ADS1291_14 Datasheet, PDF (25/75 Pages) Texas Instruments – Power, 2-Channel, 24-Bit Analog Front-End for Biopotential Measurements

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10 _{DR[2:0] = 110}
-10
ADS1291
ADS1292
ADS1292R
SBAS502B – DECEMBER 2011 – REVISED SEPTEMBER 2012
DR[2:0] = 000
-30
-50
-70
-90
-110
-130
0 0.5 1 1.5 2 2.5 3 3.5 4
Normalized Frequency (f_IN/f_MOD)
Figure 30. Transfer Function of On-Chip Decimation Filters
Until 4f_MOD for DR[2:0] = 000 and DR[2:0] = 110
REFERENCE
Figure 31 shows a simplified block diagram of the ADS1291, ADS1292, and ADS1292R internal reference. The
reference voltage is generated with respect to AVSS. The VREFN pin must always be connected to AVSS.
1 mF
VCAP1
Bandgap
R₁₍₁₎
2.42 V or
4.033 V
R₃₍₁₎
R₂₍₁₎
VREFP
10 mF
VREFN
0.1 mF
To ADC Reference Inputs
AVSS
(1) For V_REF = 2.42 V: R₁ = 100 kΩ, R₂ = 200 kΩ, and R₃ = 200 kΩ. For V_REF = 4.033 V: R₁ = 84 kΩ, R₂ = 120 kΩ, and R₃ = 280 kΩ.
Figure 31. Internal Reference
The external band-limiting capacitors determine the amount of reference noise contribution. For high-end ECG
systems, the capacitor values should be chosen such that the bandwidth is limited to less than 10 Hz so that the
reference noise does not dominate the system noise. When using a 3-V analog supply, the internal reference
must be set to 2.42 V. In case of a 5-V analog supply, the internal reference can be set to 4.033 V by setting the
VREF_4V bit in the CONFIG2 register.
Alternatively, the internal reference buffer can be powered down and VREFP can be applied externally. Figure 32
shows a typical external reference drive circuitry. Power-down is controlled by the PD_REFBUF bit in the
CONFIG2 register. This power-down is also used to share internal references when two devices are cascaded.
By default the device wakes up in external reference mode.
Copyright © 2011–2012, Texas Instruments Incorporated
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