AD8115AST datasheet(25/32 Pages) AD

25 / 32 page

AD8114/AD8115

Rev. B | Page 25 of 32

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 AD8114/AD8115 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 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 provide shielding,

physical 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 frequency 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

preventing 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 connected directly 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

output series back-termination resistors. These signals should

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

from the IC package.

Optimized for video applications, all signal inputs and outputs

are terminated with 75 Ω resistors. Stripline techniques are used

to achieve a characteristic impedance of 75 Ω on the signal

input and output lines. 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 impedances,

are well shielded.

w = 0.008"

(0.2mm)

a = 0.008"

(0.2mm)

b = 0.0514"

(1.3mm)

h = 0.025"

(0.63mm)

t = 0.00135" (0.0343mm)

TOP LAYER

SIGNAL LAYER

POWER LAYER

BOTTOM LAYER

Figure 50. Cross Section of Input and Output Traces

The board has 32 BNC type connectors: 16 inputs and 16 outputs.

The connectors are arranged in a crescent around the device. As

can be seen from Figure 53, this results in all 16 input signal

traces and all 16 signal output traces having the same length.

This is useful in tests such as all-hostile crosstalk where the

phase relationship and delay between signals needs to be

maintained from input to output.

The three power supply pins AVCC, DVCC and AVEE should

be connected to good quality, low noise, ±5 V supplies. Where

the same ±5 V power supplies are used for analog and digital,

separate cables should be run for the power supply to the

evaluation board’s analog and digital power supply pins.

As a general rule, each power supply pin (or group of adjacent

power supply pins) should be locally decoupled with a 0.01 µF

capacitor. If there is a space constraint, it is more important to

decouple analog power supply pins before digital power supply

pins. A 0.1 µF capacitor, located reasonably close to the pins,

can be used to decouple a number of power supply pins. Finally

a 10 µF capacitor should be used to decouple power supplies as

they come onto the board.

AD8115AST датащи(PDF) 25 Page - Analog Devices

AD8115AST датащи(HTML) 25 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

датащи скачать

ссылки URL