Datasheet

−160
−150
−140
−130
−120
−110
−100
−90
−80
−70
−60
−50
−40
−30
−20
−10
0
0.001 0.01 0.1 1
Normalized Frequency (f
IN
/f
MOD
)
Power Spectral Density (dB)
G001
Max (INP INN) <-
V
REF
Gain
Full-Scale Range =
±V
REF
Gain
; =
2 V
REF
Gain
AVDD 0.2- -
GainV
MAX_DIFF
2
>CM>AVSS+0.2+
GainV
MAX_DIFF
2
ADS1299
SBAS499A –JULY 2012–REVISED AUGUST 2012 www.ti.com
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, and so forth. This range is described in Equation 2:
where:
V
MAX_DIFF
= maximum differential signal at the PGA input
CM = common-mode range (2)
For example:
If V
DD
= 5 V, gain = 12, and V
MAX_DIFF
= 350 mV
Then 2.3 V < CM < 2.7 V
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 5-V supply, with a reference of 4.5 V and a gain of 12 for EEGs, is optimized for power with a differential
input signal of approximately 300 mV.
ADC ΔΣ Modulator
Each ADS1299 channel has a 24-bit, ΔΣ ADC. This converter uses a second-order modulator optimized for low-
noise applications. The modulator samples the input signal at the rate of (f
MOD
= f
CLK
/ 2). As in the case of any
ΔΣ modulator, the ADS1299 noise is shaped until f
MOD
/ 2, as shown in Figure 24. 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 ΔΣ converter feature drastically reduces the complexity of
the analog antialiasing filters typically required with nyquist ADCs.
Figure 24. Modulator Noise Spectrum Up To 0.5 × f
MOD
20 Submit Documentation Feedback Copyright © 2012, Texas Instruments Incorporated
Product Folder Link(s): ADS1299