MCP3204-BIP MICROCHIP [Microchip Technology], MCP3204-BIP Datasheet - Page 17

• Manufacturer Part Number: MCP3204-BIP
• Description: 2.7V 4-Channel/8-Channel 12-Bit A/D Converters with SPI Serial Interface
• Manufacturer: MICROCHIP [Microchip Technology]

6.2
When the MCP3204/3208 initiates the sample period,
charge is stored on the sample capacitor. When the
sample period is complete, the device converts one bit
for each clock that is received. It is important for the
user to note that a slow clock rate will allow charge to
bleed off the sample capacitor while the conversion is
taking place. At 85°C (worst case condition), the part
will maintain proper charge on the sample capacitor for
at least 1.2ms after the sample period has ended. This
means that the time between the end of the sample
period and the time that all 12 data bits have been
clocked out must not exceed 1.2ms (effective clock fre-
quency of 10kHz). Failure to meet this criterion may
induce linearity errors into the conversion outside the
rated specifications. It should be noted that during the
entire conversion cycle, the A/D Converter does not
require a constant clock speed or duty cycle, as long as
all timing specifications are met.
6.3
If the signal source for the A/D Converter is not a low
impedance source, it will have to be buffered or inaccu-
rate conversion results may occur. See Figure 4-2. It is
also recommended that a filter be used to eliminate any
signals that may be aliased back in to the conversion
results. This is illustrated in Figure 6-3 where an op
amp is used to drive the analog input of the
MCP3204/3208. This amplifier provides a low imped-
ance source for the converter input and a low pass fil-
ter, which eliminates unwanted high frequency noise.
Low pass (anti-aliasing) filters can be designed using
Microchip’s free interactive FilterLab™ software. Fil-
terLab will calculate capacitor and resistors values, as
well as determine the number of poles that are required
for the application. For more information on filtering sig-
nals, see the application note AN699 “Anti-Aliasing
Analog Filters for Data Acquisition Systems.”
FIGURE 6-3:
used to implement a 2nd order anti-aliasing filter for
the signal being converted by the MCP3204.
V
1999 Microchip Technology Inc.
IN
R
1
Maintaining Minimum Clock Speed
Buffering/Filtering the Analog Inputs
C
1
R
C
2
2
0.1µF
Reference
4.096V
The MCP601 Operational Amplifier is
R
MCP601
3
REF198
+
-
R
ADI
4
Tant.
1µF
0.1µF
IN+
IN-
MCP3204
V
REF
V
DD
10µF
1µF
Preliminary
6.4
When laying out a printed circuit board for use with ana-
log 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 1µF is recommended.
Digital and analog traces should be separated as much
as possible on the board and no traces should run
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 possi-
ble 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
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 “Lay-
out Tips for 12-Bit A/D Converter Applications” .
FIGURE 6-4:
configuration in order to reduce errors caused by
current return paths.
FilterLab is a trademark of Microchip Technology Inc. in
the U.S.A and other countries. All rights reserved.
Layout Considerations
Device 1
MCP3204/3208
V
DD
Device 2
Connection
traces arranged in a ‘Star’
V
DD
Device 3
DD
Device 4
DS21298B-page 17
connections to