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Jedec spec enables more accurate serial ADCs

Posted: 16 Sep 2008     Print Version  Bookmark and Share

Keywords:8B/10B encoding  frame clock  data clock  transmission-line-pair 

Digital designers may be all too familiar with the challenges of routing high-speed digital lines between ADCs and logic devices. Great care must be taken to ensure sufficient spacing between the high-speed traces as well as to make sure the digital signals do not cross analogue boundaries. Poor layout will result in the digital switching noise that feeds back into the ADC's analogue inputs, degrading overall system performance.

With board real estate at a premium and FPGA pins a valuable commodity, the advantages of serial-data converter interfaces over parallel are clear. Typical serial communication of high-speed digital data used in ADCs requires three pairs of transmission lines for LVDS, with one pair for the data itself.

To accurately collect this data, a data clock is required. To establish data-sample boundaries, a framing clock is also needed for frame alignment. For high-speed ADCs, aligning the data clock, the frame clock and the data usually requires a delay lock loop in both the transmitter and receiver to align the data clock properly. This alignment becomes very difficult at GHz speeds. Ultimately, this six-wire method of serial transmission is not generally done above 1.2GHz, limiting either the speed of an ADC or its resolution.

Originally invented in the 1980s by IBM, 8B/10B encoding eliminates the need for a frame clock and a data clock, which makes single transmission-line-pair communications possible at frequencies above 2GHz, as shown in Figure 1.

The unique features of 8B/10B encoding allow the data clock to be embedded in the data itself, and the framing to be maintained with comma characters through initial frame synchronisation.

Only recently has a specification been developed that defines the protocol and electrical characteristics required to standardise the implementation of this coded interface for data converters. Jedec specification JESD204 has enabled a new generation of faster and more accurate serial ADCs, such as Linear Technology Corp.'s LTC2274, 16bit, 105MSps ADC.

IBM's 8B/10B encoding eliminates the need for a frame clock and a data clock, making single transmission-line-pair communications possible at frequencies above 2GHz.

Benefits of new encoding
The 8B/10B encoded data is friendly to clock-recovery circuits because it is run-length limited. It also accommodates AC coupling because it is DC-balanced. This encoding involves transforming an 8bit octet into a 10bit code group. In each code group, the difference between the number of ones and zeros is never more than two. By monitoring the number of ones and zeros in consecutive code groups, a running disparity is calculated.

The transmitter and receiver use this disparity to encode and decode the data. For each input octet, there are two possible 10bit output codes. The selection of the code to be transmitted is dependent on the running disparity, and is intended to keep the average number of ones and zeros equal. This property of 8B/10B encoding ensures the DC offset of the signal to be zero.

Once the data is encoded, it is serialised and transmitted, beginning with bit zero of the first code group. The JESD204 specification requires that the first code group corresponds to the most significant byte of data. The second code group relates to the least significant byte of data. Combined, these two code groups make up one frame of data, which constitutes one sample from a 16bit ADC (Figure 2).

The JESD204 specification requires that the first code group corresponds to the most significant byte of data. The second code group relates to the least significant byte of data.

In-synch frame systems
Although the clock may be recovered from the data stream with a PLL, it is still necessary for the receiver to determine the location of the frame boundaries. The JESD204 standard defines a synchronisation process to establish initial frame alignment between the transmitter and the receiver. When the receiver is in need of synchronisation, it will request this action by activating the synchronisation input to the ADC. The ADC will then transmit a series of predetermined 8B/10B control symbols, also referred to as comma characters, so the receiver may identify the frame boundaries.

The JESD204 specification designates the K28.5 control symbol as the comma to be used for initial synchronisation. When a synchronisation request is received by the LTC2274, a series of K28.5 comma characters are transmitted until the receiver gets at least four valid K28.5 code groups, after which the receiver will de-assert the synchronisation request signal. Upon deactivation of the synchronisation request, the LTC2274 will continue to transmit the synchronisation preamble until the end of the frame.

At the start of the next frame, the LTC2274 will transmit data characters. This ensures that the data always begins in the same fashion, with the first code group associated with the most significant octet and the second code group associated with the least significant octet (Figure 2). By using these comma characters to align the data, the need for a frame clock is eliminated. By using synchronisation and run length limited 8B/10B encoding, serial transmission without using a bit clock or a frame clock becomes possible.

Another advantage of using an 8B/10B encoding is that it is DC-balanced. This is because the running disparity is used to maintain an equal number of ones and zeros over two alternate code groups, so the DC average of the signal is statistically zero. This allows single pair transmission lines to be used with transformers, optic couplers, DC blocking capacitors and other high pass devices.

Data scrambling
The JESD204 specification also outlines an optional scrambler that scrambles the data before it is encoded for transmission. This helps to avoid unwanted spectral peaks that can occur with high-speed serial transmission. By scrambling the data, the encoded octets are data independent, which will eliminate spectral artifacts that may occur with certain data dependent signals.

