mcp3208t-ci-sl Microchip Technology Inc., mcp3208t-ci-sl Datasheet - Page 19

• Manufacturer Part Number: mcp3208t-ci-sl
• Description: 2.7v 4-channel/8-channel 12-bit A/d Converters With Spi Serial Interface
• Manufacturer: Microchip Technology Inc.

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 1.2 ms 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 1.2 ms (effective clock
frequency of 10 kHz). Failure to meet this criterion may
introduce 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.
FIGURE 6-3:
filter for the signal being converted by the MCP3204.
© 2007 Microchip Technology Inc.
Maintaining Minimum Clock
Speed
V
The MCP601 Operational Amplifier is used to implement a second order anti-aliasing
IN
R
1
C
R
C
1
2
0.1 µF
2
Reference
4.096V
R
MCP601
3
MCP1541
+
-
R
4
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 into the conversion
results, as 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 impedance source for the
converter input, and a low pass filter, which eliminates
unwanted high frequency noise.
Low pass (anti-aliasing) filters can be designed using
Microchip’s free interactive FilterLab™ software. Filter-
Lab will calculate capacitor and resistor values, as well
as determine the number of poles that are required for
the application. For more information on filtering sig-
nals, see AN699, “Anti-Aliasing Analog Filters for Data
Acquisition Systems”.
1 µF
IN+
IN-
Buffering/Filtering the Analog
Inputs
MCP3204
V
REF
MCP3204/3208
V
DD
10 µF
1 µF
DS21298D-page 19