ADV7281 datasheet(17/32 Pages) AD

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Data Sheet

ADV7281

Rev. A | Page 17 of 32

INPUT NETWORKS

An input network (external resistor and capacitor circuit) is

required on the AINx input pins of the decoder. The components

of the input network depend on the video format selected for

the analog input.

SINGLE-ENDED INPUT NETWORK

Figure 11 shows the input network to use on each AINx input

pin of the ADV7281/ADV7281-M/ADV7281-MA when any

of the following video input formats is used:

•

Single-ended CVBS

•

YC (S-Video)

•

YPrPb

51Ω

AIN3

INPUT

CONNECTOR

VIDEO INPUT

FROM SOURCE

24Ω

100nF

EXT

ESD

Figure 11. Single-Ended Input Network

The 24 Ω and 51 Ω resistors supply the 75 Ω end termination

required for the analog video input. These resistors also create

a resistor divider with a gain of 0.68. The resistor divider attenu-

ates the amplitude of the input analog video and scales the input

to the ADC range of the ADV7281/ADV7281-M/ADV7281-MA.

This allows an input range to the ADV7281/ADV7281-M/

ADV7281-MA of up to 1.47 V p-p. Note that amplifiers within

the ADC restore the amplitude of the input signal so that

signal-to-noise ratio (SNR) performance is maintained.

The 100 nF ac coupling capacitor removes the dc bias of the analog

input video before it is fed into the AINx pin of the ADV7281/

ADV7281-M/ADV7281-MA. The clamping circuitry within the

ADV7281/ADV7281-M/ADV7281-MA restores the dc bias

of the input signal to the optimal level before it is fed into the

ADC of the ADV7281/ADV7281-M/ADV7281-MA.

DIFFERENTIAL INPUT NETWORK

Figure 12 shows the input network to use when differential

CVBS video is input on the AINx input pins of the ADV7281/

ADV7281-M/ADV7281-MA.

AIN1

AIN2

INPUT

CONNECTOR

INPUT

CONNECTOR

VIDEO INPUT

FROM SOURCE

EXT

ESD

1.3kΩ

100nF

1.3kΩ

100nF

430Ω

Figure 12. Differential Input Network

Fully differential video transmission involves transmitting

two complementary CVBS signals. Pseudo differential video

transmission involves transmitting a CVBS signal and a source

ground signal.

Differential video transmission has several key advantages over

single-ended transmission, including the following:

•

Inherent small signal and large signal noise rejection

•

Improved EMI performance

•

Ability to absorb ground bounce

Resistor R1 provides the RF end termination for the differential

CVBS input lines. For a pseudo differential CVBS input, a value

of 75 Ω is recommended for R1. For a fully differential CVBS

input, a value of 150 Ω is recommended for R1.

The 1.3 kΩ and 430 Ω resistors create a resistor divider with a

gain of 0.25. The resistor divider attenuates the amplitude of the

input analog video, but increases the input common-mode range

of the ADV7281/ADV7281-M/ADV7281-MA to 4 V p-p. Note

that amplifiers within the ADC restore the amplitude of the

input signal so that SNR performance is maintained.

The 100 nF ac coupling capacitor removes the dc bias of the analog

input video before it is fed into the AINx pin of the ADV7281/

ADV7281-M/ADV7281-MA. The clamping circuitry within the

ADV7281/ADV7281-M/ADV7281-MA restores the dc bias of

the input signal to the optimal level before it is fed into the

ADC of the ADV7281/ADV7281-M/ADV7281-MA.

The combination of the 1.3 kΩ and 430 Ω resistors and the

100 nF ac coupling capacitors limits the current flow into the

ADV7281/ADV7281-M/ADV7281-MA during short-to-battery

(STB) events (see the Short-to-Battery Protection section).

To achieve optimal performance, the 1.3 kΩ and 430 Ω resistors

must be closely matched; that is, all 1.3 kΩ and 430 Ω resistors

must have the same resistance tolerance, and this tolerance must

be as low as possible.

SHORT-TO-BATTERY PROTECTION

In differential mode, the ADV7281/ADV7281-M/ADV7281-MA

are protected against short-to-battery (STB) events by ac

coupling capacitors (see Figure 12). The input network resistors

are sized to reduce the current flow during an STB event, thus

preventing damage to the resistors. The R1 resistor is protected

because no current or limited current flows through it during

an STB event.

The ADV7281/ADV7281-M provides two STB diagnostic pins

that can be used to generate an interrupt when an STB event

occurs. For more information, see the Short-to-Battery (STB)

Diagnostics section.

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