ADS5295IPFP datasheet(82/93 Pages) TI2

82 / 93 page

DIFF

OCM

High

Low

High

OUTP

OUTM

OUT

(1)

External

100-

Load

ADS5295

SBAS595 – DECEMBER 2012

www.ti.com

LVDS OUTPUT DATA AND CLOCK BUFFERS

The equivalent circuit of each LVDS output buffer is shown in Figure 75. After reset, the buffer presents an

output impedance of 100

Ω to match with the external 100-Ω termination.

The VDIFF voltage is nominally 350 mV, resulting in an output swing of ±350 mV with a 100-Ω external

termination. The buffer output impedance behaves in the same way as a source-side series termination. By

absorbing reflections from the receiver end, this impedance helps improve signal integrity.

(1) ROUT = 100 Ω.

Figure 75. LVDS Buffer Equivalent Circuit

OUTPUT DATA FORMAT

Two output data formats are supported: twos complement and offset binary. These formats can be selected by

the BTC_MODE serial interface register bit. In the event of an input voltage overdrive, the digital outputs go to

the appropriate full-scale level. For a positive overload, the 12-bit output data (D[11:0]) is FFFh in offset binary

output format and 7FFh in twos complement output format. For a negative input overload, the output data is 000h

in offset binary output format and 800h in twos complement output format.

BOARD DESIGN CONSIDERATIONS

Grounding

A single ground plane is sufficient to give good performance, provided the analog, digital, and clock sections of

the board are cleanly partitioned. See the EVM User Guide (ADS5295, 8-Channel ADC Evaluation Module,

SLAU442) for details on layout and grounding.

Supply Decoupling

Minimal external decoupling can be used without loss in performance because the ADS5295 already includes

internal decoupling. Note that decoupling capacitors can help filter external power-supply noise; thus, the

optimum number of capacitors would depend on the actual application. The decoupling capacitors should be

placed very close to the converter supply pins.

Exposed Pad

In addition to providing a path for heat dissipation, the pad is also electrically connected to the digital ground

internally. Therefore, the exposed pad must be soldered to the ground plane for best thermal and electrical

performance.

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