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Power Supplies

Frequency

Hz

-

0

-10

-20

-30

-40

-50

-60

-70

-120

100k

1M

10M

100M

1G

-80

-90

-100

-110

f =400MSPS

S

AVDD5

AVDD3

DVDD3

ADS5474

SLAS525 – JULY 2007

The ADS5474 uses three power supplies. For the analog portion of the design, a 5-V and 3.3-V supply (AVDD5

and AVDD3) are used, while the digital portion uses a 3.3-V supply (DVDD3). All the ground pins are marked as

GND, although analog and digital grounds are not tied together inside the package. The use of low-noise power

supplies with adequate decoupling is recommended. Linear supplies are preferred to switched supplies;

switched supplies tend to generate more noise components that can be coupled to the ADS5474. However, the

PSRR value and plot shown in Figure 49 were obtained without bulk supply decoupling capacitors. When bulk

(0.1

μF) decoupling capacitors are used, the board-level PSRR is much higher than the stated value for the

ADC. The user may be able to supply power to the device with a less-than-ideal supply and still achieve very

good performance. It is not possible to make a single recommendation for every type of supply and level of

decoupling for all systems. If the noise characteristics of the available supplies are understood, a study of the

PSRR data for the ADS5474 may provide the user with enough information to select noisy supplies if the

performance is still acceptable within the frequency range of interest. The power consumption of the ADS5474

does not change substantially over clock rate or input frequency as a result of the architecture and process. The

total maximum ensured power supersedes the summation of the maximum individual supply currents.

Figure 49. PSRR versus Supply Injected Frequency

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