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AD9683

Data Sheet

Rev. 0 | Page 24 of 44

DIGITAL OUTPUTS

JESD204B TRANSMIT TOP LEVEL DESCRIPTION

The AD9683 digital output uses the JEDEC Standard No.

JESD204B, Serial Interface for Data Converters. JESD204B is a

protocol to link the AD9683 to a digital processing device over a

serial interface of up to 5 Gbps link speeds. The benefits of the

JESD204B interface include a reduction in required board area

for data interface routing and the enabling of smaller packages

for converter and logic devices. The AD9683 supports single

lane interfaces.

JESD204B Overview

The JESD204B data transmit block assembles the parallel data from

the ADC into frames and uses 8B/10B encoding as well as optional

scrambling to form serial output data. Lane synchronization is

supported using special characters during the initial establishment

of the link, and additional synchronization is embedded in the

data stream thereafter. A matching external receiver is required

to lock onto the serial data stream and recover the data and clock.

For additional details on the JESD204B interface, refer to the

JESD204B standard.

The AD9683 JESD204B transmit block maps the output of the

ADC over a single link. The link is configured to use a single

pair of serial differential outputs that is called a lane. The

JESD204B specification refers to a number of parameters to

define the link, and these parameters must match between the

JESD204B transmitter (AD9683 output) and receiver.

The JESD204B link is described according to the following

parameters:

•

S = samples transmitted per single converter per frame

cycle (AD9683 value = 1)

•

M = number of converters per converter device

(AD9683 value = 1)

•

L = number of lanes per converter device

(AD9683 value = 1)

•

N = converter resolution (AD9683 value = 14)

•

N’ = total number of bits per sample (AD9683 value = 16)

•

CF = number of control words per frame clock cycle per

converter device (AD9683 value = 0)

•

CS = number of control bits/conversion sample

(configurable on the AD9683 up to two bits)

•

K = number of frames per multiframe (configurable on

the AD9683)

•

HD = high density mode (AD9683 value = 0)

•

F = octets per frame (AD9683 value = 2)

•

C = control bit (overrange, overflow, underflow; available

on the AD9683)

•

T = tail bit (available on the AD9683)

•

SCR = scrambler enable/disable (configurable on the AD9683)

•

FCHK = checksum for the JESD204B parameters

(automatically calculated and stored in register map)

Figure 59 shows a simplified block diagram of the AD9683

JESD204B link. The AD9683 uses one converter and one lane.

The converter data is output to SERDOUT0+/SERDOUT0−.

By default, in the AD9683, the 14-bit converter word is divided

into two octets (eight bits of data). Bit 0 (MSB) through Bit 7 are

in the first octet, and the second octet contains Bit 8 through Bit 13

(LSB) and two tail bits. The tail bits can be configured as zeros, a

pseudorandom number sequence, or control bits indicating

overrange, underrange, or valid data conditions.

The two resulting octets can be scrambled. Scrambling is

optional; however, it is available to avoid spectral peaks when

transmitting similar digital data patterns. The scrambler uses a

self synchronizing, polynomial-based algorithm defined by the

1 + x14 + x15 equation. The descrambler in the receiver should be

a self-synchronizing version of the scrambler polynomial.

The two octets are then encoded with an 8B/10B encoder. The

8B/10B encoder works by taking eight bits of data (an octet) and

encoding them into a 10-bit symbol. Figure 60 shows how the

14-bit data is taken from the ADC, the tail bits are added, the two

octets are scrambled, and how the octets are encoded into two

10-bit symbols. Figure 60 illustrates the default data format.

At the data link layer, in addition to the 8B/10B encoding, the

character replacement is used to allow the receiver to monitor

frame alignment. The character replacement process occurs on the

frame and multiframe boundaries, and implementation depends

on which boundary is occurring, and if scrambling is enabled.

If scrambling is disabled, the following applies:

•

If the last scrambled octet of the last frame of the multiframe

equals the last octet of the previous frame, the transmitter

replaces the last octet with the control character /A/ =

/K28.3/.

•

On other frames within the multiframe, if the last octet in

the frame equals the last octet of the previous frame, the

transmitter replaces the last octet with the control

character /F/ = /K28.7/.

If scrambling is enabled, the following applies:

•

If the last octet of the last frame of the multiframe equals

0x7C, the transmitter replaces the last octet with the

control character /A/ = /K28.3/.

•

On other frames within the multiframe, if the last octet

equals 0xFC, the transmitter replaces the last octet with the

control character /F/ = /K28.7/.

Refer to JEDEC Standard No. 204B, July 2011 for additional

information about the JESD204B interface. Section 5.1 covers

the transport layer and data format details and Section 5.2

covers scrambling and descrambling.

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