參數(shù)資料
型號: AD811JRZ-REEL
廠商: Analog Devices Inc
文件頁數(shù): 9/21頁
文件大?。?/td> 0K
描述: IC OPAMP VIDEO HP 8SOIC
標準包裝: 2,500
應用: 電流反饋
電路數(shù): 1
-3db帶寬: 140MHz
轉(zhuǎn)換速率: 2500 V/µs
電流 - 電源: 16.5mA
電流 - 輸出 / 通道: 100mA
電壓 - 電源,單路/雙路(±): ±4.5 V ~ 18 V
安裝類型: 表面貼裝
封裝/外殼: 8-SOIC(0.154",3.90mm 寬)
供應商設備封裝: 8-SO
包裝: 帶卷 (TR)
AD811
Data Sheet
Rev. F | 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 blending
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 47 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 differential
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 ; 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.
Figure 47. A Practical Video Keyer Circuit
U3
AD811
7
6
4
3
2
+
X2
X1 +VS
W1
Y1
Y2
W2
–VS
U1
AD834
1
2
3
4
8
7
6
5
X2
X1 +VS
W1
Y1
Y2
W2
–VS
U1
AD834
1
2
3
4
8
7
6
5
R8
29.4
29.4
R9
R12
6.98k
6.98k
R13
R10
2.49k
C3
FB
VOUT
FB
VG
R6
226
R7
45.3
+5V
–5V
+5V
U4
AD589
R5
113
(0 TO +1V dc)
VA (±1V FS)
–5V
R4
1.02k
R3
100
R2
174
R1
1.87k
VB (±1V FS)
+5V
–5V
C1
0.1
F
0.1
F
0.1
F
0.1
F
R14
SEE TEXT
+5V
–5V
C4
C2
R11
2.49k
LOAD
GND
LOAD
GND
200
TO Y2
TO PIN 6
AD811
SETUP FOR DRIVING
REVERSE-TERMINATED LOAD
ZO
200
VOUT
INSET
00866-E-048
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