Designed as a high-speed driver for 12-bit ADCs, the AD8137 controls SPDT GaAs (gallium-arsenide) FET-MMIC (microwave-monolithic-IC) and PIN-diode RF switches and thus provides a low-cost and versatile alternative to conventional switch drivers. This circuit achieves typical switching speeds of approximately 7 to 11 nsec, including the propagation delays of the driver and RF load.

The GaAsFET-driver circuit (Figure 1) converts a single-ended, 0 to 3.5V TTL signal into a complementary, 0 to –4V differential-output signal. The divider formed by the 50Ω source impedance, R_S, and the input termination, R_T, imposes a 50% signal reduction. To compensate, the circuit amplifies its input by approximately 2.3 times to yield the proper output amplitude of 4V p-p. The circuit also shifts the output level by –2V to provide the proper GaAsFET bias. Equation 1 determines the output voltage:

EQUATION 1

For a symmetrical output swing, gain-setting resistors R₁ and R₄ must present the same Thevenin-equivalent resistance. In Figure 1, R₄ increases by 20Ω over R₁. This increase compensates for the fact that source resistor R_S and termination resistor R_T combine in parallel to introduce additional resistance of 25Ω. Setting R₄ to 1.02 kΩ (the closest standard value to 1.025 kΩ) ensures that the circuit will provide approximately equal gains at the differential outputs.

The AD8137's V_OCM input (Pin 2) offers a convenient method of shifting the outputs' dc common-mode level. In Figure 1, R₂ and R₃ form a voltage divider, which sets the dc output level to –2V. Connecting the AD8137's inverting input to a reference voltage of 1.75V establishes the midpoint of the input signal and allows for proper switching of the AD8137's input stage.

Figure 2 shows the GaAs FET driver's turn-on switching speed of approximately 5 nsec for isolation to insertion loss—that is, 50% of the TTL input to 90% detected RF. Figure 3 shows turn-off switching speed of approximately 11 nsec for insertion loss to isolation—that is, 50% of the TTL input to 10% detected RF.

As Figure 4 shows, with only minor modifications, the GaAs switch-driver circuit can drive PIN-diode loads that require both positive and negative bias currents. IC₁'s V_OCM input connects to ground to provide symmetrical outputs of 63.5V about ground and sinks and sources 10 mA of bias current. Altering feedback resistors R₃ and R₄ to 2 kV provides an output swing of 63.5V. Resistors R₅ and R₆ set the steady-state PIN-diode current, I_SS, as Equation 2 shows:

EQUATION 2

Capacitors C₅ and C₆ set the spiking current I_S, which removes stored charge in the PIN diodes. You can optimize a given diode switch's response time by using the AD8137's output slew rate for dV/dt and Equation 3 ,

EQUATION 3

to calculate spiking current.

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