Leveraging its expertise in analog process technology, Texas Instruments introduced a new family of ultra-low distortion amplifiers designed for driving the highest performance analog-to-digital converters (ADCs) in the market today. The new product family features fully differential amplifiers equipped with the industry's lowest distortion, surpassing competitive devices by 10 to 20 decibels (dB) across the entire frequency spectrum. The new amplifiers will enable higher ADC resolution at faster sampling speeds in wireless base station receivers, high-end test equipment, medical imaging and industrial sensing applications.

The new THS4502 amplifier, the flagship product in the family, provides fully differential signaling with ultra-low distortion for support of 12- and 14-bit ADCs operating at sampling rates of 80 megasamples per second (MSPS) and beyond. The device features distortion ratings significantly better than those of competing devices, including a third-order intermodulation distortion (IMD3) of -95 dB below the carrier (dBc), and a third-order output intercept point (OIP3) of 50 dB per milliwatt (dBm) at 30 MHz. These distortion ratings enable the device to provide up to three additional bits of resolution at the same operating frequency relative to the competition, yielding exceptional signal resolution for high-performance applications.

The THS4502 amplifier offers flexibility and performance headroom for use in a wide variety of designs. Key signaling features include a unity gain bandwidth of 360 MHz and a slew rate of 2800 volts per microsecond. Dual input supply voltages of -5 and 5 V provide differential outputs with a current of 100 milliamps (mA). The THS4502 offers a power-down feature, while an alternate device, the THS4503, is identical but without power-down.

Two other new devices in the same family provide similar performance and low distortion using a single supply. All devices in the family can shift signaling levels to a new common mode voltage when necessary. In addition, the THS4500, with power-down, and the THS4501, without power-down, can accommodate 0-V, ground-referenced input signal voltages when powered by a single 5-V power supply. All of the devices are based on BICOM II technology, the second generation of TI's innovative and highly successful BICOM manufacturing process.

"The new THS4500 family of high-performance amplifiers shows TI's dedication to providing solutions for driving high-speed ADCs," said Paul Nossaman, strategic marketing manager for High-Speed Amplifiers. "These new devices will enable designers to get the highest resolution outputs from the highest-performance ADCs in the market."

Texas Instruments, Inc., Web at www.ti.com.

The High-Speed Amplifier market is growing quickly. The communications market, especially optical networking, wireless, baystation receivers, test and measurement, industrial controls, and automatic test equipment, fuels the growth. TI designed this product to take on the competition and add a few extras of their own.

Fully differential amplifiers are found mostly in front of high-speed, high-resolution, analog-to-digital data converters. These data converters will typically have differential inputs that need to be driven at a common-mode voltage that is midway between the full scale range. These amplifiers will need to have the distortion and noise levels to support the dynamic range requirements that the system requires. For example, preceding the data converters in wireless base stations, there are 12-bit and 14-bit data converters sampling at about 80 MSPS. The intermediate frequencies of these base stations is increasing from 20MHz to 70MHz, with some as high as 190MHz. It's at 190MHz where these fully differential amplifiers will be important.

Differential signaling offers a number of performance advantages in high-speed analog signal processing systems, including immunity to external common-mode noise, suppression of even-order nonlinearities, and increased dynamic range. Fully differential buffer amplifiers have a topology that helps reduce even-order harmonics. It uses a Taylor series expansion so that when you go through the series it cancels out the even-order harmonics. It doesn't remove all the even-order harmonics but it helps reduce the total harmonic distortion. Fully-differential amplifiers not only serve as the primary means of providing gain to a differential signal chain, but also provide a monolithic solution for converting single-ended signals into differential signals for easier, higher performance processing.

TI has other fully differential amplifiers including the THS41xx family. It is from an earlier process that they call BiCOM 1, which is a first generation complementary bipolar process. Those parts work at +/-15V and provide 100 -150 MHz at minus 3 dB bandwidth, and up to about 650V/microsecond slew rate. TI's next generation complementary bipolar process (BiCOM II), provides a lower voltage process, so these parts work up to ý5V. Typically, the lower the voltage the fewer parasitics and the smaller geometries will provide better performance. The BiCOM II process allowed 400MHz of 3dB bandwidth, at a much improved slew rate of about 3,000 V/micro-seconds, and that lead to lower distortion.

There are many applications that interface to 12-, 14-, and 16-bit converters, and they sample at much higher rates. A key specification customers are looking for is to drive down signal noise ratio, dynamic range or effective number of bits. Third order intermodulation (IMD3) is the key technique that TI has been looking at to drive those specifications down. It is a key spec needed by base station requirements. For IMD3 you take two tones 200 kHz apart and show the criteria there, at 2Vp-p. Then you look at the third order products and see what the distortion levels are, noticing how far down the third order products are from the top of the fundamentals of those two tones.

TI performed some single-ended to differential conversion tests. In the base station receiver chains, the signal is single-ended through most of the path until it gets to the data converter which is where the differential conversion is needed to drive the differential inputs of the data converter. Typically, that has been done with a transformer. TI found that many of their customers wanted to get rid of the transformer and use an amplifier because an amplifier can still provide gain, and it could be placed in front of the data converter. TI made some improvements with an IMD3 spec showing about a 20 dB improvement over the competition.

With this release TI has introduced two families, the 4500/01 and the 4502/03 families. The THS4502 and 4500 offer a power down function while the THS4501 and 4503 don't have the power down capability. This circuit is for customers who want to be able to power down and save power in some circuits, being especially useful for portable hand-held testers.

The 4500 family also supports engineers that want amplifiers that use just +5V and ground. This will help them because it means they won't need to put a negative supply on the board. This may not be important to test and measurement engineers who have +/-12V and even +/-15V on the board and need a wide dynamic range. However, base station engineers will appreciate not having a negative supply because all the DSPs and digital electronics are working at +5V or +3V.

TI states that all the devices can shift signaling levels to a new common mode voltage when necessary. And that is basically what the Voc-in pin does. It performs a type of level shifting. For example, if you have a single-ended amplifier, a 5V and ground supply, and the input single to the amplifier is ground referenced, then you have to level-shift that signal to the output common mode voltage of the data converter. The THS4500 products AC-couple the signal, and then level-shift that around the amplifier with a resistive divider. You might say that this type level shifting is more elegant because it occurs without external components. At the same time it allows you to DC-couple, if you had signals down to DC that you needed to measure. This probably wouldn't be a concern for base station designs because they use a 20MHz IF with a 5MHz band, and don't care about DC most of the time. However, test and measurement engineers care about DC up to about 10MHz. So they would like to be able to DC-couple.

A couple other points to note, which are listed on the data sheet; the THS4500 family does not incorporate automatic thermal shutoff protection, so the designer must ensure that the design does not violate the absolute maximum junction temperature of the device. TI also states that careful attention to board layout, parasitic and external component types, should be observed. You can review their data sheet for recommendations that will optimize performance, and other pointers including setting the output common mode voltage.

The THS4500, THS4501, THS4502, and THS4503 high-speed fully differential amplifiers are available now in volume from TI and its authorized distributors. The devices are packaged in either 8-pin small outline integrated circuit (SOIC) or 8-pin micro small outline package (MSOP) packages. Suggested resale pricing is $3.77 each in quantities of 1,000.

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