AD8146/AD8147/AD8148

Rev. 0 | Page 15 of 24

CALCULATING THE INPUT IMPEDANCE

OUTPUT COMMON-MODE CONTROL

The AD8146 allows the user to control each of the three

common-mode output levels independently through the three

mode output level of each of their respective amplifiers with

330 MHz of small signal bandwidth and an internally fixed gain

of 1. In this way, additional control and communication signals

can be embedded on the common-mode levels as users see fit.

The effective input impedance of a circuit such as that in

being driven by a single-ended or differential signal source. For

balanced differential input signals, the differential input impedance,

impedance becomes

amplifier defaults to approximately midsupply. An internal

resistive divider with an impedance of approximately 12.5 kΩ

sets this level. To limit common-mode noise in dc common-

mode applications, external bypass capacitors should be

⎟

⎟

⎟

⎠

⎞

⎜

⎜

⎜

⎜

⎝

⎛

+

×

−

=

F

G

F

G

dm

IN,

R

R

R

R

R

2

1

SYNC-ON COMMON-MODE

The single-ended input impedance of the AD8146 and the

AD8147 is therefore 750 Ω, and the single-ended input

impedance of the AD8148 is 833 Ω.

The AD8147 and AD8148 are specifically targeted at driving

RGB video signals over UTP cable using a sync-on common-

mode technique. The common-mode outputs of each of the R,

G, and B differential outputs are set using circuitry contained

within the device. This circuitry embeds the horizontal and

vertical sync pulses on the three common-mode outputs in a

way that also results in low radiated energy. For a more detailed

description of the sync scheme, see the Applications section.

The input impedance of the circuit is effectively higher than it

would be for a conventional op amp connected as an inverter

because a fraction of the differential output voltage appears at

the inputs as a common-mode signal, partially bootstrapping

The sync-on common-mode circuit generates a current based

on the SYNC LEVEL input pin (Pin 18). With the SYNC LEVEL

input tied to GND, the common-mode output of all drivers is

applied between GND and SYNC LEVEL that determines the

maximum deviation of the common-mode outputs from their

midsupply level. If, for instance, SYNC LEVEL = 0.5 V and the

supply voltage is 5 V, the common-mode outputs fall within an

defined in the Applications section.

INPUT COMMON-MODE VOLTAGE RANGE IN

SINGLE-SUPPLY APPLICATIONS

The driver inputs are designed to facilitate level-shifting of

ground-referenced input signals on a single power supply. For a

single-ended input, this implies, for example, that the voltage at

voltage of the amplifier is also set to 0 V.

It is important to ensure that the common-mode voltage at the

by negative feedback, the input common-mode voltage of the

calculated as

In most cases, the sync-on common-mode circuit can be used

GND, and the exposed paddles of the AD8147 and AD8148

are used as the GND references for the incoming sync pulses.

When ±2.5 V supplies are used, however, external protection is

required to limit the positive excursion to less than 2.5 V. For

more details, see the Applications section.

3

OCM

ACM

V

V

V

+

=

in the current mode level-shifting circuit are well matched to

eliminate false switching transients, maximizing common-

mode balance and minimizing radiated energy.