produces a voltage spike at turn-on and/or turn-off of power
can destroy the ADC081000. The circuit of Figure 8 will pro-
vide supply overshoot protection.
Many linear regulators will produce output spiking at power-
on unless there is a minimum load provided. Active devices
draw very little current until their supply voltages reach a few
hundred millivolts. The result can be a turn-on spike that can
destroy the ADC081000, unless a minimum load is provided
for the supply. The 100Ω resistor at the regulator output in
Figure 8 provides a minimum output current during power-up
to ensure there is no turn-on spiking.
In this circuit, an LM317 linear regulator is satisfactory if its
input supply voltage is 4V to 5V . If a 3.3V supply is used, an
LM1086 linear regulator is recommended. Also, be sure that
the impedance of the power distribution system is low to min-
imize resistive losses and minimize noise on the power sup-
The output drivers should have a supply voltage, VDR, that is
within the range specified in the Operating Ratings table. This
voltage should not exceed the VA supply voltage and should
never spike to a voltage greater than (VA + 100mV).
If the power is applied to the device without a clock signal
present, the current drawn by the device might be below 100
mA. This is because the ADC081000 gets reset through
clocked logic and its initial state is random. If the reset logic
comes up in the "on" state, it will cause most of the analog
circuitry to be powered down, resulting in less than 100 mA
of current draw. This current is greater than the power down
current because not all of the ADC is powered down. The de-
vice current will be normal after the clock is established.
7.2 Thermal Management
The ADC081000 is capable of impressive speeds and per-
formance at very low power levels for its speed. However, the
power consumption is still high enough to require attention to
thermal management. For reliability reasons, the die temper-
ature should be kept to a maximum of 130°C. That is, tA
(ambient temperature) plus ADC power consumption times
θJA (junction to ambient thermal resistance) should not ex-
ceed 130°C. This is not a problem if the ambient temperature
is kept to a maximum of +85°C, the device is soldered to a
PC Board and the sample rate is at or below 1 Gsps.
Note that the following are general recommendations for
mounting exposed pad devices onto a PCB. This should be
considered the starting point in PCB and assembly process
development. It is recommended that the process be devel-
oped based upon past experience in package mounting.
The package of the ADC081000 has an exposed pad on its
back that provides the primary heat removal path as well as
excellent electrical grounding to the printed circuit board. The
land pattern design for lead attachment to the PCB should be
the same as for a conventional LQFP, but the exposed pad
must be attached to the board to remove the maximum
amount of heat from the package, as well as to ensure best
product parametric performance.
To maximize the removal of heat from the package, a thermal
land pattern must be incorporated on the PC board within the
footprint of the package. The exposed pad of the device must
be soldered down to ensure adequate heat conduction out of
the package. The land pattern for this exposed pad should be
at least as large as the 5 x 5 mm of the exposed pad of the
package and be located such that the exposed pad of the
device is entirely over that thermal land pattern. This thermal
land pattern should be electrically connected to ground. A
clearance of at least 0.5 mm should separate this land pattern
from the mounting pads for the package pins.
Since a large aperture opening may result in poor release, the
aperture opening should be subdivided into an array of small-
er openings, similar to the land pattern of Figure 9.
FIGURE 9. Recommended Package Land Pattern
To minimize junction temperature, it is recommended that a
simple heat sink be built into the PCB. This is done by includ-
ing a minimum copper pad of 2 inches by 2 inches (5.1 cm by
5.1 cm) on the opposite side of the PCB. This copper area
may be plated or solder coated to prevent corrosion, but
should not have a conformal coating, which could provide
some thermal insulation. Thermal vias should be used to con-
nect these top and bottom copper areas. These thermal vias
act as "heat pipes" to carry the thermal energy from the device
side of the board to the opposite side of the board where it
can be more effectively dissipated. The use of 9 to 16 thermal
vias is recommended.
The thermal vias should be placed on a 1.2 mm grid spacing
and have a diameter of 0.30 to 0.33 mm. These vias should
be barrel plated to avoid solder wicking into the vias during
the soldering process as this wicking could cause voids in the
solder between the package exposed pad and the thermal
land on the PCB. Such voids could increase the thermal re-
sistance between the device and the thermal land on the
board, which would cause the device to run hotter.
On a board of FR-4 material and the built in heat sink de-
scribed above (4 square inch pad and 9 thermal vias), the die
temperature stabilizes at about 30°C above the ambient tem-
perature in about 20 seconds.
If it is desired to monitor die temperature, a temperature sen-
sor may be mounted on the heat sink area of the board near
the thermal vias. Allow for a thermal gradient between the
temperature sensor and the ADC081000 die of θJC times typ-
ical power consumption = 2.8 x 1.43 = 4°C. Allowing for a 5°
C (including an extra 1°C) temperature drop from the die to
the temperature sensor, then, would mean that maintaining a
maximum pad temperature reading of 125°C will ensure that
the die temperature does not exceed 130°C, assuming that
the exposed pad of the ADC081000 is properly soldered
down and the thermal vias are adequate.
8.0 LAYOUT AND GROUNDING
Proper grounding and proper routing of all signals are essen-
tial to ensure accurate conversion. A single ground plane
should be used, as opposed to splitting the ground plane into
analog and digital areas.
Since digital switching transients are composed largely of
high frequency components, the skin effect tells us that total
ground plane copper weight will have little effect upon the
logic-generated noise. Total surface area is more important