AD9681 datasheet(25/40 Pages) AD

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Data Sheet

AD9681

Rev. A | Page 25 of 40

Figure 51 shows an example of the LVDS output using the

ANSI-644 standard (default) data eye and a time interval error

(TIE) jitter histogram with trace lengths of less than 24 inches

on standard FR-4 material.

Figure 52 shows an example of trace lengths exceeding 24 inches

on standard FR-4 material. Note that the TIE jitter histogram

reflects the decrease of the data eye opening as the edge deviates

from the ideal position.

200ps

250ps

300ps

350ps

400ps

450ps

500ps

500

400

300

200

100

–500

–400

–300

–200

–100

–0.8ns

–0.4ns

0ns

0.4ns

0.8ns

EYE: ALL BITS

ULS: 7000/400354

Figure 51. Data Eye for LVDS Outputs in ANSI-644 Mode with Trace Lengths

of Less Than 24 Inches on Standard FR-4 Material, External 100 Ω Far End

Termination Only

It is the responsibility of the user to determine if the waveforms

meet the timing budget of the design when the trace lengths exceed

24 inches. Additional SPI options allow the user to further increase

the internal termination (increasing the current) of all eight outputs

to drive longer trace lengths, which can be achieved by program-

ming Register 0x15. Although this option produces sharper rise

and fall times on the data edges and is less prone to bit errors, it

also increases the power dissipation of the DRVDD supply.

500

400

300

200

100

–500

–400

–300

–200

–100

–0.8ns

–0.4ns

0ns

0.4ns

–0.8ns

EYE: ALL BITS

ULS: 8000/414024

10k

12k

–800ps –600ps –400ps –200ps

0ps

200ps

400ps

600ps

Figure 52. Data Eye for LVDS Outputs in ANSI-644 Mode with Trace Lengths of

Greater Than 24 Inches on Standard FR-4 Material, External 100 Ω Far End

Termination Only

The default format of the output data is twos complement.

Table 10 shows an example of the output coding format. To change

the output data format to offset binary, see the Memory Map

section, Register 0x14, Bit[0].

Table 10. Digital Output Coding

Input (V)

Condition (V)

Offset Binary Output Mode

Twos Complement Mode

VIN+ − VIN−

<−VREF − 0.5 LSB

0000 0000 0000 0000

1000 0000 0000 0000

VIN+ − VIN−

−VREF

0000 0000 0000 0000

1000 0000 0000 0000

VIN+ − VIN−

0 V

1000 0000 0000 0000

0000 0000 0000 0000

VIN+ − VIN−

+VREF − 1.0 LSB

1111 1111 1111 1100

0111 1111 1111 1100

VIN+ − VIN−

>+VREF − 0.5 LSB

1111 1111 1111 1100

0111 1111 1111 1100

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