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

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). 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 the 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 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 DVDD3 PSRR is

superior to both the AVDD5 and AVDD3 so was not graphed.

Because there are two diodes connected in reverse between AVDD3 and DVDD3 internally, a power-up

sequence is recommended. When there is a delay in power up between these two supplies, the one that lags

could have current sinking through an internal diode before it powers up. The sink current can be large or small

depending on the impedance of the external supply and could damage the device or affect the supply source.

The best power up sequence is one of the following options (regardless of when AVDD5 powers up):

1) Power up both AVDD3 and DVDD3 at the same time (best scenario), OR

2) Keep the voltage difference less than 0.8V between AVDD3 and DVDD3 during the power up (0.8V is not a

hard specification - a smaller delta between supplies is safer).

If the above sequences are not practical then the sink current from the supply needs to be controlled or

protection added externally. The max transient current (on the order of

µsec) for DVDD3 or AVDD3 pin is 500mA

to avoid potential damage to the device or reduce its lifetime.

Values for analog and clock input given in the Absolute Maximum Ratings are valid when the supplies are on.

When the power supplies are off and the clock or analog inputs are still alive, the input voltage and current needs

to be limited to avoid device damage. If the ADC supplies are off, the max/min continuous DC voltage is +/- 0.95

V and max DC current is 20 mA for each input pin (clock or analog), relative to ground.

Figure 49. PSRR versus Supply Injected Frequency

28

Submit Documentation Feedback

Copyright © 2007–2008, Texas Instruments Incorporated

Product Folder Link(s): ADS5474