Digital Outputs

ADS5481

ADS5482, ADS5483

The ADC provides eight LVDS-compatible, offset binary, DDR data outputs (2 bits per LVDS output driver) and a

data-ready LVDS signal (DRY). It is recommended to use the DRY signal to capture the output data of the

ADS548x (use as a Clock Output). DRY is source-synchronous to the DATA outputs and operates at the same

frequency, creating a full-rate DDR interface that updates data on both the rising and falling edges of DRY. It is

recommended that the capacitive loading on the digital outputs be minimized. Higher capacitance shortens the

data-valid timing window. The values given for timing (see Figure 1) were obtained with a 5-pF parasitic board

capacitance to ground on each LVDS line. When setting the time relationship between DRY and DATA at the

receiving device, it is generally recommended that setup time be maximized, but this partially depends on the

setup and hold times of the device receiving the digital data. Since DRY and DATA are coincident, it will likely be

necessary to delay either DRY such that DATA setup time is maximized.

The LVDS outputs all require an external 100-

Ω load between each output pair in order to meet the expected

LVDS voltage levels. For long trace lengths, it may be necessary to place a 100-

Ω load on each digital output as

close to the ADS548x as possible and another 100-

Ω differential load at the end of the LVDS transmission line to

terminate the transmission line and avoid signal reflections. The effective load in this case reduces the LVDS

voltage levels by half. The current of all LVDS drivers is set externally with a resistor connected between the

LVDSB (LVDS Bias) pin and ground. Normal LVDS current is 3.5mA per LVDS pair, set with a 10k

Ω external

resistor. For systems with excessive load capacitance on the LVDS lines, reducing the resistor value in order to

increase the LVDS Bias current is allowed to create a stronger LVDS drive capability. For systems with short

traces and minimal loading, increasing the resistor in order to decrease the LVDS current is allowable in order to

save power. Table 3 provides a sampling of LVDSB resistor values should deviation from the recommended

LVDS output current of 3.5mA be considered. It is not recommended to exceed the range listed in the table. If

the LVDS bias current is adjusted, the differential load resistance should also be adjusted to maintain voltage

levels within the specification for the LVDS outputs. The signal integrity of the LVDS lines on the board layout

should be scrutinized to ensure proper LVDS signal integrity exists.

Table 3. Setting the LVDS Current Drive

LVDSB RESISTOR TO GND,

Ω

LVDS NOMINAL CURRENT, mA

6k

5.6

8k

4.3

10k (value for normal recommended operation)

3.5

12k

2.8

14k

2.3

16k

2.0

18k

1.7

20k

1.5

Copyright © 2008, Texas Instruments Incorporated

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Product Folder Link(s): ADS5481 ADS5482, ADS5483