AD8111AST datasheet(20/28 Pages) AD

20 / 28 page

AD8110/AD8111

–20–

REV. 0

On the output side, the crosstalk can be reduced by driving a

lighter load. Although the AD8110/AD8111 is specified with

excellent differential gain and phase when driving a standard

150

Ω video load, the crosstalk will be higher than the minimum

obtainable due to the high output currents. These currents will

induce crosstalk via the mutual inductance of the output pins

and bond wires of the AD8110/AD8111.

From a circuit standpoint, this output crosstalk mechanism

looks like a transformer with a mutual inductance between the

windings that drives a load resistor. For low frequencies, the

magnitude of the crosstalk is given by:

|XT| = 20 log10 (Mxy

× s/R

where Mxy is the mutual inductance of output x to output y,

and RL is the load resistance on the measured output. This

crosstalk mechanism can be minimized by keeping the mutual

inductance low and increasing RL. The mutual inductance can

be kept low by increasing the spacing of the conductors and

minimizing their parallel length.

PCB Layout

Extreme care must be exercised to minimize additional crosstalk

generated by the system circuit board(s). The areas that must be

carefully detailed are grounding, shielding, signal routing and

supply bypassing.

The packaging of the AD8110/AD8111 is designed to help keep

the crosstalk to a minimum. Each input is separated from each

other input by an analog ground pin. All of these AGNDs should

be directly connected to the ground plane of the circuit board.

These ground pins provide shielding, low impedance return

paths and physical separation for the inputs. All of these help to

reduce crosstalk.

Each output is separated from its two neighboring outputs by an

analog ground pin in addition to an analog supply pin of one

polarity or the other. Each of these analog supply pins provides

power to the output stages of only the two nearest outputs.

These supply pins and analog grounds provide shielding, physi-

cal separation and a low impedance supply for the outputs.

Individual bypassing of each of these supply pins with a 0.01

µF

chip capacitor directly to the ground plane minimizes high fre-

quency output crosstalk via the mechanism of sharing common

impedances.

Each output also has an on-chip compensation capacitor that

is individually tied to the nearby analog ground pins AGND00

through AGND07. This technique reduces crosstalk by prevent-

ing the currents that flow in these paths from sharing a common

impedance on the IC and in the package pins. These AGNDxx

signals should all be directly connected to the ground plane.

The input and output signals will have minimum crosstalk if

they are located between ground planes on layers above and

below, and separated by ground in between. Vias should be

located as close to the IC as possible to carry the inputs and

outputs to the inner layer. The only place the input and output

signals surface is at the input termination resistors and the out-

put series back termination resistors. These signals should also

be separated, to the extent possible, as soon as they emerge from

the IC package.

Evaluation Board

A four-layer evaluation board is available for the AD8110/AD8111.

The exact same board and external components are used for

each device. The only difference is the device itself, which offers

a selection of a gain of unity or gain of two through the analog

channels. This board has been carefully laid out and tested to

demonstrate the specified high speed performance of the device.

Figure 44 shows the schematic of the evaluation board. Figure

45 shows the component side silk-screen. The layouts of the

board’s four layers are given in:

Component Layer—Figure 46

Signal Routing Layer—Figure 47

Power Layer—Figure 48

Bottom Layer—Figure 49

The evaluation board package includes the following:

• Fully populated board with BNC-type connectors.

• Windows™ based software for controlling the board from a

PC via the printer port.

• Custom cable to connect evaluation board to PC.

• Disk containing Gerber files of board layout.

Optimized for video applications, all signal inputs and outputs

are terminated with 75

Ω resistors. Stripline techniques are used

to achieve a characteristic impedance on the signal input and

output lines also of 75

Ω. Figure 50 shows a cross-section of

one of the input or output tracks along with the arrangement of

the PCB layers. It should be noted that unused regions of the

four layers are filled up with ground planes. As a result, the

input and output traces, in addition to having controlled imped-

ances, are well shielded.

All trademarks are properties of their respective holders.

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