[Design Application]
Up Close And Personal With High-Speed Crosspoint Switches
Maintaining Signal Fidelity When Switching Large Arrays Of Fast Signals Calls For Modern ICs And Good Pc-Board Design Smarts.
Many complex video, audio, and telephony systems require electronic signals to arbitrarily switch within a given network of M inputs and N outputs. There are important trade-offs to consider, however, when selecting a CMOS or bipolar crosspoint-IC solution.
The common belief that CMOS is inherently lower in cost and power when compared to bipolar is no longer true, particularly in video applications. The designer must consider the total solution cost of each approach before proceeding with a design. In most applications, all inputs and outputs of a CMOS crosspoint IC need to be buffered. Bipolar ICs already have these buffers integrated into the die.
To minimize the effects of crosstalk and signal-frequency roll-off, consider the layout of the board. Multilayer pc boards are usually required to maintain a characteristic impedance, as well as to isolate and shield multiple signal paths from generating or receiving interference signals.
In years past, switching was accomplished by connecting a matrix of inputs and outputs, either manually or by using electromechanical devices. These switching networks were often referred to as a "crossbar switch," or in more modern times, as a "crosspoint switch."
With the advent of semiconductors, crosspoint-switching arrays were made using discrete transistors. Later, ICs were utilized, particularly with analog multiplexers. Over time, these techniques became costly and sometimes unreliable. Today's design is simplified by using monolithic crosspoint-switch ICs already designed with an M-by-N matrix size and configuration-state storage in digital registers. Most crosspoint-IC logic can be programmed with serial or parallel data to set the required connection of inputs to outputs. Thanks to these components, it's now possible to design cost-effective crosspoint-switch arrays using single or multiple crosspoint ICs that have excellent performance and reliability.
Bipolar vs. MOS Crosspoint Switches: The first solid-state crosspoint solutions available on the market were manufactured using MOS processes. CMOS or DMOS are commonly used processes. Originally targeted for audio and telephony applications, these devices are still in use. A basic MOS crosspoint is designed with the well-known "T-switch" cell (Fig. 1), which is essentially a NMOS 2:1 multiplexer consisting of T switches S0 and S1. Its advantages lie in its simplicity, zero dc offset, low-static power dissipation, and cost. Since the inputs and outputs of the switches are symmetrical, the switch is inherently bidirectional, which is essential in many telecommunication applications. In effect, the bidirectional design connects the input source to the load when the switch is on, limiting the number of outputs to which an input can be fanned out. This bidirectional signal path is formed by the inherent RDS-ON resistance from input to output. The magnitude of RDS-ON in the pass transistors (M1 to M4) depends on the voltage level of the applied signal. Figure 1b shows the nonlinear relationship between RDS-ON and signal voltage.
To minimize this effect, T switches are designed using a CMOS process (Fig. 2). The model is approximated by a three-pole low-pass filter where CS and CD are the source and drain capacitance in the on state. CMOS lets complementary transistors (M1´ to M4´) be added in parallel with M1 to M4, helping mitigate the nonlinear affects of RDS-ON on the signal. The transfer functions of the complementary transistors "overlap" each other, aiding to minimize RDS-ON and improve the switch linearity (Fig. 2b). This tends to reduce the level of dc errors and ac distortion in the T switch.
Typically, to get RDS-ON as low as possible, very large devices are required for each CMOS transistor. This often results in a large input capacitance of ~10 pF, a high-disable output capacitance of ~10 pF, and a substantial charge injection of ~5 pC when switching between channels. If not properly compensated, the consequence of the charge injection can be large glitches of ~1 V. Compensation for some of these shortcomings can be achieved by shrinking the switch transistors and buffering the inputs and outputs on the die.
Unfortunately, CMOS processes aren't well suited for implementing high-performance, analog-buffer amplifiers. Crosspoint ICs designed using CMOS generally don't include drivers (gain) to operate directly into the back-terminated connections used in telecommunication and video applications. Designers using CMOS crosspoint switches must use external buffers on the inputs and outputs to alleviate RDS-ON effects and charge-injection-induced glitches, as well as to drive back-terminated loads. The designer must consider buffers and drivers with specifications to drive large capacitive loads up to 100 pF, with little degradation in frequency response and distortion. This requirement adds cost and power consumption to the design, while complicating the pc-board layout and assembly.
Crosspoint-IC solutions are available for more demanding applications like composite video and computer graphics, which have integrated buffers and drivers. These are fabricated using a fast complementary bipolar process with efficient vertical NPN and PNP transistors that are well matched. One particular process that is well suited for these applications is Analog Devices' eXtra Fast Complementary Bipolar process (XFCB), based on silicon-on-insulator (SOI) using bonded wafers. This process not only allows for small- geometry transistors, but supplies extremely fast performance with low supply currents and voltages (±5 V).
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