AD5539 查看數據表（PDF） - Analog Devices

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AD5539

Operational Amplifier Ultrahigh Frequency

Analog Devices 

AD5539

The transient response of the signal channel at VX = +2 V,

VY = VOUT = + or –1 V is shown in Figure 46; with the VCA

driving a 75 Ω load. The rise and fall times are both approxi-

mately 7 ns.

A few final circuit details: in general, the control amplifier com-

pensation capacitor for Pin 2, CC, must have a minimum value

of 3000 pF (3 nF) to provide both circuit stability and maximum

control bandwidth. However, if the maximum control bandwidth

is not needed, then it is advisable to use a larger value of CC,

with typical values between 0.01 and 0.1 µF. Like many aspects

of design, the value of CC will be a tradeoff: higher values of CC

will lower the high frequency distortion, reduce the high fre-

quency crosstalk and improve the signal channel phase response.

Conversely, lower values of CC will provide a higher control

channel bandwidth at the expense of degraded linearity in the

output response when amplitude modulating a carrier signal.

The control channel bandwidth will vary in inverse proportion to

the value of CC, providing a typical bandwidth of 2 MHz with a

CC of 0.01 µF and a VX voltage of +1.7 volts.

Both the bandwidth and pulse response of the control channel

can be further increased by using a feedforward capacitor, Cff,

with a value between 5 and 20 percent of CC. Cff should be care-

fully adjusted to give the best pulse response for a particular step

input applied to the control channel. Note that since Cff is con-

nected between a linear control input (Pin 1) and a logarith-

Figure 46. Transient Response of the Voltage-Controlled

Amplifier VX = +2 Volts, VY = ±1 Volt

mic node, the settling time of the control channel with a pulse

input will vary with different control input step levels.

Diode D1 clamps the logarithmic control node at Pin 2 of the

AD539, (preventing this point from going too negative); this

diode helps decrease the circuit recovery time when the control

input goes below ground potential.

THE AD539/5539 COMBINATION AS A FAST, LOW

FEEDTHROUGH, VIDEO SWITCH

Figure 47 shows how the AD539/5539 combination can be used

to create a fast video speed switch suitable for many high fre-

REV. B

Figure 47. An Analog Multiplier Video Switch

–15–

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