Drazen Jurisic

Low-noise, low-sensitivity, active-RC allpole filters using impedance tapering

In this paper it is shown that active-RC filters whose sensitivity to component tolerances can be minimized by impedance tapering, will also have low output thermal noise. It is shown that impedance tapering will also reduce output thermal noise in OTA-C filters. Copyright

Low-sensitivity current-mode active-RC filters using impedance tapering

This paper is concerned with a new design method of low-sensitivity current-mode filters, which results from low-sensitivity voltage-mode filter design using impedance tapering. The current-mode filters are obtained by application of a network transposition to their voltage-mode counterparts, in which the passive-RC network remains the same, and both filters are expected to have identical sensitivity properties. However, current-mode filters may be easier to realize in IC form and are expected to have higher bandwidths, greater linearity and wider dynamic range than the voltage-mode filters. In this paper, the 2nd-order class-4 current-mode filters (with positive feedback) are considered. Design procedures are given for the design of low-sensitivity low-pass (LP), high-pass (HP), band-pass (BP) and band-rejection (BR)) filters, which are all realizable in class-4. A sensitivity analysis is examined using PSpice Monte Carlo runs.

A Novel Third-Order Leap-Frog Active Filter

This paper presents the realization of third-order low-pass active-RC filters using a new Leap-Frog (LF) topology. New structure is a simplified LF structure with the elements calculated directly from the transfer function coefficients. Several versions of the circuits are presented and compared. The comparison to other common third-order filter sections is done, as well. The new LF filter has the reduced number of components, reduced complexity and straightforward design procedure compared to classical filters. As an illustration of the efficiency of the proposed new LF filter, the sensitivity analysis using Schoeffler sensitivity measure as well as output thermal noise analysis was performed on examples with Butterworth and Chebyshev 0.5dB pass-band ripple transfer functions. Using PSpice with a TL081 opamp model, the filter performance is simulated and the results compared and verified by measurements on a discrete-component breadboard filter. All equations needed for the step-by-step design are given.

Tuning elliptic filters with a ‘tuning biquad’

This paper presents an optimum tuning procedure for high-order low-pass (LP) elliptic filters. Since elliptic filters are often used to satisfy very tight specifications, they often need to be tuned accurately. In this paper, we describe the tuning of one biquad, the ‘tuning biquad’, in a cascade of biquads. It is shown by Matlab simulations that the best choice for the tuning biquad consists of the pole pair with the highest pole Q (‘maximum-Q poles’) combined with the zero pair with the lowest frequency (‘minimum-frequency zeros’). We also show how standard tuning procedures, such as those for the Tow-Thomas biquad, lead to excellent results. As an example, the tuning procedure is performed on a normalized sevent-horder elliptic LP filter.

Low-sensitivity active-RC high- and band-pass second-order Sallen & Key allpole filters

The design procedure of low-sensitivity, low-pass (LP) 2/sup nd/- and 3/sup rd/-order class-4 Sallen and Key active resistance-capacitance (RC) allpole filters, using impedance tapering, has already been published. In this paper desensitization using impedance tapering is applied to HP and BP 2/sup nd/-order filters. It is shown that HP filters have dual properties to LP filters in the sense of sensitivity. Among various topologies of BP filters, the best topology is proposed. The sensitivity of a filter transfer function to component tolerances is examined using the Schoeffler sensitivity measure as a basis for comparison. Monte Carlo runs are performed as a double-check. The component values, selected for impedance tapering, account for the considerable decrease in sensitivities to component tolerances for the LP as well as for the HP and BP filters.

Dynamic range improvement of new leap-frog filter using numerical optimization

In this paper the dynamic-range optimization of a new class of low-pass active-RC filters, based on the leap-frog topology is presented, using the MATLAB Optimization Toolbox. The new structure is a simplified leap-frog structure with a reduced number of components, reduced complexity, and permitting a straightforward design procedure. It is most suitable for the nonbalanced-to-ground version of low-sensitivity filters. As an illustration of the efficiency of the proposed new leap-frog filter, a sensitivity analysis using Monte Carlo runs was performed on examples with Butterworth and Chebyshev 0.5dB pass-band ripple low-pass filters. A comparison with other common filters such as standard leap-frog filters and single-amplifier Biquads demonstrates the sensitivity improvement obtained with the new filters. In this paper we present a numerical method of optimizing these new filters for improved dynamic range.

