ADS1294 datasheet(27/88 Pages) TI

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AVDD

0.2

GainV

MAX_DIFF

>CM>AVSS+0.2+

GainV

MAX_DIFF

Max(INP

INN)<

REF

Gain

Full-ScaleRange=

REF

Gain

;

REF

Gain

100

110

120

130

140

150

NormalizedFrequency(Hz)

100

ADS1294, ADS1294R

ADS1296, ADS1296R

ADS1298, ADS1298R

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SBAS459H

– JANUARY 2010 – REVISED OCTOBER 2011

Input Common-Mode Range

The usable input common-mode range of the front end depends on various parameters, including the maximum

differential input signal, supply voltage, PGA gain, etc. This range is described in Equation 2:

where:

VMAX_DIFF = maximum differential signal at the input of the PGA

CM = common-mode range

(2)

For example:

If VDD = 3V, gain = 6, and VMAX_DIFF = 350mV

Then 1.25V

< CM < 1.75V

Input Differential Dynamic Range

The differential (INP

– INN) signal range depends on the analog supply and reference used in the system. This

range is shown in Equation 3.

(3)

The 3V supply, with a reference of 2.4V and a gain of 6 for ECGs, is optimized for power with a differential input

signal of approximately 300mV. For higher dynamic range, a 5V supply with a reference of 4V (set by the

VREF_4V bit of the CONFIG3 register) can be used to increase the differential dynamic range.

ADC

ΔΣ Modulator

Each channel of the ADS129x 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 for High-Resolution

mode and fMOD = fCLK/8 for Low-Power mode. As in the case of any ΔΣ modulator, the noise of the ADS129x is

shaped until fMOD/2, as shown in Figure 29. The on-chip digital decimation filters explained in the next 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 the analog antialiasing filters that

are typically needed with Nyquist ADCs.

Figure 29. Modulator Noise Spectrum Up To 0.5

× fMOD

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