IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 29, NO. 8, AUGUST 1994 921 Programmable S witched-Current Wave Analog Filters Albert0 Ydfera, Student Member, IEEE, Adoracidn Rueda, Member, IEEE, and Jose L. Huertas, Senior Member, IEEE Abstruct- This paper presents a methodology to realize Programmable Switched-Current filters. A universal wave filter structure is built based on a low-pass (LP) to band-pass (BP) frequency transformation in the z-domain that allows obtaining different filtering functions from a single low-pass reference filter without altering the global circuit topology. Two different parameters, modified by changing the gain of current mirrors, independently control the filter bandwidth and center frequency. A 2.88-mm2 IC prototype has been fabricated in a 1.6-pm CMOS digital technology that is capable of implementing three LP’s (and their three complementary HP’s) and nine BP’s (and their nine complementary BR) Chebyschev filters. The realization of the 24 filtering functions requires less than 15% of additional area than that required to implement only one BP function. The chip operates from a 5-V supply, dissipates 0.83 mW/pole, and met the expected performance levels for all filter functions. I. INTRODUCTION OW-sensitive switched-current (SI) filters are based on L the emulation of doubly-terminated LC ladder networks because of these circuits’ low sensitivity to their element variations. Two different approaches have been explored to implement these SI filters. One applies the synthesis pro- cedures broadly used for switched-capacitor (SC) filters [ 11, [2], the other, which has proven to be particularly interesting for SI techniques [3], [4], is based on the wave digital filter (WDF) concept developed by Fettweis [5]. Wave digital filters emulate the behavior of passive lossless reference filters by transforming passive L and C elements into one-port digital elements defined by an incident signal, a reflected signal, and a port resistance determined by the value of the corresponding analog element and the sampling frequency. The interconnection of one-port elements with different port resistances requires a multiport element called an adaptor. In addition to their excellent passband sensitivity properties, WDF’s have two inherent advantages. They are based on the bilinear transformation between continuous and discrete fre- quency domains, and they have a high degree of homogeneity involving very regular and simple structures formed by the interconnection of easy-to-design components (linear multiport and unit delay) instead of requiring high quality integrators. In spite of both their wide acceptance in digital appli- cations and their theoretical superiority, wave filters have Manuscript received December 29, 1993; revised March 30, 1994. The authors are with Dto. de Disefio Anal6gic0, Centro Nacional de Microelectrhica, Universidad de Sevilla, Edificio CICA Cmarfia, 41012- Sevilla, Spain. IEEE Log Number 9402 145, been found impractical in analog when either continuous- time or switched-capacitors are used. However, since the basic operations required to implement Wave Analog Filters (WAF) are addition, subtraction, multiplication by a constant, and delaying, a current mode technique involving only accurate current amplifiers and simple memory cells appears highly suitable. We have already proved that the wave filter concept is worth considering as a practical manner to implement SI analog filters in low-voltage standard CMOS technologies [3], This paper intends to explore further this technique, pro- viding a method to synthesize programmable SI filters which is demonstrated with a working IC prototype. We will show how different filtering functions can be obtained from a single low-pass reference filter relying on a universal wave filter structure. Section I1 describes the general procedure to build programmable structures, based on an LP to BP frequency transformation in the z-domain, which gives rise to a universal wave filter structure. Two different parameters of the transformation independently control the bandwidth and center frequency of the filter. The basic building blocks used in the programmable switched-current WAF are shown in Section 111, together with some synthesis issues, which by exploiting the freedom in the choice of filter parameter values, leads to implementations that are less dependent on element tolerances and nonidealities. Section IV presents an IC prototype that implements a programmable 6-th order bilinear Chebyschev filter and its experimental results. Finally, Section V gives some conclusions. [41. 11. PPROGRAMMABLE FILTER STRUCTURE A ) Wave Filter Synthesis A general procedure to realize a wave filter basically con- sists of two steps. First, a reference passive filter is constructed to meet the required frequency specifications. Then, a wave model is derived which emulates the reference filter. This wave model establishes the relationships between wave signals through modular building blocks (delays and adaptors). The wave model is described in the z-domain and the reference fil- ter in the +-domain, in such a manner that the correspondence between domains is governed by the bilinear transformation, where T is the sampling period. 0018-9200/94$04.00 0 1994 IEEE