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Data Sheet

AD811

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 consisting

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 46. However, this cannot be achieved by lowering the

feedback resistor because below a critical value (not much less

than 500 Ω) the peaking of the AD811 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 48 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 more information on

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

refer to the AN-216 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.

Figure 48. A Plot of the AC Response of the Video Keyer

–90

–80

–70

–60

–50

–40

–30

–20

–10

FREQUENCY (Hz)

100k

10k

10M

100M

R14 = 49.9

Ω

R14 = 137

Ω

GAIN

ADJACENT CHANNEL

FEEDTHROUGH

Rev. G | Page 17 of 20

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