AD8129/AD8130
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.
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 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.
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.
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.
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.
+V
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.
AD8129
0.1
μ
F
10μF
V
IN
3
7
3
1
8
+V
+
+
PD
S
AGILENT
HSMS 2822
6
V
OUT
1
2
4
5
V
IN
–V
S
2
10μF
0.1μF
–V
Figure 146. Schottky Diodes Across the Inputs
Limits the Input Differential Voltage
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.
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.
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.
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.
Rev. C | Page 37 of 40
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