ADS1294CZXGR datasheet(49/114 Pages) TI2

49 / 114 page

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-60

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-100

-120

-140

Normalized Frequency (f / f

)

0.5

4.5

3.5

2.5

1.5

0.5

1.5

2.5

Normalized Frequency (f / f

)

0.05

0.1

0.15

0.35

0.2

0.25

0.3

½H(f) =

sin

N f

MOD

sin

MOD

½H(z) =

- -N

- -1

ADS1294, ADS1294R, ADS1296, ADS1296R, ADS1298, ADS1298R

www.ti.com

SBAS459K – JANUARY 2010 – REVISED AUGUST 2015

9.3.2.4 Digital Decimation Filter

The digital filter receives the modulator output and decimates the data stream. By adjusting the amount of

filtering, tradeoffs are made between resolution and data rate: filter more for higher resolution, filter less for

higher data rates. Higher data rates are typically used in ECG applications to implement software pace detection

and ac lead-off detection.

The digital filter on each channel consists of a third-order sinc filter. The decimation ratio on the sinc filters is

adjusted by the DR bits in the CONFIG1 register (see Table 16 for details). This setting is a global setting that

affects all channels; therefore, in these devices, all channels operate at the same data rate.

9.3.2.4.1

Sinc Filter Stage (sinx / x)

The sinc filter is a variable-decimation rate, third-order, low-pass filter. Data are supplied to this section of the

filter from the modulator at the rate of fMOD. The sinc filter attenuates the high-frequency noise of the modulator,

then decimates the data stream into parallel data. The decimation rate affects the overall data rate of the

converter.

Equation 4 shows the scaled Z-domain transfer function of the sinc filter.

(4)

The frequency-domain transfer function of the sinc filter is shown in Equation 5.

where

•

N = decimation ratio

(5)

The sinc filter has notches (or zeroes) that occur at the output data rate multiples. At these frequencies, the filter

has infinite attenuation. Figure 52 shows the frequency response of the sinc filter and Figure 53 shows the rolloff

of the sinc filter. With a step change at input, the filter requires 3 × tDR conversion cycles to settle. After a rising

edge of the START pin or completion of the START command, the filter takes tSETTLE periods to give the first

data output. The settling time of the filters at various data rates are discussed in the Start Mode subsection of the

SPI Interface section. Figure 54 and Figure 55 show the filter transfer function to fMOD / 2 and fMOD / 16,

respectively, at different data rates. Figure 56 shows the transfer function extended out to 4 × fMOD. As shown in

the figures, the passband of the ADS129x repeats itself at every fMOD muiltple. Choose input R-C antialiasing

filters for the system that sufficiently attenuate any interference in frequencies around multiples of fMOD.

Figure 52. Sinc Filter Frequency Response

Figure 53. Sinc Filter Roll-Off

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ADS1294CZXGR Datasheet(PDF) 49 Page - Texas Instruments

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