MCP3204 datasheet(17/20 Pages) MICROCHIP

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Preliminary

DS21298B-page 17

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 1.2ms 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

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 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:

The MCP601 Operational Amplifier is

used to implement a 2nd order anti-aliasing filter for

the signal being converted by the MCP3204.

6.4

Layout Considerations

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

DD 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

“Lay-

out Tips for 12-Bit A/D Converter Applications”.

FIGURE 6-4:

traces

arranged

‘Star’

configuration in order to reduce errors caused by

current return paths.

FilterLab is a trademark of Microchip Technology Inc. in

MCP3204

10µF

IN-

IN+

VIN

REF

4.096V

Reference

ADI

REF198

1µF

0.1µF

Tant.

0.1µF

MCP601

Connection

Device 1

Device 2

Device 3

Device 4

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