AN-1538| Application Note

Interfacing National's DS90CR218A and LM98714

National Semiconductor Application Note 1538 Peter Ho November 2006

1.0 Introduction
Digital image processing systems like color copier, scanner etc. involve moving large amount of data between the sensor controller unit to the image processor across a 60 to 150 cm cable in an EMI efficient fashion. National Semiconductor's DS90CR218A, a LVDS channellink receiver together with LM98714, a 16-Bit, 45 MSPS signal processing AFE (Analog Front End) provide a high performance solution to digitize the CCD/CIS sensor outputs, process it and make it available to the image processor. The LVDS signaling and serialization of the LM98714 output solve the EMI and cable size problems associated with wide, highspeed LVCMOS/LVTTL interfaces operating from 12MHz to 45MHz.

While operating in LVDS mode, the LM98714 serializes the ADC output to 3 data channels with an additional line for the output clock. The DS90CR218A de-serializes the three input LVDS data streams into 21 bits of CMOS/TTL output data. The receiver supports an input clock frequency of 12 to 85 Mhz. This application note examines the issues that system designers may face when interfacing the DS90CR218A and LM98714. It also offers guidance and solutions on solving these issues that will deliver for a reliable and cost effective LVDS data link.

2.0 System Block Diagram

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3.0 Advantages of LVDS
LVDS (Low Voltage Differential Signaling) is a low swing, differential signaling technology which allows high speed data transmission while consuming significantly less power than conventional single ended technologies. In addition, LVDS is less susceptible to common-mode noise, and it also tends to radiate less noise than single-ended signals due to the canceling of magnetic fields. The current-mode driver used in LVDS is also less prone to ringing and switching spikes (switching noise), further reducing noise.
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The current-mode, low swing, low noise nature of LVDS technology means data can be switched very quickly without creating noise and power consumption issues, which are common trade-offs for high-speed data transmission applications. Both the LM98714 and DS90CR218A support LVDS signaling and together enable a low EMI system that is less prone to signal quality issues.

AN-1538

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