OFFSET, GAIN AND CONVERSION
SPEED ADJUSTMENTS (OPTIONAL)
to the ground plane connections. For example, the ADC701
output data lines will sink current (statically and/or dynami-
cally) when in the low state. This current comes from the
power supply that runs the interface logic, and so must
return to that supply’s ground. If the ground termination is
placed such that this digital current will flow away from the
ADC701, then the existing ground plane will suffice to carry
the current. On the other hand, if the ground termination
must be placed such that the digital current flows across the
ADC or SHC layout, then it would be advisable to break the
analog ground plane under the package (to stop the flow of
current across the package) and to provide a separate trace
(several centimeters wide) on another PC board layer to
carry the digital return current from pins 11 and 19 to the
termination point. If the ADC701 must interface into a fairly
noisy digital environment, then another approach is to keep
the first layer of latches and/or buffers connected to the
ADC701 power and ground planes, so that the ADC itself is
connected to “quiet” circuits with short return paths. This
transfers the interface problem to the outputs of the latches,
where it can be managed with less impact on the analog
components.
Adjustment of the reference voltage is the most straightfor-
ward way to adjust the ADC gain. For the internal reference,
this is accomplished by connecting a 20kΩ potentiometer as
shown in Figure 3. This will provide a gain trim range of
about ±3%. It is also possible to use external series or
parallel resistance in the input network, but that is more
cumbersome and usually will degrade the gain stability over
temperature due to tempco (temperature coefficient) mis-
matches among the resistors.
ADC offset may be adjusted by connecting a 500kΩ poten-
tiometer to pins 21 and 22, with the wiper connected
through a series 30kΩ resistor to ground as shown in Figure
3. This will provide an offset trim range of approximately
±0.25% FSR. For a larger trim range of offset or gain, it is
recommended that trims be accomplished elsewhere in the
system.
The Clock Adjust input (pin 18) is intended primarily for
small adjustments of the conversion time. However, this
will rarely be necessary because the ADC701 is guaranteed
to convert up to 512kHz over the specified temperature
range without external clock adjustment.
PHYSICAL INSTALLATION
The packages may be soldered directly into a PC board or
mounted in low-profile machined pin sockets with good
results. Use of tall (long lead length) sockets, adapters or
headers is not recommended unless a local ground plane and
bypass capacitors can be mounted directly under the pack-
ages.
POWER AND GROUND CONNECTIONS
Experience with testing and applying the ADC701 shows
that it will perform well in most board layouts, provided that
appropriate care is taken with grounding and bypassing.
Power supplies may be shared between the ADC701, SHC702
and other analog circuitry without difficulty. It is recom-
mended that each power pin be locally bypassed to the
ground plane with a high quality tantalum capacitor of at
least 1µF. If at all possible, power should be derived from
well-regulated linear supplies—switching power supplies
will require much more effort for proper decoupling and are
not recommended for this or any high performance wide-
band analog system.
In a room-temperature environment or inside an enclosure
with moderate airflow, the ADC701 and SHC702 normally
do not require heat-sinking. However, to keep the devices
running as cool as possible, it is helpful to install a thin heat-
transfer plate under the packages to conduct heat into the
ground plane. The plate may be made from metal (copper,
aluminum or steel) or from a special heat-conductive mate-
rial such as Sil-Pad(1). The Sil-Pad material has the advan-
tage of being electrically insulating and somewhat pliable,
so that it will tend to distribute pressure evenly and conform
to the package—an advantage in systems where the board
may be flexed or subjected to vibration.
The +5V Digital supply pins, though not as sensitive to
noise as the +5V Analog pin, should nonetheless be kept as
quiet as possible. If the system digital supply is noisy, then
it is best to use the system +5V analog supply for all of the
+5V connections on the ADC701 and SHC702 rather than
trying to separate them. If only one +5V supply is available
and it is shared with other system logic, then extra bypassing
and/or supply filtering may be required.
PC BOARD LAYOUT
An optimized layout has been designed for the DEM-
ADC701-E demonstration fixture. For information concern-
ing the demo board and the layout, contact your local sales
representative.
The –5V supply will operate with any voltage between –
4.75 and –6V. If –5V is not available from the system
supplies, then an industry-standard 7905 regulator may be
used to derive –5V from the –15V supply.
All ground pins on both the ADC701 and the SHC702
should be connected directly to a common ground plane.
This is true for both analog and digital grounds. However,
it is also helpful to recognize where the digital ground
currents flow in the system, and to provide PC board return
paths for potentially troublesome digital currents in addition
®
10
ADC701/SHC702
BB [ BURR-BROWN CORPORATION ]
