AD8123

Rev. A | Page 13 of 16

USING THE AD8123 WITH COAXIAL CABLE

types of cable, including coaxial cable. Figure 18 presents the

AD8123 is used with good quality 75 Ω video cable. Figure 24

20Ω

5.11kΩ

20kΩ

–5V

+5V

24.3kΩ

47.5kΩ

1.16kΩ

10kΩ

1.24kΩ

is close to unity. A passive offset circuit would require an offset

injection voltage that is much larger in magnitude than the

also be developed independently.

The AD8123 differential input can accept signals carried over

unbalanced cable, as shown in Figure 25, for an unbalanced

75 Ω coaxial cable termination.

75Ω

INPUT FROM

75Ω CABLE

AD8123

INPUT STAGE

Figure 25. Terminating a 75 Ω Cable

DRIVING 75 Ω VIDEO CABLE WITH THE AD8123

When the RGB outputs must drive a 75 Ω line rather than a

high impedance load, an additional gain of two is required to

make up for the double termination loss (75 Ω source and load

terminations). There are two options available for this.

One option is to place the additional gain of 2 at the drive end

by using the AD8148 triple differential driver to drive the cable.

The AD8148 has a fixed gain of 4 instead of the usual gain of 2

and thereby provides the required additional gain of 2 without

having to add additional amplifiers to the signal chain. The

AD8148 also contains sync-on-common-mode encoding. If

sync-on-common-mode is not required, it can be deactivated

on the AD8148 by connecting its SYNC LEVEL input to ground.

The other option is to include a triple gain-of-2 buffer, such as the

ADA4862-3, on the AD8123 RGB outputs, as shown in Figure 26

for one channel (power supplies not shown). The ADA4862-3

provides the gain of 2 that compensates for the double-

termination loss.

ONE VIDEO

OUTPUT

FROM AD8123

ONE CHANNEL OF ADA4862-3

75Ω

Figure 26. Using ADA4862-3 on AD8123 Outputs

DRIVING A CAPACITIVE LOAD

When driving a high impedance capacitive input, it is necessary

to place a small series resistor between each of the three AD8123

video outputs and the load to buffer the input capacitance of the

device being driven. Clearly, the resistor value must be small

enough to preserve the required bandwidth.

FILTERING THE RGB OUTPUTS

In some cases, it is desirable to place low-pass filters on the

AD8123 video outputs to reduce high frequency noise. A 3-pole

Butterworth filter with cutoff frequency in the neighborhood of

140 MHz is sufficient in most applications. Figure 27 and Figure 28

present filters for the high impedance load case (driving a delay

line, crosspoint switch, ADA4862-3) and the double-termination

case (75 Ω source and load resistances), respectively. In the high

impedance load case, the load capacitance must be absorbed in

the capacitor that is placed across the load. For example, in

Figure 27, if the high-Z load were the input to an ADA4862-3,

which has an input capacitance of 2 pF, the filter capacitor value

in parallel with the input would be 15 pF to obtain 17 pF.

*INPUT CAPACITANCE OF LOAD MUST BE

ABSORBED INTO THIS VALUE.

HIGH-Z

150nH

5.6pF

17pF*

AD8123

OUTPUT

Figure 27. 140 MHz Low-Pass Filter on AD8123 Output Feeding High-Z Load

180nH

75Ω

15pF

15pF

AD8123

OUTPUT

Figure 28. 135 MHz Low-Pass Filter on AD8123 Output Feeding

Doubly Terminated Load

These filters are by no means the only choices but are presented

here as examples. In the high-Z load case, it is important to

keep the filter source resistance large enough to buffer the

capacitive loading presented by the first capacitor in the filter.