AD811

REV. D

–14–

than 4 as in the VCA circuit of Figure 40. However, this cannot

be achieved by lowering the feedback resistor, since 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 con-

trolled by this feedback resistor. It would be possible to operate

at a gain of X4 and then attenuate the signal at the output.

Instead, we have chosen to attenuate the signals by 6 dB at the

input to the AD811; this is the function of R8 through R11.

Figure 42 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

the –3 dB bandwidth is 15 MHz when using a 137

Ω resistor for

R14 and 70 MHz with R14 = 49.9

Ω. For further information

regarding the design and operation of the VCA and video keyer

circuits, refer to the application note “Video VCA’s and Keyers

Using the AD834 & AD811” by Brunner, Clarke, and Gilbert,

available FREE from Analog Devices.

100M

–60

–80

100k

–70

10k

–30

–50

–40

–20

–10

0

10M

1M

FREQUENCY – Hz

ADJACENT

CHANNEL

FEEDTHROUGH

GAIN

R14 = 137

R14 = 49.9

Figure 42. A Plot of the AC Response of the Video Keyer