EL4453C
Video Fader
Applications Information
Contd
The gain of the feedback dividers are H
A
and
H
B
and 0
s
H
s
1 The transfer function of the
part is
V
OUT
e
A
O
c
((V
IN
A
a
) –H
A
c
V
OUT
)
c
(1
a
(V
FADE
a
)
b
(V
FADE
b
)) 2
a
((V
IN
B
a
)–H
B
c
V
OUT
)
c
(1
b
(V
FADE
a
)
a
(V
FADE
b
)) 2
a
(Sum
a
) –(Sum
b
))
with
b
1
s
(V
FADE
a
) – (V
FADE
b
)
s
a
1 nu-
merically
A
O
is the open-loop gain of the amplifier and is
about 600 The large value of A
O
drives
((V
IN
A
a
)–H
A
c
V
OUT
)
c
(1
a
(V
FADE
a
)
– (V
FADE
b
)) 2
a
((V
IN
B
a
) –H
B
c
V
OUT
)
c
(1
b
(V
FADE
a
)
a
(V
FADE
b
)) 2
a
(Sum
a
) – (Sum
b
))
0
The EL4453C is stable for a direct connection be-
tween V
OUT
and V
IN
A
b
or V
IN
B
b
yielding a
gain of
a
1 The feedback divider may be used for
higher output gain although with the traditional
loss of bandwidth It is important to keep the
feedback dividers’ impedances low so that stray
capacitance does not diminish the feedback loop’s
phase margin The pole caused by the parallel im-
pedance of the feedback resistors and stray ca-
pacitance should be at least 150 MHz typical
strays of 3 pF thus require a feedback impedance
of 360X or less Alternatively a small capacitor
across R
F
can be used to create more of a fre-
quency-compensated divider The value of the ca-
pacitor should scale with the parasitic capaci-
tance at the FB input It is also practical to place
small capacitors across both the feedback resis-
tors (whose values maintain the desired gain) to
swamp out parasitics For instance two 10 pF
capacitors across equal divider resistors for a gain
of two will dominate parasitic effects and allow a
higher divider resistance Either input channel
can be set up for inverting gain using traditional
feedback resistor connections
At 100% gain an input stage operates just like
an op-amp’s input and the gain error is very low
around
b
0 2% Furthermore nonlinearities are
vastly improved since the gain core sees only
small error signals not full inputs Unfortunate-
ly distortions increase at lower fade gains for a
given input channel
The Sum pins can be used to inject an additional
input signal but it is not as linear as the V
IN
paths The gain error is also not as good as the
main inputs being about 1% Both sum pins
should be grounded if they are not to be used
x
Rearranging and substituting
V
OUT
e
F
c
V
IN
A
a
F
c
V
IN
B
a
Sum
F
c
H
A
a
F
c
H
B
Where F
e
(1
a
(V
FADE
a
) – (V
FADE
b
)) 2
F
e
(1
b
(V
FADE
a
)
a
(V
FADE
b
)) 2 and
Sum
e
(Sum
a
) –(Sum
b
)
In the above equations F represents the fade
amount with F
e
1 giving 100% gain on V
IN
A
but 0% for V
IN
B F
e
0 giving 0% gain for
V
IN
A but 100% to V
IN
B F is 1
b
F the comple-
ment of the fade gain When F
e
1
V
OUT
e
V
IN
A
a
Sum
H
A
and the amplifier passes V
IN
A and Sum with a
gain of 1 H
A
Similarly for F
e
0
V
OUT
e
V
IN
B
a
Sum
H
B
and the gains vary linearly between fade ex-
tremes
9
ELANTEC [ ELANTEC SEMICONDUCTOR ]
