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Analog Devices Introduces Breakthrough Differential Amplifier For High Speed, High Resolution ADCs
AD8351 is industryıs first single-chip amplifier to drive 14-bit ADCs at 70 MHz
| The manufacturer says . . . | ChipCenter's Paul O'Shea says . . . |
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Analog Devices announced the industryıs first single-chip, low-distortion, fully differential amplifier able to drive high-resolution (12- to 14-bit) analog-to-digital converters (ADCs) at high-speed (70 MHz) ı a critical requirement for use in next generation wireless infrastructure equipment. The AD8351 also supports 10bit distortion performance at 240MHz making it equally ideal for test equipment. Analog Devices achieved these previously unattained performance levels by using the third generation of its proprietary eXtra Fast Complimentary Bi-polar (XFCB) and Silicon on Insulator (SOI) manufacturing processes. The result is a single chip that prevents signal bottlenecks at the ADC, making it an ideal solution for a broad range of applications, including differential ADC drivers, IF sampling receivers, RF/IF gain blocks, SAW filter interfacing, single-ended to differential conversion and instrumentation. ıThe move to higher IF sampling radio architectures for 3G Universal Mobile Telecommunications System, CDMA 2000 systems and associated instrumentation and test equipment means that our industry must find an acceptable balance between performance and power,ı said Lew Counts, Vice President, Advanced Linear Products at Analog Devices. ıThe AD8351 differential amplifier, which not only has outstanding performance but consumes only 135 milliWatts at full bandwidth, addresses this requirement and further demonstrates Analog Devicesı pioneering role in developing leading edge processes and linking those with tangible, breakthrough products.ı The AD8351 simplifies analog to digital conversion by allowing the user to adjust the common mode voltage and set the gain from 0dB to 26dB (voltage gains from one to 20) through a single external resistor. The AD8351 provides a wide bandwidth (-3dB) of 2.2GHz with a gain of 12dB, low noise spectral density of 2.2nV/rtHz, 2nd / 3rd order harmonic distortion of ı79/ -81dBc at 70MHz, slew rate of 11,000V/micro sec and exceptional settling/ overdrive recovery. Operation is specified from a single three or five volts supply drawing just 27mA quiescent current. The device will operate over the temperature range of -40ıC to +85ıC. The AD8351 has been fully specified at the common IF frequencies 10MHz, 70MHz, 140MHz and 240MHz, and is suitable for fully differential signal paths, single ended to differential applications and DC coupled applications. Analog Devices, Inc., Ray Stata Technology Center, 804 Woburn Street, Wilmington, MA 01887. Tel: 800-ANALOGD, (800-262-5643). |
The growth of base stations in wireless infrastructure and the continuing need of test equipment to verify the cutting edge products was reason enough for Analog Devices to design a differential amp to meet market demands. This differential RF IF amp is optimized to drive high-resolution high-speed ADCs for those two markets by providing the performance and the low distortion. Many high speed ADCs have differential inputs that are used in base station IFs and instrumentation. For these applications, balanced inputs are very useful for minimizing second harmonic distortions. Three years ago ADI introduced the AD8138, which was the first differential amp optimized to interface with these high-speed ADCs. It was perhaps, the first real differential op amp. It was primarily aimed at 20 MSPS applications. However, it wasnıt a product that would be useful with 12- and 14-bit converters in base station IFs. ADI decided to build an amp that would go with their 12-bit 40MSPS, the AD9042. What they ended up with was this entirely new product called the AD8351. Itıs a differential amp but has an entirely different circuit than the AD8138, its predecessor. This differential amp lists specs that are inclusive of gain setting. In other words, there are no add-ons; you donıt have to build a special network for it. This is a complete differential amp with just one resistor used to set the gain. The result is a part that is good for about ı80 dBc at 70MHz, and shows ı60 dBm at 240MHz, and ı3dB as 2.2 GHz, at a gain of 4. You can set the gain to 1 with no resistor but if you use the resistor, the gain ratio can be set to 30. So at a gain of 4, which is common, itıs good for 2.2GHz, and the bandwidth holds up at the 2Vp-p. ADI put in a spec of 11,000V/micro-second slew rate, which is an indirect way of saying that you donıt have to worry about the 2Vp-p with this amp ı because itıs fast. It has been characterized for all specs for all temperatures and sampled throughout the industry. They looked at distortion at temperatures ranging fromı50 to +80C, and then looked at the noise ı itıs 2.2nV/rtHz at a gain of 4 and thatıs for the whole differential amp system. This differential amp system is specified at the three popular IF frequencies and at several gains. So you donıt have to figure out what would happen if you use a gain resistor of so many ohms and the effect that would have on noise. Using this thinking, ADI made it easy for the system designer. It is a differential amp that is fully specified, which isnıt true if you use an op amp. If you have an op amp with letıs say, 2.5 nV of noise, by the time you build a differential amp from it you know you will have more noise than 2.5nV/rtHz. This product uses ADIs third generation SOI (XFCB ı extra fast) process and it has a new circuit design that allows ADCs to have glitches on the input. The AD8351 is immune to input artifacts that you can get with ADCs. Itıs a new idea and because of the design and process it draws about 150 mW on a 5V supply. In comparison there are some single op amps that are not as fast and donıt have the slew rate, but they draw current about that level, which is <30mA at 5V. Additionally, there are some very high power amps that are designed for the cable TV industry that can do this but they have many watts of power. If you are building a multichannel per rack base stations ı you canıt afford to have more power in the pre-amp than you have in the ADCs. The other interesting thing about this part is that it works equally well as a single to differential amplifier. Thatıs usually a big problem for designers ı determining how to go from single to differential. The AD8351 is currently sampling and available in a compact 10-pin microSO package. The AD8351 is priced at $3.69 per unit in 1,000-piece quantities. Data Sheet: Data Sheet
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