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ISL43840 Datasheet(PDF) 10 Page - Intersil Corporation |
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ISL43840 Datasheet(HTML) 10 Page - Intersil Corporation |
10 / 14 page 10 Detailed Description The ISL43840 analog switch offers a precise switching capability from a bipolar ±2V to ±6V or a single 2V to 12V supply with low on-resistance (39 Ω) and high speed operation (tON =38ns, tOFF = 19ns) with dual 5V supplies. It has an enable bar pin to simultaneously open all signal paths. The device is especially well suited for applications using ±5V supplies. With ±5V supplies the performance (RON, Leakage, Charge Injection, ect.) is best in class. High frequency applications also benefit from the wide bandwidth, and the very high off isolation and crosstalk rejection. Supply Sequencing And Overvoltage Protection With any CMOS device, proper power supply sequencing is required to protect the device from excessive input currents which might permanently damage the IC. All I/O pins contain ESD protection diodes from the pin to V+ and to V-(see Figure 8). To prevent forward biasing these diodes, V+ and V- must be applied before any input signals, and input signal voltages must remain between V+ and V-. If these conditions cannot be guaranteed, then one of the following two protection methods should be employed. Logic inputs can easily be protected by adding a 1k Ω resistor in series with the input (see Figure 8). The resistor limits the input current below the threshold that produces permanent damage, and the sub-microamp input current produces an insignificant voltage drop during normal operation. This method is not applicable for the signal path inputs. Adding a series resistor to the switch input defeats the purpose of using a low RON switch, so two small signal diodes can be added in series with the supply pins to provide overvoltage protection for all pins (see Figure 8). These additional diodes limit the analog signal from 1V below V+ to 1V above V-. The low leakage current performance is unaffected by this approach, but the switch resistance may increase, especially at low supply voltages. Power-Supply Considerations The ISL43840 construction is typical of most CMOS analog switches, in that they have three supply pins: V+, V-, and GND. V+ and V- drive the internal CMOS switches and set their analog voltage limits, so there are no connections between the analog signal path and GND. Unlike switches with a 13V maximum supply voltage, the ISL43840 15V maximum supply voltage provides plenty of room for the 10% tolerance of 12V supplies (±6V or 12V single supply), as well as room for overshoot and noise spikes. This switch device performs equally well when operated with bipolar or single voltage supplies.The minimum recommended supply voltage is 2V or ±2V. It is important to note that the input signal range, switching times, and on- resistance degrade at lower supply voltages. Refer to the electrical specification tables and Typical Performance Curves for details. V+ and GND power the internal logic (thus setting the digital switching point) and level shifters. The level shifters convert the logic levels to switched V+ and V- signals to drive the analog switch gate terminals. Logic-Level Thresholds V+ and GND power the internal logic stages, so V- has no affect on logic thresholds. This switch family is TTL compatible (0.8V and 2.4V) over a V+ supply range of 2.7V to 10V. At 12V the VIH level is about 3.3V. This is still below the CMOS guaranteed high output minimum level of 4V, but noise margin is reduced. For best results with a 12V supply, use a logic family that provides a VOH greater than 4V. The digital input stages draw supply current whenever the digital input voltage is not at one of the supply rails. Driving the digital input signals from GND to V+ with a fast transition time minimizes power dissipation. High-Frequency Performance In 50 Ω systems, signal response is reasonably flat even past 100MHz (see Figures 17 and 18). Figures 17 and 18 also illustrates that the frequency response is very consistent over varying analog signal levels. An OFF switch acts like a capacitor and passes higher frequencies with less attenuation, resulting in signal feed through from a switch’s input to its output. Off Isolation is the resistance to this feed through, while Crosstalk indicates the amount of feed through from one switch to another. Figure 19 details the high Off Isolation and Crosstalk rejection provided by this family. At 10MHz, Off Isolation is about 55dB in 50 Ω systems, decreasing approximately 20dB per decade as frequency increases. Higher load impedances decrease Off Isolation and Crosstalk rejection due to the voltage divider action of the switch OFF impedance and the load impedance. FIGURE 8. INPUT OVERVOLTAGE PROTECTION V- VCOM VNO OPTIONAL PROTECTION V+ LOGIC DIODE OPTIONAL PROTECTION DIODE OPTIONAL PROTECTION RESISTOR FOR LOGIC INPUTS 1k Ω ISL43840 |
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