The data is scrambled using a 1 + x14 + x15 polynomial. This pseudorandom pattern repeats itself every 215—1 cycles. The nature of this polynomial and scrambling scheme is that it can be used with a self-synchronous descrambler. The FPGA must have a descrambling algorithm to descramble the data after the 8B/10B decoder. This feature is designed into the LTC2274 as an option that can improve performance in certain situations.

Monitoring alignment
It may be helpful to check synchronisation of data periodically. If the receiver requests a synchronisation pattern from the transmitter in the normal fashion, there will be a data loss associated with the transmitter sending a synchronisation preamble. To prevent loss of data, the JESD204 specification defines an alternative method of frame alignment, available in the LTC2274 through its frame alignment monitoring mode. This allows synchronisation to be checked, without losing data or asserting the synchronisation request input on the ADC. The JESD204 standard defines two methods of frame alignment monitoring (Figure 3).

These are the modes of frame alignment monitoring for data resynchronisation.

The first frame alignment mode occurs when the data is not being scrambled. When the second code group in the current frame is equal to the second code group in the previous frame, the current code group will be replaced by K28.7. It is then the responsibility of the receiver to replace the K28.7 octet with the octet from the previous sample. If a third of the second data octet is equal to the previous two, the actual octet will be transmitted. This mode of frame alignment is highly dependent on the data, and is not guaranteed to occur within any length of time.

The second frame alignment mode occurs when the data octets are scrambled prior to encoding. Whenever the second code group of any frame is equal to D28.7, it will be replaced by K28.7. The receiver will then need to replace the K28.7 with the correct data character, D28.7. Since the effects of the scrambler are random, this method of frame alignment is less data dependent. Statistically, a K28.7 should occur one in every 256 frames.

In either mode, it can be determined that there is an error if the control character K28.7 is found in the first octet. If this occurs, the receiver can either realign the frame or activate the synchronisation request signal to resynchronise with the transmitter. When realigning the frame without initiating a synchronisation request, the K28.7 should always appear in the second code group. If it is found in any other position, the following code group will signify the beginning of the first code group of the next frame. This feature allows for data resynchronisation without loss of data from the ADC.

If the received data is shifted by one of more bits, this will result in invalid 8B/10B code groups. The receiver should then reassert the frame synchronisation request signal which will cause the transmitter to send a stream of comma characters. If the data shifts by a whole code group, frame alignment monitoring can be used to detect this shift. A shift in the data of a whole code group will result in data corruption that DSP should be able to detect.

Performance
Using 8B/10B encoding for high-speed serial-data transfer, ADCs can now operate at higher sampling rates, and with greater resolutions. The LTC2274 from Linear is a 105MSps, 16bit ADC that uses 8B/10B encoding to transmit its 16bit output word serially to the receiver with a data output rate of 2.1Gbps (20 encoded bits at 105MSps).

The Jedec serial interface is compatible with many FPGA high-speed interfaces including Rocket IO, Stratix II GX I/O and ECP2M I/O. Reference designs using the LTC2274 are already available from many FPGA manufacturers.

One of the biggest challenges in the design of these new converters was achieving the high AC specifications while integrating the high-speed serial interface on the same die. The LTC2274 achieves SNR performance of 77.6dBFS (full scale) and spurious free dynamic range (SFDR) of 100dB at base band (Figure 4).

The LTC2274 uses 8B/10B in transmitting its 16bit output to the receiver (20 encoded bits at 105MSps).

These AC specifications enable low-level signals to be resolved in the presence of large interferers or blockers, which are especially critical for multi-channel receiver applications. Ultra low jitter of 80fs RMS enables undersampling of input frequencies up to 500MHz while maintaining good noise performance, allowing the ADC to sample closer to the antenna.

With the LTC2274, serial test patterns can be produced to facilitate testing of the serial interface and verify BER. This feature is invaluable for debugging the interface, but is not required by the Jedec specification.

ADCs such as the LTC2274 with its serial interface make excellent sense for cost-sensitive applications, where FPGA pin count dominates the cost of the design. High-performance communications equipment such as basestation receivers and digital predistortion transmitters can achieve significant cost savings using the dedicated Serdes port on the FPGA, while benefiting from the high SNR and SFDR performance for multi-carrier receiver designs. Spectrum analysers can improve overall system performance with the capability to isolate the digital and analogue circuitry. Multichannel applications such as ATE and medical imaging will benefit from the reduced pin count for ease of routing and additional space savings.

The JESD204 standard for data converters makes it possible for high-speed, high-resolution ADCs to transmit high-speed data across a single pair of transmission lines. By using a run-length-limited signal to recover the data clock, and comma characters for initial frame synchronisation, the difficulties of standard serial transmission are mitigated. As a result, the 8B/10B signal has a DC offset of zero, and can be transmitted through any high pass device, such as DC blocking capacitors. The JESD204 standard also allows for frame alignment without loss of data through frame alignment monitoring. It also provides a means of reducing nonharmonic spurs by using a scrambling polynomial.

- Clarence Mayott
Applications Engineer, Linear Technology Corp.





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