AD811
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.
R14 = 49.9⍀
0
–10
–20
–30
–40
–50
–60
–70
–80
GAIN
R14 = 137⍀
ADJACENT
CHANNEL
FEEDTHROUGH
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
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 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.
10k
100k
1M
FREQUENCY – Hz
10M
100M
Figure 42. A Plot of the AC Response of the Video Keyer
–14–
REV. D
ADI [ ADI ]
