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Linear unveils SiP signal chain receiver modules

Posted: 05 Feb 2008     Print Version  Bookmark and Share

Keywords:SiP signal chain receiver modules  ADC  FPGAs 


Linear Technology has introduced the LTM9001, the first in a series of System-in-Package (SiP) signal chain receiver modules leveraging Linear's µModule packaging technology.

The new family of integrated receiver subsystems is aimed to bridge the expertise gap between the RF world and digital domain to provide ease of use and shortened time to market. The LTM9001 is a semi-customisable IF/baseband receiver subsystem that includes a 16bit ADC sampling up to 160MSps, an anti-aliasing filter and fixed-gain differential ADC driver.

Sampling high dynamic range signals in RF applications requires extensive experience to maximise the full capabilities of the ADC. An intimate knowledge of the amplifier output stage and ADC front-end is required to match the impedance, while careful attention to layout is required to minimise coupling of the digital outputs into the sensitive analogue input. Degraded ADC performance in many cases can be attributed to bad layout. These considerations can challenge the engineer whose expertise lies in the RF or digital worlds.

Shorter design time
LTM9001 transfers this burden of component selection, input impedance matching, filter design and layout from designer to device, shortening overall design time, system troubleshooting and ultimately improving time to market.

The beauty of the LTM9001 lies in its semi-customisation (customisation requires minimum order quantity). Using pin-compatible product families, the LTM9001 can be configured for various sampling rates and the differential ADC driver can be substituted for fixed gain versions ranging from 8dB up to 26dB. The anti-aliasing filters can also be configured as low-pass or bandpass filter versions, accepting input frequencies as high as 300MHz.

The LTM9001 is packaged in a space-saving 11.25-by-11.25mm LGA package, utilising a multilayer substrate that shields sensitive analogue lines from the digital traces. The LGA pads are positioned on the bottom of the package so that input and clock signals are separated from the digital outputs, simplifying the layout for the designer. Bypass capacitance is placed inside the module and close to the die, providing a space, cost and performance advantage over traditional packaging. With no external capacitance required, the LTM9001 consumes approximately half the space of the discrete implementation.

Simple and powerful
In concept, there are three elements to the LTM9001: a high-speed ADC, an anti-alias filter and a fixed-gain ADC driver. In practice, it is difficult to drive an ADC of this level of performance. The high speed and low noise of the ADC require special care, a wealth of experience and in-depth knowledge of the amplifier output stage and the ADC front-end. For most customers, the disciplines required for the engineering on either side of the ADC, namely RF/IF design and digital design, do not include mastery of the art of ADC interface design.

Even bypass capacitors, which are integrated in the LTM9001, can have tremendous impact on the system performance. Inside the part, the capacitors sit close to the ADC supply and reference nodes, minimising the distance to ground and thus, maximising performance. The LTM9001 is a black box that solves all of these problems, says Linear Technology.

LTM9001 requires no external components. An equivalent implementation would require over 30 components and take twice the board area. In the application, the IF signal chain is usually designed as a differential signal path with 200Ω impedance; the inputs to the LTM9001 are differential with 200Ω impedance. The nuances of interfacing the RF/IF path to an ADC are all taken care of inside the µModule receiver. The clock input includes a 100Ω termination, simplifying the interface to a differential clock distribution chain. The digital outputs mate directly to an FPGA with either differential LVDS pairs or CMOS, which also includes the 50Ω termination. Finally, the locations of the pads on the LGA package guide the user to an optimum layout. The analogue inputs and clock inputs are surrounded by ground pads and there is a solid row of ground pads separating the inputs from the digital outputs—leading the user to a well-isolated, low noise design.

Target markets are similar to those of Linear's discrete ADCs: high performance, wireless base stations, specialised communications equipment, satellite communications, communications test equipment, spectrum analysers and DAQ systems.

The LTM9001 is available in production volumes today and is priced at Rs.3,237.23 ($82) each in 1,000-piece quantities.

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