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SW-277 Datasheet(PDF) 2 Page - M/A-COM Technology Solutions, Inc. |
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SW-277 Datasheet(HTML) 2 Page - M/A-COM Technology Solutions, Inc. |
2 / 4 page Drivers for GaAs FET Switches and Digital Attenuators Rev. V5 Application Note S2079 • North America Tel: 800.366.2266 • Europe Tel: +353.21.244.6400 • India Tel: +91.80.4155721 • China Tel: +86.21.2407.1588 2 Visit www.macomtech.com for additional data sheets and product information. M/A-COM Technology Solutions Inc. and its affiliates reserve the right to make changes to the product(s) or information contained herein without notice. Built-in Drivers Some of M/A-COM's newer switches and attenuators feature simplified control using CMOS (0 V, 2.7 V) or TTL (0 V, 5 V) logic, with no need for negative control voltages. The Appendix to this application note lists some popular M/A-COM switches. The SW-277, SW-349, SW-394, and SW-399 include level shifting components for compatibility with positive CMOS or TTL control voltages, but these switches still require complementary control logic. The SW-335, SW65-0xxx series and related switches incorporate a CMOS driver circuit in the same package, along with the GaAs switching elements, for true single line control. Many future switches from M/A- COM will likely incorporate driver circuitry and switching elements together in small, low cost plastic packages. The AT-226, AT-264, and AT-242 digital attenuators feature internal level shifting to provide control with a single CMOS input line for each attenuation bit. The AT65-0xxx series miniature digital attenuator modules incorporate CMOS driver circuitry to accomplish this. SWD-109 & SWD-119 Drivers M/A-COM's SWD-109 and quad-channel SWD-119 provide the complementary control voltages necessary for driving GaAs FET switches and digital attenuators using a single control input per bit. Both the SWD-109 and SWD-119 incorporate buffering stages so that the drivers will switch with either standard TTL or CMOS logic level input. The devices employ standard CMOS analog fabrication techniques for low power consumption. The devices consist of input buffers, inverters to generate complementary logic values, voltage translators, and output buffers, all designed to allow the designer the flexibility to optimize switch and attenuator performance. To design a board with RF switches and attenuators, consider that modulation of the source-drain resistance in the FET's by input RF can lead to output compression and intermodulation distortion. Although GaAs FET switches and attenuators will operate well with nominal 0 V and -5 V for control, careful selection of the control voltages in the ranges of - 8 V < VFEToff < - 5 V, and 0 V < VFETon < 2 V can improve the maximum RF level (P1dB). With proper selection of positive and negative supply voltage, the SWD-109 and SWD-119 can both provide output control voltages in these ranges. Another consideration in design with switches and attenuators is the elimination of crosstalk that can arise from RF leakage onto control lines. Most board designers take care of this by adding capacitance to ground on the control lines, shunting any RF energy to ground. The SW-109 and SWD-119 output buffers can drive load capacitance up to 25 pF. Other Circuits as Drivers You can use TTL and CMOS logic IC's to drive GaAs FET switches and attenuators. An ideal driver would run from a single supply voltage, consume little current, and introduce very little switching delay. One driver technique that works well floats the channel of the FET's on the MMIC switch above ground potential through the addition of pull-up resistors and DC blocking and bypass capacitors. As shown in Figure 3, the circuit takes a voltage of 0 VDC, applied to either control port, and shifts it to -5 VDC at the attached FET gates to turn them off. A voltage of +5 VDC shifts to 0 VDC at the FET gates to turn them on. Figure 3: GaAs SPDT Switch with CMOS Driver Control A Control B RF Common to RF 1 RF Common to RF 2 TTL Low TTL High ON OFF TTL High TTL Low OFF ON RF Common R1 +5 VDC C1 Q2 Q3 RF1 C5 Q1 Q4 C4 RF2 R3 +5V C3 Control "B" Control "A" R2 +5V C2 TTL Control +5 VDC C6 VCC GND CD54HCT04 |
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