EL4453 データシートの表示（PDF） - Intersil

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EL4453

Video Fader

Intersil 

EL4453

FIGURE 1.

Applications Information

The EL4453 is a complete two-quadrant fader/gain control

with 80MHz bandwidth. It has four sets of inputs; a

differential signal input VINA, a differential signal input VINB,

a differential fade-controlling input VFADE, and another

differential input Sum which can be used to add in a third

input at full gain. This is the general connection of the

EL4453 (Figure 1).

The gain of the feedback dividers are HA and HB, and

0 ≤ H ≤ 1. The transfer function of the part is:

VOUT = AO × [((VINA+) – HA × VOUT) × (1 + (VFADE+)

- (VFADE-)) / 2 + ((VINB+) – HB × VOUT) × (1 - (VFADE+)

+ (VFADE-)) / 2 + (Sum+) – (Sum-))],

with -1 ≤ (VFADE+)–(VFADE-) ≤ +1 numerically.

AO is the open-loop gain of the amplifier, and is about 600.

The large value of AO drives:

((VINA+) – HA × VOUT) × (1 + (VFADE+) – (VFADE-)) / 2

+ ((VINB+) – HB × VOUT) × (1- (VFADE+) + (VFADE-)) / 2

+ (Sum+) – (Sum-))→0.

Rearranging and substituting:

VOUT

=

-F-----×----V-----I--N----A-----+-----F-----×-----V----I--N-----B-----+-----S----u----m----

F × HA + F × HB

Where F = (1 + (VFADE+)–(VFADE-)) ⁄ 2

F = (1–(VFADE+) + (VFADE-)) ⁄ 2

and Sum = (Sum+) – (Sum-)

In the above equations, F represents the fade amount, with F

= 1 giving 100% gain on VINA but 0% for VINB; F = 0 giving

0% gain for VINA but 100% to VINB. F is 1 - F, the

complement of the fade gain. When F = 1,

VOUT

=

V-----I--N----A-----+-----S-----u----m---

HA

and the amplifier passes VINA and Sum with a gain of 1/HA.

Similarly, for F = 0,

VOUT

=

V-----I--N----B-----+-----S-----u----m---

HA

and the gains vary linearly between fade extremes.

The EL4453 is stable for a direct connection between VOUT

and VINA- or VINB-, yielding a gain of +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 divider’s impedances low so that stray capacitance

does not diminish the feedback loop’s phase margin. The

pole caused by the parallel impedance of the feedback

resistors and stray capacitance should be at least 150MHz;

typical strays of 3pF thus require a feedback impedance of

360Ω or less. Alternatively, a small capacitor across RF can

be used to create more of a frequency-compensated divider.

The value of the capacitor should scale with the parasitic

capacitance at the FB input. It is also practical to place small

capacitors across both the feedback resistors (whose values

maintain the desired gain) to swamp out parasitics. For

instance, two 10pF 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 -0.2%.

Furthermore, nonlinearities are vastly improved since the

gain core sees only small error signals, not full inputs.

Unfortunately, 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 VIN 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.

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