MCP3004-ISLASHST datasheet(19/34 Pages) MICROCHIP

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DS21295C-page 19

MCP3004/3008

6.2

Maintaining Minimum Clock

Speed

When the MCP3004/3008 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 10 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 in to the conversion

results, as is illustrated in Figure 6-3, where an op amp

is used to drive, filter and gain the analog input of the

MCP3004/3008. This amplifier provides a low imped-

ance source for the converter input, plus 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 resistors 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 MCP3004.

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 1 µF is recommended.

Digital and analog traces should be separated as much

as possible on the board, with no traces running under-

neath the device or bypass capacitor. Extra precau-

tions 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 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:

VDD traces arranged in a ‘Star’

configuration in order to reduce errors caused by

current return paths.

MCP3004

VDD

10 µF

IN-

IN+

VIN

VREF

4.096V

Reference

1µF

0.1 µF

MCP601

MCP1541

Connection

Device 1

Device 2

Device 3

Device 4

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