MCP3202_06 datasheet(16/28 Pages) MICROCHIP

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MCP3202

DS21034D-page 16

6.2

Maintaining Minimum Clock Speed

When the MCP3202 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 cap 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 criteria may induce linear-

ity errors into the conversion outside the rated specifi-

cations. 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. It is also recom-

mended that a filter be used to eliminate any signals

that may be aliased back into the conversion results.

This is illustrated in Figure 6-3 below where an op amp

is used to drive the analog input of the MCP3202. This

amplifier provides a low impedance output for the con-

verter input and a low pass filter, which eliminates

unwanted high frequency noise.

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 information on filtering signals,

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 MCP3202.

6.4

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 should

always be used with this device and should be placed

as close as possible to the device pin. A bypass capac-

itor value of 0.1 µF 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 VDD connections to

devices in a “star” configuration can also reduce noise

by eliminating current return 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”

(DS00688).

FIGURE 6-4:

traces

arranged

‘Star’

configuration in order to reduce errors caused by

current return paths.

MCP3202

μF

IN-

IN+

0.1

μF

MCP601

VDD

Connection

Device 1

Device 2

Device 3

Device 4

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