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Sillicon Photomultipliers (SiPMs) with quantum efficiency maximized for a wavelength between 420 and 470 nm have been developed at ITC-irst Trento (Italy), and are being tested for their application in the construction of a ultra high resolution small animal PET tomograph. The devices have an area of 1 mm x 1 mm and 625 microcells. The breakdown voltage is around 30 V, and the gain of the order of [10.sup.6]. The intrinsic timing resolution is 70 ps rms at the single photoelectron level. The first tests as readout for scintillators show an energy resolution of 21% FWHM with Na-22 employing LSO crystals. The first matrices of SiPMs have been produced and are being tested. Index Terms--Geiger APD, photodetector, silicon photomultiplier, small animal pet.

Continuous-time (CT) filters are widely used in signal processing but, owing to the large spread in the process parameters which determine their time constants, they require a tuning system to align their frequency characteristics. Several techniques have already been proposed in the literature and are commonly employed, but usually they require either a doubling of the filter circuitry or some complex self-calibration scheme, resulting in sensible area overhead or high computational complexity. Here, we present a novel tuning approach which can be applied to both master-slave and self-calibration arrangements and is based on the evaluation of a few samples of the input-output cross-correlation function of the filter, when the input signal is a finite length sequence of pseudorandom pulses. The key advantages of the proposed technique are basically the ease of generation of the input bit-stream and the simplicity of the circuitry required to sample the filter output and to perform the cross-correlation operation. As a consequence, the area overhead associated to the tuning circuitry is kept very small. Some simulation results of the application to the tuning of a low-Q telecommunication filter are given and an extension to the tuning of high-Q active filters is also explored in order to assess the effectiveness and robustness of the proposed technique. Index Terms--Continuous-time (CT) filters, mixed analog-digital integrated circuits, tunable filters, tuning.

The complementary metal-oxide-semiconductor (CMOS) differential amplifier with active load and single-ended output is one of the most widespread analog building blocks in modern mixed-signal circuits for signal processing applications, because of its good performance in terms of common-mode rejection and voltage gain, combined with an extremely simple circuit structure, which performs directly differential to single-ended conversion. The authors present a straightforward approach to studying the behavior of such a simple circuit and try to explain some of its features in an intuitive way without resorting to tedious calculations. In particular, this study considers the dc operating point and the common- and differential-mode voltage gains, and the results of the proposed analysis are in good agreement with those provided by both accurate analytical solution and simulation of the circuit. Among the other results, this approach emphasizes some interesting, though very often neglected, aspects of the circuit behavior. Index Terms--Active load, complementary metal-oxide-semiconductor (CMOS) analog circuits, common-mode gain, differential amplifiers, differential-mode gain.

One of the main benefits of negative feedback is the ability to control the impedances in linear active networks (LANs). Traditionally in undergraduate courses, the design of a feedback circuit is cast in terms of a two-port analysis, since the closed-loop properties of the circuit are expressed in terms of the corresponding open-loop ones. Therefore, the designer can directly identify the proper feedback topology and adjust the amount of feedback based on the specified closed-loop performances. However, there are practical situations in which a two-port analysis may lead to incorrect results if applied inappropriately or cannot be applied at all. In this paper, the authors show how it is possible to achieve a better insight of these problems in the light of Blackman's method. Moreover, after a derivation of the formula as a consequence of the superposition principle applied to a LAN, they discuss, with reference to some typical examples, the conditions that must be met for the correct application of the two-port method. Index Terms--Blackman's formula, feedback circuits, impedance evaluation, return ratio, two-port analysis.