Datasheet

MCP3302/04
DS21697F-page 20  2011 Microchip Technology Inc.
5.5 Buffering/Filtering the Analog
Inputs
Inaccurate conversion results may occur if the signal
source for the A/D converter is not a low-impedance
source. Buffering the input will overcome the
impedance issue. It is also recommended that an
analog filter be used to eliminate any signals that may
be aliased back into the conversion results. This is
illustrated in Figure 5-9, where an op amp is used to
drive the analog input of the MCP3302/04. This
amplifier provides a low-impedance source for the
converter input and a low-pass filter, which eliminates
unwanted high-frequency noise. Values shown are for
a 10 Hz Butterworth Low-Pass filter.
Low-pass (anti-aliasing) filters can be designed using
Microchip’s interactive FilterLab
®
software. FilterLab
will calculate capacitor and resistor values, as well as
determine the number of poles that are required for the
application. For more detailed information on filtering
signals, see AN699 “Anti-Aliasing, Analog Filters for
Data Acquisition Systems”, at www.microchip.com.
FIGURE 5-9: The MCP601 Operational
Amplifier is used to implement a 2nd order anti-
aliasing filter for the signal being converted by
the MCP3302/04.
5.6 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 from V
DD
to ground should always be used with this device and
should be placed as close as possible to the device pin.
A bypass capacitor value of 0.1 µF is recommended.
Digital and analog traces on the board should be
separated as much as possible, with no traces running
underneath the device or the 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 V
DD
connections to
devices in a “star” configuration can also reduce noise
by eliminating current return paths and associated
errors (see Figure 5-10). Layout tips for using the
MCP3302, MCP3304, or other ADC devices, are avail-
able in AN688, “Layout Tips for 12-Bit A/D Converter
Applications”, from www.microchip.com.
FIGURE 5-10: V
DD
traces arranged in a
‘Star’ configuration in order to reduce errors
caused by current return paths.
MCP330X
V
DD
10 µF
IN-
IN+
-
+
V
IN
2.2 µF
1µF
V
REF
4.096V
Reference
0.1 µF
1µF
0.1 µF
MCP601
7.86 k
14.6 k
MCP1541
C
L
V
DD
Connection
Device 1
Device 2
Device 3
Device 4