AD8129ARM datasheet(25/28 Pages) AD

25 / 28 page

REV. 0

AD8129/AD8130

–25–

Power Dissipation

The AD8129/AD8130 can operate with supply voltages from

+5 V to

±12 V. The major reason for such a wide supply range

is to provide a wide input common-mode range for systems

that might require this. This would be encountered when sig-

nificant common-mode noise couples into the input path. For

applications that do not require a wide input or output dynamic

range, it is recommended to operate with lower supply voltages.

The AD8129/AD8130 is also available in a very small Micro_SO-8

package. This has higher thermal impedance than larger packages

and will operate at a higher temperature with the same amount

of power dissipation. Certain operating conditions that are within

the specification range of the parts can cause excess power dissi-

pation. Caution should be exercised.

The power dissipation is a function of several operating condi-

tions. These include the supply voltage, the input differential

voltage, the output load and the signal frequency.

A basic starting point is to calculate the quiescent power dissipa-

tion with no signal and no differential input voltage. This is just

the product of the total supply voltage and the quiescent operat-

ing current. The maximum operating supply voltage is 26.4 V

and the quiescent current is 13 mA. This causes a quiescent

power dissipation of 343 mW. For the Micro_SO package, the

JA specification is 142

°C/W. So the quiescent power will cause

about a 49

°C rise above ambient in the Micro_SO package.

The current consumption is also a function of the differential

input voltage. (See TPCs 109 and 110.) This current should be

added on to the quiescent current and then multiplied by the

total supply voltage to calculate the power.

The AD8129/AD8130 can directly drive loads of as low as

100

Ω, such as a terminated 50 Ω cable. The worst-case power

dissipation in the output stage occurs when the output is at

midsupply. As an example, for a 12 V supply and the output

driving a 250

Ω load to ground, the maximum power dissipation

in the output will occur when the output voltage is 6 V.

The load current will be 6 V/250

Ω = 24 mA. This same current

will flow through the output across a 6 V drop from +VS. This

will dissipate 144 mW. For the Micro_SO-8 package, this causes a

temperature rise of 20

°C above ambient. Although this is a worst-

case number, it is apparent that this can be a considerable

additional amount of power dissipation.

Several changes can be made to alleviate this. One is to use the

standard SO-8 package. This will lower the thermal impedance

to 121

°C/W, which is a 15% improvement. Next is to use a

lower supply voltage unless absolutely necessary.

Finally, do not use the AD8129/AD8130 to directly drive a

heavy load when it is operating on high supply voltages. It is

best to use a second op amp after the output stage. Some of the

gain can be shifted to this stage so that the signal swing at the

output of the AD8129/AD8130 is not too large.

Layout, Grounding and Bypassing

The AD8129/AD8130 are very high-speed parts that can be

sensitive to the PCB environment in which they have to oper-

ate. Realizing their superior specifications requires attention

to various details of standard high-speed PCB design practice.

The first requirement is for a good solid ground plane that cov-

ers as much of the board area around the AD8129/AD8130 as

possible. The only exception to this is that the ground plane

around the FB pin should be kept a few mm away, and ground

should be removed from inner layers and the opposite side of

the board under this pin. This will minimize the stray capaci-

tance on this node and help preserve the gain flatness versus

frequency.

The power supply pins should be bypassed as close as possible

to the device to the nearby ground plane. Good high-frequency

ceramic chip capacitors should be used. This bypassing should

be done with a capacitance value of 0.01

µF to 0.1 µF for each

supply. Further away, low frequency bypassing should be provided

with 10

µF tantalum capacitors from each supply to ground.

The signal routing should be short and direct in order to avoid

parasitic effects. Where possible, signals should be run over

ground planes to avoid radiating, or to avoid being susceptible

to other radiation sources.

AD8129ARM Datasheet(PDF) 25 Page - Analog Devices

AD8129ARM Datasheet(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

Datasheet Download

Link URL