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AD811

Rev. E | Page 16 of 20

A VIDEO KEYER CIRCUIT

By using two AD834 multipliers, an AD811, and a 1 V dc source,

a special form of a two-input VCA circuit called a video keyer

can be assembled. Keying is the term used in reference to blend-

ing two or more video sources under the control of a third

signal or signals to create such special effects as dissolves and

overlays. The circuit shown in Figure 48 is a two-input keyer,

with video inputs VA and VB, and a control input VG. The

transfer function (with VOUT at the load) is given by

VOUT = GVA + (1−G)VB

where G is a dimensionless variable (actually, just the gain of the

A signal path) that ranges from 0 when VG = 0 to 1 when VG =

1 V. Thus, VOUT varies continuously between VA and VB as G

varies from 0 to 1.

Circuit operation is straightforward. Consider first the signal

path through U1, which handles video input VA. Its gain is

clearly 0 when VG = 0, and the scaling chosen ensures that it has

a unity value when VG = 1 V; this takes care of the first term of

the transfer function. On the other hand, the VG input to U2 is

taken to the inverting input X2 while X1 is biased at an accurate

1 V. Thus, when VG = 0, the response to video input VB is already

at its full-scale value of unity, whereas when VG = 1 V, the differ-

ential input X1−X2 is 0. This generates the second term.

The bias currents required at the output of the multipliers are

provided by R8 and R9. A dc level-shifting network comprising

R10/R12 and R11/R13 ensures that the input nodes of the

AD811 are positioned at a voltage within its common-mode

range. At high frequencies, C1 and C2 bypass R10 and R11,

respectively. R14 is included to lower the HF loop gain and is

needed because the voltage-to-current conversion in the

AD834s, via the Y2 inputs, results in an effective value of the

feedback resistance of 250 Ω; this is only about half the value

required for optimum flatness in the AD811’s response. (Note

that this resistance is unaffected by G: when G = +1, all the

feedback is via U1, while when G = 0 it is all via U2). R14

reduces the fractional amount of output current from the

multipliers into the current-summing inverting input of the

AD811 by sharing it with R8. This resistor can be used to adjust

the bandwidth and damping factor to best suit the application.

AD811

–

X1 +VS

–VS

AD834

1234

8765

X1 +VS

–VS

AD834

1234

8765

29.4

Ω

29.4

Ω

R12

6.98k

Ω

6.98k

Ω

R13

R10

2.49k

Ω

VOUT

226

Ω

45.3

Ω

+5V

–5V

+5V

AD589

113

Ω

(0 TO +1V dc)

VA (±1V FS)

–5V

1.02k

Ω

100

Ω

174

Ω

1.87k

Ω

VB (±1V FS)

+5V

–5V

0.1

µF

0.1

µF

0.1

µF

0.1

µF

R14

SEE TEXT

+5V

–5V

R11

2.49k

Ω

LOAD

GND

LOAD

GND

200

Ω

TO Y2

TO PIN 6

AD811

SETUP FOR DRIVING

REVERSE-TERMINATED LOAD

200

Ω

VOUT

INSET

Figure 48. A Practical Video Keyer Circuit

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