AD811 查看數據表（PDF） - Analog Devices

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AD811

High Performance Video Op Amp

Analog Devices 

To generate the 1 V dc needed for the 1−G term, an AD589

reference supplies 1.225 V ± 25 mV to a voltage divider consist-

ing of resistors R2 through R4. Potentiometer R3 should be

adjusted to provide exactly 1 V at the X1 input.

In this case, an arrangement is shown using dual supplies of

±5 V for both the AD834 and the AD811. Also, the overall gain

is arranged to be unity at the load when it is driven from a

reverse-terminated 75 Ω line. This means that the dual VCA has

to operate at a maximum gain of +2, rather than +4 as in the

VCA circuit of Figure 47. However, this cannot be achieved by

lowering the feedback resistor because below a critical value

(not much less than 500 Ω) the AD811’s peaking may be

unacceptable. This is because the dominant pole in the open-

loop ac response of a current feedback amplifier is controlled by

this feedback resistor. It would be possible to operate at a gain of

×4 and then attenuate the signal at the output. Instead, the

signals have been attenuated by 6 dB at the input to the AD811;

this is the function of R8 through R11.

Figure 49 is a plot of the ac response of the feedback keyer when

driving a reverse-terminated 50 Ω cable. Output noise and

adjacent channel feedthrough, with either channel fully off and

the other fully on, is about −50 dB to 10 MHz. The feedthrough

AD811

at 100 MHz is limited primarily by board layout. For VG = 1 V,

the −3 dB bandwidth is 15 MHz when using a 137 Ω resistor for

R14 and 70 MHz with R14 = 49.9 Ω. For more information on

the design and operation of the VCA and video keyer circuits,

refer to the application note “Video VCAs and Keyers: Using the

AD834 and AD811” by Brunner, Clarke, and Gilbert, available

on the Analog Devices, Inc. website at www.analog.com.

10

0

GAIN

–10

R14 = 49.9Ω

R14 = 137Ω

–20

–30

–40

–50

ADJACENT CHANNEL

–60

FEEDTHROUGH

–70

–80

–90

10k

100k

1M

10M

FREQUENCY (Hz)

100M

Figure 49. A Plot of the AC Response of the Video Keyer

Rev. E | Page 17 of 20

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