for low-frequency filtering applications

This paper presents the design of a two-stage pseudo-differential operational transconductance amplifier (OTA) and its application in low-frequency continuous time filters. The OTA was designed in a 0.18 μm, 0.45 V V T CMOS process. An improved bulk-mode common-mode feedback (CMFB) circuit has been designed which does not load the OTA compared to prior art. A self cascode load structure and partial positive feedback provide higher gain. The bulk terminals of all transistors have been biased to lower their threshold voltages (VT) and maximize signal swing. The OTA operates at a supply voltage of 0.5 V and consumes only 28 μW of power. Rail-to-rail input is made possible by using the transistor's bulk terminal as the input. For a load of 20 pF the OTA has a measured DC gain of 63 dB and a gain-bandwidth product of 570 kHz. To demonstrate the use of the OTA in practical circuits, three active RC filters were designed: a 10 kHz Butterworth filter, a 10 kHz Bessel filter, and a 2.5 kHz Tschebycheff filter.

Current-mode integrator for voltage-controllable low frequency continuous-time filters

A novel differential current-mode integrator (CMI) for voltage-controllable low frequency continuous-time filters is presented. An example fifth-order lowpass filter using the proposed CMI and on-chip capacitors was implemented in an AMI 1.2 µm CMOS process, and it achieved −3 dB cutoff frequencies ranging from 160 Hz to 5.6 kHz, by changing a single control voltage.

Design considerations and implementation of very low frequency continuous-time CMOS monolithic filters

The design of low frequency continuous-time filters is considered. Problems encountered at such frequencies are discussed, along with possible solutions. Two case histories reporting experimentally verified designs are presented to show that good quality low frequency filters can be designed reliably in CMOS technology.

IC Voltage to Current Transducers with Very Small Transconductance

This paper deals with the design of very small ac transconductance voltage to current transducers intended for the design of low frequency continuous-time filters, very large resistors and other applications. The first type of Operational Transconductance Amplifiers (OTA) is based on a triode biased transistor and a current division technique. The second one uses partial positive feedback which allows to reduce transistor dimensions but the sensitivity to transistor mismatches increases. The proposed techniques can be used for the design of high-order low frequency IC filters, ladder or based on biquads, with moderated transistor dimensions while the dynamic range-cutoff frequency performance is comparable to previously reported structures. A 10 Hz third order lowpass ladder filter has been designed with these techniques, and it shows a dynamic range of 62 dB. Besides, a novel biasing technique for capacitive sources coupled preamplifiers is proposed. Experimental results for a prototype, fabricated in a 1.2 μm CMOS process, have shown very low distortion components (THD< 1 level below 15 μ V_RMS and dynamic range of 63 dB. The power consumption is only 10 μwatts and the supply voltages are ± 1.5 volts.

Noise improvement beyond the "kT/C limit"" in low frequency C-T filters

A technique to improve the noise performance of low frequency continuous-time filters is presented. The technique is based on scaling the filter's capacitor currents using current mirrors. First a representative current-scaled filter topology is analyzed using a generic current mirror model. It is concluded that the transconductance of the current mirror transistors should be small if a noise improvement is to be achieved. Noise properties of down-scaling bipolar and MOS current mirrors are presented along with comparisons to the generic model. Several current-scaled 100 Hz first-order low-pass filters have been designed and optimized for low noise, using the obtained analysis results. Simulation results indicate that if a class A/B biasing scheme is used for the down-scaling current mirrors, then a noise improvement of at least 8 dB can be obtained, when compared to a nonscaled reference filter. Comparing the simulated noise to the fundamental limit for a passive RC-filter (kT/C), a 5 dB lower noise level is demonstrated.