AN501 VISHAY
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www.vishay.com FaxBack 408-970-5600 6-2 Document Number: 70608 03-Aug-99 The DG536 is housed in a small, 44-pin J-lead package, thus minimizing board size requirements. The DG535 is packaged in a 28-pin DIP. Chip select pins (CS and CS ) permit easy stacking of devices for multi-channel multiplexing systems (see Applications Section, Figure 19). An additional feature, a DIS pin, is an open drain terminal with the source tied to the device substrate. The DIS terminal represents a high impedance to the substrate (normally ground) when the DG535/536 is disabled and a low impedance to ground when the DG535/536 is enabled. This output can be used to indicate which device in a large matrix has been enabled (see Figure 13), or it can be used to switch off circuitry following the multiplexer stages. Minimizing Parasitic Effects The insertion loss and bandwidth of the switch are improved with DMOS transistors that offer a low on-resistance and low intrinsic capacitance (see Vishay Siliconix SD5000 data sheet). On the DG536 channel-to-channel crosstalk is minimized by physically separating each input channel with a GND pin which extends to the device substrate. This, in conjunction with careful PC board layout (see Figure 6), can yield channel-to-channel crosstalk figures better than –92 dB at 5 MHz. Further ac performance benefits are obtained through the n-channel DMOS transistor T configurations (Figure 2). This maximizes the off-isolation, since SW 2 provides a shunt path to ground for any signals fed through the parasitic capacitance associated with SW 1. SW 3 (working in phase with SW1) provides an extra stage of off-isolation and prevents the shunt switch (SW 2) from affecting consecutive channels. Two-Level Switching The two-level switching system of the DG535/536 (SW4 and SW 5) works out of phase, effectively isolating half of the switch outputs from the drain (output) of the multiplexer. These series switches serve several functions: They provide an extra stage of off-isolation. They reduce the drain output capacitance significantly and increase the multiplexing transition speed. They reduce the off-leakage current, which reduces the offset voltage that develops from the total off-leakage current flowing through the load resistance and/or switch on-resistance. This enables lower analog signal levels to be handled accurately. Silicon Gate Polysilicon is used as the transistor gate material for the DG535/536, as opposed to more conventional metal-gate designs. This technology minimizes the charge coupling of the control-logic signals to the switch output due to the self-aligning properties of the process. Metal-gate technology relies on photolithographically aligning the gate metal with the channel diffusions, resulting in greater overlap tolerances. As shown in Figure 3, a PN junction exists between the p-type substrate and the n-type channel diffusions. This junction should not become forward biased by the analog signal going more negative than the substrate potential (normally ground). Device damage could result from the current flow through the forward biased substrate-channel junction, exceeding the aluminum current handling capacity (i.e. 20 mA). Analog signal dc biasing or offsetting the device power supplies can prevent this problem. These methods are discussed in the applications section of this paper (Figure 7 and Figure 8). Gate Source Drain Symbol Source/Body p-Substrate — Aluminum — Gate Oxide — Polysilicon — Field Oxide ÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊ ÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊ ÊÊÊÊÊÊÊÊÊÊÊÊÊÊÊ ÉÉÉÉÉÉÉ ÉÉÉÉÉÉÉ ÉÉÉÉÉÉÉ ÉÉÉÉÉÉÉ ÉÉ ÉÉ ÊÊ ÊÊ Gate Drain FIGURE 3.Cross-Section of an N-Channel, Silicon-Gate DMOS Transistor
Document Number: 70608 03-Aug-99 www.vishay.com FaxBack 408-970-5600 6-3 DG535/536 DC Characteristics On-Resistance On-resistance must be low to ensure low insertion loss, especially when the switch drives low load resistances. As shown in Figure 4, the on-resistance remains low and fairly constant over the usable analog signal range. This makes the DG535/536 useful for audio applications that require low harmonic distortion. rDS(on) vs. VD and Temperature VD (V) 400 360 280 01 0 320 240 120 200 160 8642 V+ = +15 V GND = 0 V 125/C0095C 25/C0095C –55/C0095C FIGURE 4.On-Resistance vs. Analog Signal Characteristics rDS(on)(k )
1 Vp-p