Ladder-Biquad filter partitioning for on-chip tuning

One of the problems with on-chip active-RC filters is that in order to satisfy tight specifications accurately, they need to be fine-tuned. This becomes more difficult with increasing filter order. In this paper, we introduce a simplified tuning method which is obtained by a new structure consisting of a filter section of second- or third-order (Biquad or Bitriplet which is referred to as a ‘tuning block’), in cascade with a ladder-RLC filter (passive LC, or simulated-active RC). The cut-off frequency of the resulting filter can be simply tuned by adjusting one component of the tuning block. The new ladder tuning-block (LTB) is compared with two conventional structures, a cascade of Biquads and a single-ladder filter. Fine tuning these two structures is considerably more complicated. It is shown that beside the advantage of ease of tuning, the sensitivity to component tolerances of the new structure is lower than that of the two conventional ones, for the price of a slightly higher filter order.

Optimal design of low-sensitivity, low-power 2 nd -order BP filters

In this paper we present an optimal design procedure for 2nd-order BP filters with a single operational amplifier. We compare two known design procedures and investigate how a combination of two design methods yields an optimal 2nd-order BP filter. The new design provides a filter with low sensitivity to passive components. We present this straightforward design procedure in the form of a tabulated step-by-step design framework and demonstrate it with a 2nd-order Chebyshev filter. The resulting sensitivity is investigated using the Schoeffler sensitivity measure.

Elliptic biquadratic sections using second generation current conveyors (CCIIs)

In this paper we propose a new, programmable, easy-tunable, current-mode general-purpose Biquad (C-GP-B), using second-generation current conveyors (CCIIs), based on the well-known direct form II, which, among other transfer functions, is also suitable to realize band-rejection (BR) or notch transfer function. It is convenient for elliptic transfer function realizations because it can realize finite transfer function zeroes. Since it has low sensitivity and grounded capacitors it is suitable for integrated circuit (IC) design. The new Biquad is then compared in sensitivity performance with other two CCII-based versions of biquadratic sections obtained by different approaches. Since all presented Biquads use CCIIs they operate in the current mode, and are therefore suitable to realize filter circuits operating at higher frequencies, consuming less power, and needing lower supply voltage. As an example, the design procedure, and the Monte Carlo sensitivity analysis using PSpice were performed for a fourth-order elliptic low-pass (LP) filter.

Systems for analog and mixed signal processing in integrated technology based on network theory

Despite the overwhelming trend of digitization of electronic systems, and many indisputable advantages of having digital signal processing compared to analog, hardware towards the real, outside world, is a part of the overall mixed system and remains mostly analog. This analog part, in most IC-system chips is well known as “analog-front-end” (AFE). Therefore, the analog electronic circuits as part of an AFE, not only will remain an important part of most integrated systems, but often represent one of the bottlenecks in achieving low power consumption and small area on an integrated circuit. This explains why the development and design of new and improved (from the standpoint of IC design) analog electrical circuits still plays an important role — and will likely continue to play an important role — in the development of new and advanced systems-on-chip. One of the types of analog circuits that have traditionally lagged behind the others in the process of designing in integrated-circuit form is frequency-selective filters. Filters on the chip, may not have inductances; in the time-continued domain they are the so-called ‘active RC filters’, which means they combine resistors and capacitors with voltage or current amplifiers. Reasons for the delay in the design of integrated active-RC filters are manifold, but one of the most important are theirs large components tolerances (as a consequence there is a need for circuit tuning), voltage noise within the circuit, limited dynamic range and the size of the chip. The new and innovative ideas arise in the design of time-continuous filters from the application of network and transmission line theory. Some special situations in communication systems require novel topologies in filter design, and solve different problems that are important in modern analog IC design. The new approaches are: (i) savings in chip area in elliptical filters in the integrated design using signal-flow graphs, (ii) reducing the sensitivity to variations in the component values by application of negative feedback, (iii) filter partitioning for efficient onchip filter tuning, and (iv) the new structure of the active-RC filter that is used to build ADSL splitter; a stability problem to be solved with different telephone-line terminations in different countries. All four systems are suitable to be designed in the analog filter IC design.