ADS1292 datasheet(23/69 Pages) TI

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Normalized Frequency (fIN/fMOD)

G001

H(z)

H(f) =

sin

N f

MOD

sin

MOD

ADS1291

ADS1292

ADS1292R

www.ti.com

SBAS502A – DECEMBER 2011 – REVISED MARCH 2012

ADC

ΔΣ Modulator

Each channel of the ADS1291, ADS1292, and ADS1292R has a 24-bit

ΔΣ ADC. This converter uses a second-

order modulator optimized for low-power applications. The modulator samples the input signal at the rate of fMOD

= fCLK/4 or fCLK/16, as determined by the CLK_DIV bit. In both cases, the sampling clock has a typical value of

128 kHz. As in the case of any

ΔΣ modulator, the ADS1291, ADS1292, and ADS1292R noise is shaped until

fMOD/2, as shown in Figure 25. The on-chip digital decimation filters explained in the Digital Decimation Filter

section can be used to filter out the noise at higher frequencies. These on-chip decimation filters also provide

antialias filtering. This feature of the

ΔΣ converters drastically reduces the complexity of analog antialiasing filters

that are typically needed with nyquist ADCs.

Figure 25. Power Spectral Density (PSD) of a

ΔΣ Modulator (4-Bit Quantizer)

DIGITAL DECIMATION FILTER

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

filtering, tradeoffs can be 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 for 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 can

be adjusted by the DR bits in the CONFIG1 register (see the Register Map section for details). This setting is a

global setting that affects all channels and, therefore, in a device all channels operate at the same data rate.

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

(8)

The frequency domain transfer function of the sinc filter is shown in Equation 9.

where:

N = decimation ratio

(9)

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