2008 Microchip Technology Inc.
DS21295D-page 23
MCP3004/3008
6.4
Layout Considerations
When laying out a printed circuit board for use with
analog components, care should be taken to reduce
noise wherever possible. A bypass capacitor should
always be used with this device and should be placed
as close as possible to the device pin. A bypass
capacitor value of 1 F is recommended.
Digital and analog traces should be separated as much
as possible on the board, with no traces running
underneath the device or bypass capacitor. Extra
precautions should be taken to keep traces with high-
frequency signals (such as clock lines) as far as
possible from analog traces.
Use of an analog ground plane is recommended in
order to keep the ground potential the same for all
devices on the board. Providing VDD connections to
devices in a “star” configuration can also reduce noise
by eliminating return current paths and associated
errors (see
Figure 6-4). For more information on layout
tips when using A/D converters, refer to AN688,
“Layout Tips for 12-Bit A/D Converter Applications”
.
FIGURE 6-4:
VDD traces arranged in a
‘Star’ configuration in order to reduce errors
caused by current return paths.
6.5
Utilizing the Digital and Analog
Ground Pins
The MCP3004/3008 devices provide both digital and
analog ground connections to provide additional
the analog and digital circuitry is separated internal to
the device. This reduces noise from the digital portion
of the device being coupled into the analog portion of
the device. The two grounds are connected internally
through the substrate which has a resistance of 5 -10.
If no ground plane is utilized, both grounds must be
connected to VSS on the board. If a ground plane is
available, both digital and analog ground pins should
be connected to the analog ground plane. If both an
analog and a digital ground plane are available, both
the digital and the analog ground pins should be
connected to the analog ground plane. Following these
steps will reduce the amount of digital noise from the
rest of the board being coupled into the A/D converter.
FIGURE 6-5:
Separation of Analog and
Digital Ground Pins.
VDD
Connection
Device 1
Device 2
Device 3
Device 4
MCP3004/08
Analog Ground Plane
DGND
AGND
VDD
0.1 F
Substrate
5 - 10
Digital Side
-SPI Interface
-Shift Register
-Control Logic
Analog Side
-Sample Cap
-Capacitor Array
-Comparator