ADS1291
ADS1292
ADS1292R
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SBAS502A –DECEMBER 2011–REVISED MARCH 2012
QUICK-START GUIDE
PCB LAYOUT
Power Supplies and Grounding
The ADS1291, ADS1292, and ADS1292R have two supplies: AVDD and DVDD. AVDD should be as quiet as
possible. AVDD provides the supply to the charge pump block and has transients at fCLK. It is important to
eliminate noise from AVDD that is non-synchronous with the ADS1291, ADS1292, and ADS1292R operation.
Each ADS1291, ADS1292, and ADS1292R supply should be bypassed with 10-μF and a 0.1-μF solid ceramic
capacitors. It is recommended that placement of the digital circuits (such as the DSP, microcontrollers, and
FPGAs) in the system is done such that the return currents on those devices do not cross the ADS1291,
ADS1292, and ADS1292R analog return path. The ADS1291, ADS1292, and ADS1292R can be powered from
unipolar or bipolar supplies.
The capacitors used for decoupling can be of the surface-mount, low-cost, low-profile multi-layer ceramic type. In
most cases the VCAP1 capacitor can also be a multi-layer ceramic, but in systems where the board is subjected
to high or low frequency vibration, it is recommend that a non-ferroelectric capacitor such as a tantalum or class
1 capacitor (for example, C0G or NPO) be installed. EIA class 2 and class 3 dielectrics (such as X7R, X5R, X8R,
and such) are ferroelectric. The piezoelectric property of these capacitors can appear as electrical noise coming
from the capacitor. When using internal reference, noise on the VCAP1 node results in performance degradation.
Connecting the Device to Unipolar (+3 V or +1.8 V) Supplies
Figure 60 illustrates the ADS1291, ADS1292, and ADS1292R connected to a unipolar supply. In this example,
the analog supply (AVDD) is referenced to analog ground (AVSS) and the digital supply (DVDD) is referenced to
digital ground (DGND).
+3 V
+1.8 V
0.1 mF
1 mF
1 mF
0.1 mF
AVDD
DVDD
VREFP
VREFN
0.1 mF
10 mF
PGA1N
(1)
4.7 nF
VCAP1
VCAP2
PGA1P
Device
PGA2N
PGA2P
4.7 nF
1 mF
1 mF
AVSS DGND
NOTE: Place the capacitors for supply, reference, VCAP1, and VCAP2 as close to the package as possible.
(1) When using the ADS1292R and the channel 1 respiration function, this capacitor must be 47 nF.
Figure 60. Single-Supply Operation
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