AD8129ARMZ datasheet(37/40 Pages) AD

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AD8129/AD8130

Rev. C | Page 37 of 40

Another problem can occur with the AD8129 operating

at a supply voltage of greater than or equal to ±12 V. The

architecture causes the supply current to increase as the input

differential voltage increases. If the AD8129 differential inputs

are overdriven too far, excessive current can flow into the device

and potentially cause permanent damage.

A practical means to prevent this from occurring is to clamp the

inputs differentially with a pair of antiparallel Schottky diodes

(see Figure 146). These diodes have a lower forward voltage of

approximately 0.4 V. If the differential voltage across the inputs

is restricted to these conditions, no excess current is drawn by

the AD8129 under these operating conditions.

If the supply voltage is restricted to less than ±11 V, the internal

clamping circuit limits the differential voltage and excessive

supply current is not drawn. The external clamp circuit is not

needed.

VIN

AGILENT

HSMS 2822

VOUT

–V

–VS

+VS

VIN

AD8129

0.1

μF

0.1

μF10μF

Figure 146. Schottky Diodes Across the Inputs

Limits the Input Differential Voltage

In both circuits, the input series resistors function to limit the

current through the diodes when they are forward biased. As a

practical matter, these resistors must be matched so that the

CMRR is preserved at high frequencies. These resistors have

minimal effect on the CMRR at low frequency.

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

can require this. This would be encountered when significant

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

that do not require a wide dynamic range for the input or output, it

is recommended to operate with lower supply voltages.

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

MSOP package. This package has higher thermal impedance

than larger packages and operates at a higher temperature with

the same amount of power dissipation. Certain operating

conditions that are within the specifications range of the parts can

cause excess power dissipation. Caution should be exercised.

The power dissipation is a function of several operating

conditions, including the supply voltage, the input differential

voltage, the output load, and the signal frequency.

A basic starting point is to calculate the quiescent power

dissipation with no signal and no differential input voltage.

This is just the product of the total supply voltage and the

quiescent operating 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

MSOP package, the θJA specification is 142°C/W. Therefore,

the quiescent power causes about a 49°C rise above ambient

in the MSOP package.

The current consumption is also a function of the differential

input voltage (see Figure 113 and Figure 114). This current

should be added onto 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 with the output

driving a 250 Ω load to ground, the maximum power dissipation

in the output occurs when the output voltage is 6 V. The load

current is 6 V/250 Ω = 24 mA. This same current flows through

the output across a 6 V drop from VS. It dissipates 144 mW. For

the 8-lead MSOP 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 8-lead SOIC package. This lowers the thermal impedance

to 121°C/W, which is a 15% improvement. Another is to use a

lower supply voltage unless absolutely necessary.

Finally, do not use the AD8129/AD8130 when it is operating on

high supply voltages to directly drive a heavy load. 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.

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