MCP3204-CISLASHP datasheet(19/34 Pages) MICROCHIP

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DS21298D-page 19

MCP3204/3208

6.2

Maintaining Minimum Clock

Speed

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.

6.3

Buffering/Filtering the Analog

Inputs

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”.

FIGURE 6-3:

The MCP601 Operational Amplifier is used to implement a second order anti-aliasing

filter for the signal being converted by the MCP3204.

MCP3204

VDD

10 µF

IN-

IN+

VIN

VREF

4.096V

Reference

1µF

0.1 µF

MCP601

MCP1541

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