GT4122 View Datasheet(PDF) - Gennum -> Semtech

Part Name

Description

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GT4122

Video Multiplier

Gennum -> Semtech 

DETAILED DESCRIPTION

The GT4122 is a broadcast quality monolithic inte-

grated circuit specifically designed to linearly mix two

video signals under the control of a third signal.

Referring to the block diagram, the input signals are applied

to conventional differential amplifiers (AMP1 and AMP2).

Each amplifier has provisions for individually adjusting the

DC offset (OFFSET).

Following each input amplifier, the signals are applied to

linear multiplier circuits (XA and XB) whose outputs are the

product of the incoming signals and controlling voltages (VCA)

or (VCB). The controlling voltage VCA is the sum of a nominal

0.5V source (VNOM) and a variable source VK while VCB is made

up of the sum of the nominal voltage VNOM and -VK.

VK and -VK are themselves proportional to the difference

between an externally applied reference voltage (V ) and

REF

an externally applied CONTROL voltage (VC). The voltages VK

and -VK are produced by a differential amplifier (AMP3) whose

gain is AK. This gain can be altered by two external resistors,

REXT and RSPAN according to the following formula:

0.85 • REXT

AK ≈ —————

RSPAN

[1kΩ < REXT < 3kΩ]

Note that REXT is connected between the REXT pin and ground

and R is connected between the pins S1 and S2.

SPAN

Each of the voltages (+V and -V ) is applied to summing

K

K

circuits (Σ2 and Σ3) whose second inputs are DC voltage

sources that can also be slightly varied. The nominal value of

these voltage sources is 0.5 volts.

When they are exactly 0.5V and when V = V , the gain of

C

REF

each signal channel of the mixer is 0.5 (50%).

By connecting the ends of an external potentiometer

(CONTROL OFFSET) between the offset pins COS1 and

COS2, the voltage sources can be altered differentially. If a

second potentiometer (50% GAIN) is connected between the

wiper of the CONTROL OFFSET potentiometer and the supply

voltage, the voltage sources can be varied in a common

mode fashion. In this way not only can the control range of

the mixer be varied but also the point at which 50% of each

input signal appears at the output.

The outputs from the multiplier circuits (XA and XB) are then

applied to a summing circuit (Σ1) whose output feeds a

wideband amplifier (AMP4) and presents the mixed signals to

the outside world.

Although there are two separate differential inputs, the usual

operational amplifier gain-setting methods can be applied to

determine the closed loop gain of the mixer. Usually the

mixer will be configured for unity gain by connecting both

inverting inputs (-IN A , -IN B) to the common output (OUT). In

this case, the general transfer function is:

VO = VA •[VNOM + AK•(VC - VREF)] + VB•[VNOM - AK•(VC -

VREF)]

(Unity gain configuration)

Where V and V are the input analog signals applied to +IN A

A

B

and +IN B respectively, and V is the CONTROL voltage.

C

Note that VNOM ranges between 0.45V < VNOM < 0.55.

+IN A

- IN A

A OS1

A OS2

+IN B

- IN B

B OS1

B OS2

- VS

+VS

GND

+

AMP 1

XA

-

VCA=0.5 + VK

+

Σ1

+

+

AMP 2

-

XB

VCB=0.5 - VK

+ Σ2 + - VK

VNOM

+

0.5V

+

Σ3 + VK

+ VNOM

-

-

AMP 4

BIAS

AMP 3

-

AK

+

COMP

OUT

REXT

VREF

VC

S1

S2

COS2

COS1

Device Function: VOUT = VINA • [VNOM + AK (VC - VREF)] + VINB • [VNOM - AK (VC - VREF)]

FUNCTIONAL BLOCK DIAGRAM

3

520 - 25 - 4

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