The latest trend in digital output interfaces for converters is to use a serialized interface that uses current mode logic (CML) output drivers. Typically converters with higher resolutions (≥14 bits), higher speeds (≥200 Msps), and the desire for smaller packages with reduced power use these types of drivers. The CML output driver is employed in JESD204 interfaces that are being used on the latest converters.
Utilizing CML drivers with serialized JESD204 interfaces allows data rates on the converters outputs to go up to 12 Gbps (with the current revision of the specification JESD204B). In addition, the number of output pins required is dramatically reduced. Routing a separate clock signal is no longer necessary since the clock becomes embedded in the 8b/10b encoded data stream.
The number of data output pins is also reduced, requiring a minimum of two. As the resolution, speed, and channel count of the converter increase, the number of data-output pins may be scaled to account for the greater throughput required. However, since the interface employed with CML drivers is typically serial, the increase in the number of pins required is much smaller than that compared with CMOS or LVDS. (The data transmitted in CMOS or LVDS is parallel, which requires a much great number of pins.)
Table 1 shows the pin counts for the three different interfaces using an 80 Msps converter with various channel counts and bit resolutions. The data assumes a synchronization clock for each channel’s data in the case of the CMOS and LVDS outputs, and a maximum data rate of 3.2 Gbps for JESD204 data transfer using the CML outputs. The reasons for the progression to CML become obvious when looking at this table and the dramatic reduction in pin count that can be achieved.
Table 1: Pin-count comparison – 80 Msps ADC
Since CML drivers are used in serialized data interfaces, the number of pins required is much smaller. Figure 3 shows a typical CML driver used for converters with JESD204 or similar data outputs. The figure gives a generalization of the typical architecture of a CML driver. It shows the optional source termination resistor and the common mode voltage. The inputs to the circuit drive the switches to the current sources which drive the appropriate logic value to the two output terminals.
Figure 3: Typical CML output driver
A CML driver is similar to the LVDS driver in that it operates in a constant-current mode. This also gives the CML driver an advantage in terms of power consumption. Operating in a constant-current mode requires fewer output pins and the total power consumption is reduced.
As with LVDS, a load termination is required as well as controlled-impedance transmission lines having a single-ended impedance of 50 ? and a differential impedance of 100 ?. The driver itself may also have terminations as shown in Figure 3 to help with any signal reflections due to the sensitivity with such high-bandwidth signals.
In converters following the JESD204 standard, there are different specifications for the differential and common-mode voltage levels, depending upon the speed of operation. Operating at speeds up to 6.375 Gbps, the differential voltage level is nominally 800 mV while the common mode is approximately 1.0 V.
When operating above speeds of 6.375 Gbps but less than 12.5 Gbps, the differential voltage level is specified at 400 mV, while the common mode is again approximately 1.0 V. As converter speed and resolution increase, CML outputs look to be the desired driver type to deliver the speeds necessary to keep pace with technology demands placed on converters for their various applications.
Digital timing: things to look out for
Each of the digital-output driver types has timing relationships which need close attention. Since there are multiple data outputs with CMOS and LVDS, this includes the routing paths of the signals in order to minimize skew. If there is too large of a difference, then proper timing at the receiver may not be achieved.
In addition, there is a clock signal that needs to be routed and aligned with the data outputs. Careful attention must be given to the routing paths between the clock output and the data outputs also to ensure that the skew is not too large.
In the case of CML in the JESD204 interface, attention must also be paid to the routing paths between the digital outputs. There are significantly fewer data outputs to manage, so this task does become easier, but it cannot be neglected altogether. There is no worry in this case with regards to timing skew between the data outputs and the clock output, since the clock is embedded in the data. However, attention must be given to an adequate clock and data recovery (CDR) circuit in the receiver.
In addition to the skew, the setup and hold times with CMOS and LVDS must also be watched carefully. The data outputs must be driven to their appropriate logic state in sufficient time before the edge transition of the clock, and must be maintained in that logic state for a sufficient time after the edge transition of the clock. This can be affected by the skew between the data outputs and the clock outputs, so it is important to maintain good timing relationships.
LVDS has the advantage over CMOS due to the lower signal swings and differential signaling. The LVDS output driver does not have to drive such a large signal to many different outputs and does not draw a large amount of current from the power supply when switching logic states as the CMOS driver would. This makes it less likely for there to be an issue delivering a change in logic state.
If there were many CMOS drivers switching simultaneously, the power-supply voltage could get pulled down and introduce issues driving the right logic values to the receiver. The LVDS drivers would maintain a constant level of current such that this particular issue would not arise. In addition, the LVDS drivers are inherently more immune to common-mode noise due to its use of differential signaling.
The CML drivers have similar benefits to LVDS. These drivers also have a constant level of current, but unlike LVDS, a much smaller number are required due to the serialization of the data. In addition, the CML drivers also offer immunity to common-mode noise since they also use differential signaling.
However, the disadvantage to LVDS and CML is that the current is constant so, even at lower sample rates, the power consumption can still be significant. The advantage over CMOS for converters with higher speeds and resolutions is where power and pin count is significantly reduced when using LVDS or CML.
As converter technology has progressed with increased speeds and resolutions, the digital output drivers have adapted and evolved to meet the requirements necessary transmit data. CML outputs are becoming more popular as the digital output interfaces in converters transition to serialized data transmission.
However, CMOS and LVDS digital outputs are still being used today in current designs. There are applications where each type of digital output is best suited and makes the most sense to use. Each type of output comes with challenges and design considerations and each type of output has its advantages.
In converters with sampling speeds less than 200 Msps, CMOS is still an appropriate technology to employ. When sampling speeds increase above 200 Msps, LVDS becomes a more viable option in many applications as compared to CMOS. To further increase efficiency and reduce power and package size, CML drivers can be employed with a serialized data interface such as JESD204.
References
•Application Note AN-586, Analog Devices, 2002, Cindy Bloomingdale and Gary Hendrickson. www.analog.com.
•JEDEC Standard JESD204 (April 2006), JEDEC Standard JESD204A (April 2008), and JEDEC Standard JESD204B (July 2011), JEDEC Solid State Technology Association, www.jedec.org.
About the author
Jonathan Harris is a product applications engineer, High-Speed Converter Group, Analog Devices, Inc. (Greensboro, NC). He has over seven years of experience as an applications engineer supporting products in the RF industry. Jonathan received his MSEE from Auburn University and his BSEE from UNC-Charlotte. He can be reached at jonathan.harris@analog.com.
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