MCP3912 datasheet(33/82 Pages) MICROCHIP

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DS20005348A-page 33

MCP3912

The gain of the transfer function of this filter is one at

each multiple of DMCLK (typically 1 MHz), so a proper

anti-aliasing filter must be placed at the inputs. This will

attenuate the frequency content around DMCLK and

keep the desired accuracy over the baseband of the

converter. This anti-aliasing filter can be a simple, first-

order RC network, with a sufficiently low time constant

to generate high rejection at the DMCLK frequency

Any unsettled data is automatically discarded to avoid

data corruption. Each data ready pulse corresponds to

fully settled data at the output of the decimation filter.

The first data available at the output of the decimation

filter is present after the complete settling time of the

filter (see Table 5-3). After the first data has been

processed, the delay between two data ready pulses

coming from the same ADC channel is one DRCLK

period. The data stream from input to output is delayed

by an amount equal to the settling time of the filter

(which is the group delay of the filter).

The achievable resolution, the -3 dB bandwidth and the

settling time at the output of the decimation filter (the

output of the ADC), is dependent on the OSR of each

sinc filter and is summarized in Table 5-3.

FIGURE 5-3:

SINC Filter Frequency

Response, OSR = 256, MCLK = 4 MHz,

PRE<1:0> = 00.

FIGURE 5-4:

SINC Filter Frequency

Response, OSR = 4096 (in pink), OSR = 512 (in

blue), MCLK = 4 MHz, PRE<1:0> = 00.

TABLE 5-3:

OVERSAMPLING RATIO AND SINC FILTER SETTLING TIME

OSR<2:0>

OSR3

OSR1

Total OSR

Resolution in Bits

(No Missing Code)

Settling Time

-3 dB Bandwidth

000

96/DMCLK

0.26*DRCLK

001

192/DMCLK

0.26*DRCLK

010

128

384/DMCLK

0.26*DRCLK

011

256

768/DMCLK

0.26*DRCLK

100

512

1536/DMCLK

0.26*DRCLK

101

512

1024

2048/DMCLK

0.37*DRCLK

110

512

2048

3072/DMCLK

0.42*DRCLK

111

512

4096

5120/DMCLK

0.43*DRCLK

-80

-60

-40

-20

-120

-100

100

1000

10000

100000

Input Frequency (Hz)

-100

-80

-60

-40

-20

-160

-140

-120

100

10000

1000000

Input Frequency (Hz)

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