rDS(on) v 2 Nominal rDS(on) = 50 VOUT R L 600 FIGURE 5.600- Audio System Insertion Loss 20 LOG 10 V OUT VIN In a 600- audio system, such as the one represented in Figure 5, the percentage of on-resistance change relative to the load resistance is only 0.33%. The insertion loss due to the switch on-resistance is 0.7dB. Leakage Current The DG535/536 features low off and on leakage currents, reducing switching errors. Power Supply Current Consumption Until now, most available video multiplexers or digital crosspoint switches relied on high-level supply currents for operation. The DG535/536 requires a total supply current of only 5 A, typical. This feature makes the DG535/536 ideal for systems with high channel density, such as 32-channel crosspoint matrices used in video mixing consoles or as ECL digital crosspoint replacements in large data transmission systems. The total supply current for a 32-channel crosspoint system using the DG535/536 is approximately 320 A, much lower than other video multiplexers. DG536 AC Characteristics (Refer to the DG535 data sheet for the 28-pin DIP performance.) Bandwidth The “on” frequency response is expressed as the frequency at which the insertion loss (at dc) increases by 3 dB. The measured bandwidth of the DG536 is greater than 300 MHz. Crosstalk Crosstalk is the amount of unwanted signal apparent at a particular node due to the parasitic capacitance of the device. As the most important parameter for many applications, crosstalk is specified in a number of ways. 1. Single-channel crosstalk is the ratio of the signal seen at the drain (output) to the signal applied to a single off-channel input. This is expressed by XTALK (SC)(dB) 20 log10 VOUT VIN Most conventional multiplexers specify this parameter on the data sheet as off-isolation. This value for the DG536 is more than twice as good as other 16-channel analog multiplexer ICs, proving the effectiveness of the T switch. 2. All hostile crosstalk is the ratio of the signal measured at the drain to the signal applied simultaneously to all 15 channels (i.e., with one channel on). 3. Chip-disabled crosstalk is the drain output to signal input ratio. The input signal is applied to all 16 off channels simulta- neously. 4. Adjacent input crosstalk is the ratio of the signal applied to a source (input) to the signal measured at any adjacent source. A low adjacent input crosstalk is required for video applications to avoid ghosting effects that may appear on video monitors or TV screens.
www.vishay.com FaxBack 408-970-5600 6-4 Document Number: 70608 03-Aug-99 /C0070 /C0072 /C0077 Parameter DG526 DG506A DG508A CD4051/2/3 DG536 CS (off) (pF) 10 6 5 10 2 CD (off) (pF) 65 46 26 60 8 Note: These are typical values taken from data sheets Switching Time The DG535/536 switching time enables use of the device at high multiplexing rates. The low transition times (tON = 300 ns maximum and tOFF = 150 ns maximum) make it ideal for fast multi-channel analog or digital multiplexing. True break-before-make (BBM) switching action is guaranteed by design. This prevents shorting (crosstalk) of time adjacent input channels during transition. Capacitance Capacitance determines the loading effect of the multiplexer on signal sources and affects transition times, as well as system bandwidth. 1. Off-state input capacitance gives the loading of the device (in the off-state) to a signal source. With a typical value of 2 pF, this allows efficient paralleling of many device channels in multi-channel crosspoint matrices with negligible loading effects. 2. On-state input capacitance also determines the loading effects of the device on-signal sources and limits the number of parallel on channels allowed in a large matrix. In large matrixes buffering of the input signals is recommended. 3. Off-state output capacitance affects the transition speed of the multiplexer. The output capacitance must be charged and discharged in turning on and off a device; thus, a low value of capacitance enables rapid transition times. Table 1 shows a comparison of DG536 capacitance to comparable 16-channel multiplexers. Circuit Board Layout To optimize the high-frequency characteristics of the DG536, care must be taken in circuit board layout and interconnections. Parasitic stray capacitances caused by poor layout could degrade performance significantly. As shown in Figure 6, use of guard planes and traces between signal paths is a good layout practice. Other layout considerations include: Short signal paths Sufficient power supply decoupling Coaxial interconnect of leads, plugs, and sockets. Sockets should be avoided DG536 Mounted on Underside of PCB SMB-PCB Mounting sockets Top Side (Ground Plane) PCB Bottom Side ÉÉÉÉÉÉÉÉ ÉÉÉÉÉÉÉÉ FIGURE 6.Circuit Board Layout for Optimal Performance (DG536)
Applications
Many applications for the DG535/536 will be in video related systems. Some examples of circuit configurations are included in this section. Video The DG536 was designed primarily for handling broadcast quality video signals. Optimum performance is achieved with a bias between +2.5 V and +3 V. Differential phase linearity is best at this bias level.
16 Signal
4 Si584 Buffers
FIGURE 11. The DG535/536 as a 16 /C0032 2 Matrix Switch FIGURE 12. 32-Channel Multiplexer FIGURE 13. 16 /C0032 16 Video Crosspoint Circuit