11.1.1.1 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 f_CLK. 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.