Digitally-programmable switched-current filters

Graduation date: 1990Digitally-programmable filters have been an ongoing research topic for a number of years. The first such filters were FIR transversal filters using Charge-Coupled Devices (CCD's) and IIR recursive filters using switched-capacitor (SC) techniques. Although both techniques achieve excellent results, they require non-standard and/or additional IC fabrication steps. Low substrate doping is often essential to obtain high charge-transfer efficiency in CCD filters. This is contrary to the trend towards higher doping levels as MOSFETs are scaled. Switched-capacitor circuits require floating linear capacitors that add processing complexity. SC circuits also use voltage operational amplifiers which limit the maximum operating frequency and the minimum power supply voltage. The recently introduced switched-current (SI) technique [1] is an attractive alternative for implementing digitally-programmable filters. SI circuits may be viewed as charge processors where Q= It as opposed to SC circuits wherein Q=CV. Hence, in SI circuits, current rather than voltage is the working variable, and time rather than a capacitance ratio is the precision quantity. No precision circuit elements are required. Therefore, a standard low-voltage scaled digital VLSI CMOS process may be used to implement analog sampled-data SI filters. As current is the working quantity in SI circuits, current signal amplification may be realized using simple current reflection techniques. Because of the low impedance nodes associated with CMOS current mirrors, higher operating frequencies are expected as compared with SC circuits. The low impedance nodes associated with the current amplifiers also suggest reduced power-supply coupling for precision mixed-mode applications. In this study, we present design techniques for digitally programming a second-order SI filter section. While providing similar capabilities to the programmable SC filters [2], the SI circuits have an additional degree of flexibility for optimization in that AC signal currents and/or DC bias currents are programmable. In order to directly compare the SI and SC techniques, the programmable SI filter has been designed to the same specifications as the programmable SC filter of [2]. The programmable second-order section has 63 possible gain (G) values, 63 possible selectivity (Q) values, and 8 possible logarithmically-spaced center frequencies (coo) per octave