AFE5804 datasheet(39/60 Pages) TI

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LVDS INTERNAL TERMINATION PROGRAMMING

LOW-FREQUENCY NOISE SUPPRESSION MODE

AFE5804

www.ti.com ................................................................................................................................................ SBOS442A – JUNE 2008 – REVISED SEPTEMBER 2008

ADDRESS

IN HEX

D15

D14

D13

D12

D11

D10

NAME

EN_LVDS_TERM

TERM_LCLK<2:0>

TERM_FRAME<2:0>

TERM_DAT<2:0>

The LVDS buffers have high-impedance current sources that drive the outputs. When driving traces with

characteristic impedances that are not perfectly matched with the termination impedance on the receiver side,

there may be reflections back to the LVDS output pins of the AFE5804 that cause degraded signal integrity. By

enabling an internal termination (between the positive and negative outputs) for the LVDS buffers, the signal

integrity can be significantly improved in such scenarios. To set the internal termination mode, the

EN_LVDS_TERM bit should be set to '1'. Once this bit is set, the internal termination values for the bit clock,

frame clock, and data buffers can be independently programmed using sets of three bits. Table 15 shows an

example of how the internal termination of the LVDS buffer driving the bit clock is programmed (the method is

similar for the frame clock and data drive strengths). These termination values are only typical values and can

vary by several percentages across temperature and from device to device.

Table 15. Bit Clock Internal Termination

INTERNAL TERMINATION BETWEEN

TERM_LCLK<2>

TERM_LCLK<1>

TERM_LCLK<0>

LCLKP AND LCLKM (

Ω)

None

260

150

125

ADDRESS

IN HEX

D15

D14

D13

D12

D11

D10

NAME

LFNS_CH<1:4>

LFNS_CH<8:5>

The low-frequency noise suppression mode is especially useful in applications where good noise performance is

desired in the frequency band of 0MHz to 1MHz (around dc). Setting this mode shifts the low-frequency noise of

the AFE5804 to approximately fS/2, thereby moving the noise floor around dc to a much lower value.

LFNS_CH<8:1> enables this mode individually for each channel.

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