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Converter smooths the way to direct IF sampling to 500 MHz

A new 80 Msps, 14-bit analog to digital converter (ADC) from Linear Technology provides designers of cellular basestations with a dramatic improvement in dynamic performance. The LTC1750 greatly simplifies the design and reduces the cost of cellular basestations by eliminating the second intermediate frequency (IF) down-conversion stage. The device provides wide input bandwidth and excellent dynamic performance for direct IF digitizing applications. The LTC1750 undersamples up to 500MHz input frequencies and delivers 84dB spurious free dynamic range (SFDR) with 140MHz inputs and 74dB SFDR with 350MHz inputs. The device's wide bandwidth and excellent AC performance are ideal for use in cellular basestations and broadband software radios, where it can directly digitize the first IF and eliminate the second IF down-conversion stage.

The LTC1750 offers the flexibility to configure the input and the output specifically for the application. An on-chip programmable gain amplifier (PGA) allows multiple input ranges to optimize the performance. The larger input range offers industry-leading low noise, and the smaller range lowers the gain requirements on the drive circuitry, making it easier to meet the IP3 requirements. If the input ever goes out of range there is automatic indicator on the overflow pin. A separate digital output supply pin allows connection to low voltage DSPs, FIFOs or logic as low as 0.5V.

The LTC1750 is one device in a family of pin compatible high speed ADCs. The LTC1749 is a pin compatible 12-bit version of the LTC1750 allowing multiple price/performance products with one design. For other high-speed applications this pin compatible family extends from 25Msps to 80Msps and from 12-bits to 14-bits. Each device is available in the 48-pin TSSOP package for both the commercial and the industrial temperature ranges.

Summary of Features:
* LTC1750: 14-Bit 80Msps Wide Bandwidth ADC
* LTC1749: 12-Bit 80Msps Wide Bandwidth ADC
* 500MHz Full Power Bandwidth S/H
* Excellent Dynamic Performance
– – LTC1750 ” 71.7dB SNR & 84dB SFDR @ 140MHz
* Selectable Input Ranges: 1.35Vp-p to 2.25Vp-p
* Output Supply Pin: 0.5V to 5V Range
* Single 5V Supply
* Power Dissipation: 1.45W
* 48-Pin TSSOP Package

Linear Technology Corporation was founded in 1981 as a manufacturer of high performance linear integrated circuits. Linear Technology products include high performance amplifiers, comparators, voltage references, monolithic filters, linear regulators, DC-DC converters, battery chargers, data converters, communications interface circuits, RF signal conditioning circuits, and many other analog functions. Applications for Linear Technology's high performance circuits include telecommunications, cellular telephones, networking products such as optical switches, notebook and desktop computers, computer peripherals, video/multimedia, industrial instrumentation, security monitoring devices, high-end consumer products such as digital cameras and MP3 players, complex medical devices, automotive electronics, factory automation, process control, and military and space systems.

Linear Technology Corporation, 1630 McCarthy Boulevard, Milpitas, CA 95035-7417. Tel: 408-432-1900

www.linear.com

LTC has been busy developing converters. This past year we've seen them introduce a family of high-speed A/D converters, from 12-bit 25 megasmaples to 14-bit 80 megasamples. This LTC1750 is another version of an 80 MSPS part that is primarily targeted at communications and specifically base station design.

The LTC1750 has one very important feature that the marketplace will really like; it allows you to eliminate an intermediate frequency (IF) down-conversion stage. Typically, in base stations there are a couple down-conversion stages, and the second down-conversion stage is around 70 MHz. The 1750 helps eliminate that second IF down-conversion and do direct IF sampling in the 250-350 MHz range. LTC accomplished this by redesigning the sample-and- hold circuitry in the front end and optimize it for really high input frequencies.

This part follows a trend in the industry to eliminate as many down-conversion stages as possible. Most designs now have only two down conversions with the aim to have no down-conversions,or, only have one stage. Obviously, the ideal part would have a very high resolution gigahertz converter, but that's a discussion for another year.

Another important aspect of this converter is its spurious free dynamic range (SFDR) vs. input range. Typically, there's a classic shape of the curve for the SFDR vs. input frequency. Up to about 70 MHz most products are usually flat at their peak SFDR number. However, that changes as the input frequency increases. There usually is a roll off, and it is either very steep or it comes down gracefully, and graceful is better. Since SFDR is about distortion, you want the signal to be relatively small, so it stays within the linear region with all the surrounding circuits. That also means you don't want to get too close to ground or too close to the rail, because distortion gets worse there. LTC's 1750 manages to roll off subtly all the way out to really high input frequencies, offering an SFDR of 82 dB at 250 MHz. That's important to customers designing base stations because they want to pack more and more callers into a given base station, and they want to be able to tell the difference between signals that are in nearby frequency ranges. So, if one caller is far away and weak, the equipment can distinguish it from noise in the presence of a call that may strong and nearby.

The company says that if offers flexibility to configure the input and the output for specific applications. So if you're trying to maximize SFDR, and you know that's the key thing you're doing, then you can set this part up for a small full-scale signal swing. However, if you're going to use it for communications test equipment or something where you are more concerned about SNR, then you can optimize the 1750 with a larger signal swing.

Generally, you may have a 3.3- or a 5-volt device, if it's 5-volt you can run it at 3.3″V. The company says you can actually run it all the way down at 0.5 volts, but there's no logic levels right now at that level, but practically, you can run this part at 1.2-volt logic levels.

There are other parts from other vendors including Analog Devices, National Semiconductor, and Fairchild Semiconductor that offer competitive parts but you will have to review the data sheets to determine which part has the right combination of SFDR, SNR, and power consumption for your needs. For example, if you are designing a pico cell base station you don't want big power supplies or heat sinks, because power is an issue and you don't need as many channels as larger systems.

Linear Tech is offering this converter part in the 14-bit LTC1750 as well as the 12-bit LTC1749. The company is expecting significant interest in both parts for different reasons. The 1750 will attract the designers that need the resolution while the 1749 will be of interest to those designers who need to reduce cost and don't need the extra resolution. Additionally, since the 12-bit part actually provides 12-bit effective performance some designers may want to consider replacing their more pricey 14-bit converters that only provide 12-bit effective performance with this less costly 1749 converter.

Pricing for 1,000-piece quantities begins at $32.30 each for the LTC1750, $19.55 each for the LTC1749.

LTC1750 data sheet

The LTC1749 is a pin compatible 12-bit version of the LTC1750. LTC1749 data sheet

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