AD8143

Data Sheet

Rev. A | Page 18 of 24

APPLICATIONS INFORMATION

OVERVIEW

The AD8143 contains three independent active-feedback

amplifiers that can be effectively applied as differential line

receivers for red-green-blue (RGB) signals or component video,

such as YPbPr, signals transmitted over unshielded-twisted-pair

(UTP) cable. The AD8143 also contains two general-purpose

comparators with hysteresis that can be used to receive digital

signals or to extract video synchronization pulses from received

common-mode signals that contain encoded synchronization

signals.

An internal linear voltage regulator derives power for the

comparators from the positive supply; therefore, the AD8143

must always have a minimum positive supply voltage of 4.5 V.

The AD8143 includes a power-down feature that can be

asserted to reduce the supply current when a particular device

is not in use.

BASIC CLOSED-LOOP GAIN CONFIGURATIONS

As described in the Theory of Operation section, placing a

resistive feedback network between an amplifier output and its

respective feedback amplifier input creates a stable negative

feedback amplifier. It is important to note that the closed-loop

gain of the amplifier used in the signal path is defined as the

amplifier’s single-ended output voltage divided by its differential

input voltage. Therefore, each amplifier in the AD8143 provides

differential-to-single-ended gain. Additionally, the amplifier used

for feedback has two high impedance inputs—the FB input,

where the negative feedback is applied, and the REF input,

which can be used as an independent single-ended input to

apply a dc offset to the output signal. Some basic gain

configurations implemented with an AD8143 amplifier are

shown in Figure 37 through Figure 39.

+

–

+

–

+5V

–5V

REF

FB

0.01

µF

0.01

µF

The gain equation for the circuit in Figure 37 is

(1)

In this configuration, the voltage applied to the REF pin appears

approach.

+

–

+

–

+5V

–5V

REF

FB

0.01

µF

0.01

µF

The gain equation for the circuit in Figure 38 is

(2)

Another configuration that provides the same gain equation as

Equation 2 is shown in Figure 39. In this configuration, it is

+

–

+

–

+5V

–5V

REF

FB

0.01

µF

0.01

µF

For stability reasons, the inductance of the trace connected to

the REF pin must be kept to less than 10 nH. The typical

inductance of 50 Ω traces on the outer layers of the FR-4 boards

is 7 nH/in, and on the inner layers, it is typically 9 nH/in. Vias

must be accounted for as well. The inductance of a typical via in

a 0.062-inch board is on the order of 1.5 nH. If longer traces are

required, a 200 Ω resistor should be placed in series with the

trace to reduce the Q-factor of the inductance.