MCP3550 datasheet(17/36 Pages) MICROCHIP

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DS21950E-page 17

MCP3550/1/3

FIGURE 4-2:

SINC Filter Response,

MCP3550-50 Device.

FIGURE 4-3:

SINC Filter Response,

MCP3550-60 Device.

FIGURE 4-4:

SINC Filter Response,

MCP3551 Device, Simultaneous 50/60 Hz

Rejection.

FIGURE 4-5:

SINC Filter Response at

Integer Multiples of the Sampling Frequency (fs).

4.3

Internal Oscillator

The MCP3550/1/3 devices include a highly stable and

accurate internal oscillator that provides clock signals

to the delta-sigma ADC with minimum jitter. The

oscillator is a specialized structure with a low

temperature coefficient across the full range of

specified operation. See Table 4-1 for oscillator

frequencies.

The conversion time is an integer multiple of the

internal clock period and, therefore, has the same

accuracy as the internal clock frequency. The internal

oscillator frequency is 102.4 kHz ±1% for the

MCP3550-50, 112.64 kHz ±1% for the MCP3551, and

122.88 kHz ±1% for the MCP3550-60 and MCP3553

devices, across the full power supply voltage and

specified temperature ranges.

The notch of the digital filter is proportional to the

internal oscillator frequency, with the exact notch

frequency equivalent to the oscillator accuracy

(< 1% deviation). This high accuracy, combined with

wide notches, will ensure that the MCP3551 will have

simultaneous 50 Hz and 60 Hz line frequency rejection

and the MCP3550-50 or MCP3550-60 devices will

have greater than 120 dB rejection (at either 50 or

60 Hz) by the digital filtering, even when jitter is

present.

The internal oscillator is held in the reset condition

when the part is in Shutdown mode to ensure very low

power consumption (< 1 µA in Shutdown mode). The

internal oscillator is independent of all serial digital

interface edges (i.e., state machine processing the

digital SPI interface is asynchronous with respect to the

internal clock edges).

-120

-100

-80

-60

-40

-20

100

150

200

Frequency (Hz)

-120

-100

-80

-60

-40

-20

120

180

240

Frequency (Hz)

-120

-110

-100

-90

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-70

-60

-50

-40

-30

-20

-10

1020 3040506070 8090 100 110

Frequency (Hz)

-140

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-80

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-40

-20

28160 56320 84480 112640 140800 168960 197120 225280 253440

Frequency (Hz)

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