AD811A Analog Devices, AD811A Datasheet - Page 14

• Manufacturer Part Number: AD811A
• Description: High Performance Video op Amp
• Manufacturer: Analog Devices

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 ) 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
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 V
the –3 dB bandwidth is 15 MHz when using a 137
R14 and 70 MHz with R14 = 49.9 . 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.
cable. Output noise and
G
resistor for
= +1 V,
–14–
Figure 42. A Plot of the AC Response of the Video Keyer
–50
–60
–70
–80
–10
–20
–30
–40
0
10k
100k
FEEDTHROUGH
ADJACENT
CHANNEL
GAIN
FREQUENCY – Hz
1M
R14 = 49.9
R14 = 137
10M
100M
REV. D