Power Supplies

f − Frequency − MHz

40

50

60

70

80

90

100

G032

AVDD5

fS = 500 MSPS

fIN = None

AVDD3

0.01

0.1

1

100

10

ADS5463

ADS54RF63

www.ti.com ................................................................................................................................................... SLAS515E – NOVEMBER 2006 – REVISED JULY 2009

The ADS5463/ADS54RF63 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

ADS5463/ADS54RF63. However, the PSRR value and the plot shown in Figure 67 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 ADS5463/ADS54RF63 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 ADS5463/ADS54RF63 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):

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

• Keep the voltage difference less than 0.8 V between AVDD3 and DVDD3 during the power up (0.8 V 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 the DVDD3 or AVDD3 pin is 500

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

The values for the analog and clock inputs 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 being actively driven, the input

voltage and current need to be limited to avoid device damage. If the ADC supplies are off, 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 67. PSRR versus Supply Injected Frequency

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Product Folder Link(s): ADS5463 ADS54RF63