LINEAR DESIGN FEATURES LTMag- V18N02-04 - LTM9002 - Todd Nelson Technical Manual

L DESIGN FEATURES

Complete Dual-Channel Receiver Combines 14-Bit, 125Msps ADCs, Fixed-Gain Amplifiers and Antialias Filters in a Single 11.25mm × 15mm μModule Package by Todd Nelson Introduction Multichannel

Extensive hands-on applications his expertise lies in the RF or digital ADC Applications

experience is a prerequisite for any worlds. Multichannel applications have sev- designer hoping to take full advan- The LTM9002 dual-channel, IF/ eral unique requirements, such as tage of an ADC’s capabilities when baseband receiver harnesses years channel matching in terms of gain, sampling high dynamic range signals of applications design experience phase, DC offset, and channel-to- in multichannel IF-sampling or in I/Q and squeezes it into an easy-to-use channel isolation. Gain and phase baseband communications channels. 11.25mm × 15mm µModule package. errors directly affect the demodula- An intimate knowledge of the ampli- Inside the package is a high perfor- tion of the I and Q channels. And fier output stage and ADC front end mance dual 14-bit ADC sampling up since direct conversion receivers is required to match the impedances, to 125Msps, antialiasing filters, two are typically DC-coupled, DC offset while careful attention to layout is fixed gain differential ADC drivers and limits the dynamic range of the re- required to minimize coupling of a dual auxiliary DAC. By combining ceiver. Multiple-input, multiple-output the digital outputs into the sensitive these components, the LTM9002 elimi- (MIMO) systems depend on multiple analog input. nates the burden of input impedance receiver channels all receiving the In fact, good layout is paramount matching, filter design, gain/phase same signal while detecting the slight to maintaining ADC performance, matching, isolation between channels variations caused by multipath delays yet ever more demanding market and high frequency layout, dramati- so gain and phase errors affect these requirements call for smaller designs cally improving time to market. Even systems as well. Like I/Q receivers, and higher channel density, which in this small package, the LTM9002 diversity receivers require excellent exacerbate layout issues. These guarantees high performance that will isolation between channels because design requirements can challenge enhance many communications and crosstalk appears as noise corruption even the most seasoned engineer if instrumentation applications. and can be more difficult to suppress

VCC = 3V VDD = 3V

VREF 0.5V TO VDD

MAIN 14-BIT

FILTER

RF SAW 125Msps ADC

DAC CLKOUT ADC CLK

DIFFERENTIAL

AMPLIFIERS

MUX OF

DIVERSITY 14-BIT

FILTER

RF SAW 125Msps ADC

Figure 1. LTM9002 used for a Main/Diversity receiver.

Linear Technology Magazine • June 2008