AD8123

Rev. A | Page 15 of 16

INPUT COMMON-MODE RANGE

When used, common-mode sync signals are generally applied

with a peak deviation of 500 mV and thereby increase the

common-mode level from 2.675 V to 3.175 V. This common-

mode level exceeds the specified input voltage swing limits of

±3.0 V; therefore, the AD8123 cannot be used with a system

that uses common-mode sync encoding with 500 mV sync peak

deviation and 2.5 V common-mode line level. While it is possible

to operate a driver powered from a single 5 V supply at a common-

mode voltage of <2.5 V to obtain a received voltage swing that is

within the specified limits, there is not much margin for other

shifts in the common-mode level due to interference pickup and

differing ground potentials. There are two ways to increase the

common-mode range of the overall system. One is to power the

driver from ±5 V supplies, and the other is to place an AD8143

in front of the AD8123, as shown in Figure 32. These techniques

may be combined or applied separately.

Most applications that use the AD8123 as a receiver use a driver

(such as one from the AD8146/AD8147/AD8148 family, the

AD8133, or the AD8134) powered from ±5 V supplies. This

places the common-mode voltage on the line nominally at 0 V

relative to the ground potential at the driver and provides optimum

immunity from any common-mode anomalies picked up along

the cable (including ground shifts between the driver and receiver

ends). In many of these applications, the AD8123 input voltage

range of typically ±3.0 V is sufficient. If wider input range is

required, the AD8143 triple receiver (input common-mode

range equals ±10.5 V on ±12 V supplies) may be placed in front of

the AD8123. Figure 32 illustrates how this is done for one channel.

100Ω

49.9Ω

1

2

3

RECEIVED

SIGNAL

+5V

ONE AD8123

INPUT

ONE AD8143 CHANNE L

POWER SUPPLIES = ±12V

–5V

HBAT-540C

SMALL SIGNAL FREQUENCY RESPONSE

Though the AD8123 large signal frequency response

interested in the small signal frequency response. The AD8123

for 200 meter and 300 meter cable lengths.

Figure 32. Optional Use of AD8143 in Front of AD8123 for

Wide Input Common-Mode Range

FREQUENCY (MHz)

–12

–11

–10

–9

–8

–7

–6

–5

–4

–3

–2

–1

0

1

2

3

0.1

1

10

100

0.01

200 METERS

300 METERS

The Schottky diodes are required to protect the AD8123 from

any AD8143 outputs that may exceed the AD8123 input limits.

The 49.9 Ω resistor limits the fault current and produces a pole

at approximately 800 MHz with the effective diode capacitance of

3 pF and the AD8123 input capacitance of 1 pF. The pole drops

the response by only 0.07 dB at 100 MHz and therefore has a

negligible effect on the signal.

When using a single 5 V supply on the driver side, the common-

The largest received differential video signal is approximately

mode voltage, resulting in a worst-case peak voltage of 2.675 V

on an AD8123 input (presuming there is no ground shift between

driver and receiver). This is within the AD8123 input voltage

swing limits, and such a system works well as long as the difference

in ground potential between driver and receiver does not cause

the input voltage swing to exceed its specified limits.

Figure 33. Small Signal Frequency Response for Various Cable Lengths

POWER-DOWN

The power-down feature is intended to be used to reduce power

consumption when a particular device is not in use and does

not place the output in a high-Z state when asserted. The input

logic levels and supply current in power-down mode are presented

in the Power Supply section of Table 1.