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LVDS INTERNAL TERMINATION PROGRAMMABILITY

ADS5281

ADS5282

SBAS397E – DECEMBER 2006 – REVISED JANUARY 2008

All three drive strengths (bit clock, frame clock, and data) are programmed using sets of three bits. Table 4

shows an example of how the drive strength of the bit clock is programmed (the method is similar for the frame

clock and data drive strengths).

ILVDS_LCLK<2>

ILVDS_LCLK<1>

ILVDS_LCLK<0>

0

0

0

3.5mA (default)

0

0

1

2.5mA

0

1

0

1.5mA

0

1

1

0.5mA

1

0

0

7.5mA

1

0

1

6.5mA

1

1

0

5.5mA

1

1

1

4.5mA

(1)

Current settings lower than 1.5mA are not recommended.

ADDRESS

IN HEX

D15

D14

D13

D12

D11

D10

D9

D8

D7

D6

D5

D4

D3

D2

D1

D0

NAME

X

EN_LVDS_TERM

1

X

X

X

TERM_LCLK<2:0>

12

1

X

X

X

TERM_FRAME<2:0>

1

X

X

X

TERM_DAT<2:0>

The LVDS buffers have high-impedance current sources driving the outputs. When driving traces whose

characteristic impedance is not perfectly matched with the termination impedance on the receiver side, there may

be reflections back to the LVDS output pins of the ADS528x 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 5 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 up to ±20% across

temperature and from device to device.

Table 5. Bit Clock Internal Termination

INTERNAL TERMINATION BETWEEN

TERM_LCLK<2>

TERM_LCLK<1>

TERM_LCLK<0>

0

0

0

None

0

0

1

260

0

1

0

150

0

1

1

94

1

0

0

125

1

0

1

80

1

1

0

66

1

1

1

55

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