TI’s fully-differential amps push A/D converters to 100 MHz

Dallas, — Texas Instruments Inc. (TI) introduced the industry's lowest noise, lowest distortion fully differential amplifier for driving high-speed analog-to-digital converters (A/D converters) up to 100 MHz. With one-third less settling time than previous industry-leading fully differential amplifiers, the 1.9-GHz THS4509 is ideal for use in demanding applications such as test and measurement, medical imaging, wireless infrastructure and industrial.

Leveraging TI's unique, high-speed BiCom-III complementary bipolar silicon-germanium (SiGe) process, the THS4509 delivers an unmatched input referred voltage noise of 2.0 nanovolts per square root hertz (nV/rtHz), second- and third-order harmonic distortion at 70 MHz of -80 dBc and -87 dBc (2 V peak to peak into 200-ohm load), respectively, and 1% settling time of 2 nanoseconds (ns) with 2-V output step. When driving the ADS5500 14-bit A/D converter at 125 mega samples per second (M/samples/s) and 70-MHz input frequency, the THS4509 and ADS5500 system achieves unprecedented levels of performance. The single-tone spurious-free dynamic range (SFDR) is 81 dBc, the two-tone SFDR with 5-MHz tone spacing is 89 dBc and the signal-to-noise ratio (SNR) is 70.1 dB full scale (FS).

BiCom-III is the industry's only SiGe process that integrates complementary npn and pnp bipolar transistors, enabling high-performance analog and mixed-signal products that combine high speed, low noise, low voltage and greater dynamic range. This enables designers to achieve optimal high-speed performance without requiring complex compensation circuits or drawing large currents.

“Designers require an amplifier that is on equal footing with the ADC in order to realize the full potential of their high-speed systems,” said Art George, vice president of TI's high-performance linear business. “With the THS4509, customers enjoy the best of two worlds, highest performance and ease of use.”

The architecture of the THS4509 decouples the gain, output common-mode voltage and output impedance-matching issues from one another, enabling the designer too easily set them independently. In addition, the device will perform single-ended input to differential output conversion to enable dc-coupled data acquisition systems. Along with the noise and distortion performance, these features enable the designer to ensure accurate low-level signal measurements and high signal fidelity, while greatly simplifying the design process and reducing the overall solution size.

Additional features of the THS4509 include large signal bandwidth (2 V peak to peak) of
1.5 GHz, small signal bandwidth of 1.9 GHz (<1 V peak to peal), slew rate of 6600 V/μs and 0.65 mA power-down mode.

The THS4509 is well suited for driving other high-performance A/D converters, including the new ADS5424 14-bit, 105-M/sample/s A/D converter.

The THS4509 is sampling now, with volume production scheduled for this quarter. The device is available in a 16-pin QFN package, priced at $3.75 in 1,000-unit quantities. An evaluation module (EVM) is also available.

Click here for TI’s THS4509 data sheet.

Readers can contact TI's Literature Response Center at 1 (800) 477-8924, or visit them on the web at

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TI's THS4509 Amplifier Drives High-Speed Converters

TI's next-generation differential amplifiers for driving high-speed A/D converters up to 100 MHz tout low noise and distortion and fast settling time.

Boasting one-third less settling time, 25% lower noise, and 10 dB to 24 dB improvement over competing amplifiers, the THS4509 is the company's first fully-differential amplifier manufactured using its BiCOM-III process, said Jim Karki, TI's strategic marketing manager for high-speed amplifiers and RF products. TI's SiGe process, which integrates complementary npn and pnp bipolar transistors, is also based on a 350-nanometer (nm) CMOS process, he said.

The process has enabled TI to boost A/D converter performance from 50 MHz to 100 MHz. Essentially, TI is shooting to raise the standards bar for amplifier performance in all of the markets it addresses, Karki said.

A/D converters are moving up the performance chain and the amplifiers that are driving them need to mirror their performance, Karki explained. “A/D converters are becoming faster, offering lower noise and better precision, and the amplifiers used to drive them need to keep up with this progress. A lot of wideband applications, especially in test and measurement and communications are moving into higher frequencies,” he added.

TI's fully-differential THS4509 amplifier can be used with other manufacturer's high-performance A/D converters, as well as TI's ADS5424 14-bit, 105-Msamples/s A/D converter, which is also being introduced today. Compared to TI's older ADS5500 14-bit A/D converter, which is shown in the diagram above in combination with the THS4509, the ADS5424 is slower but provides lower noise and improved SFDR and SNR characteristics, Karki said. Based on the application, engineers can decide which is more important — more speed or less noise.

The high-speed amplifier market will rise from roughly $715 million this year worldwide, to more than $1.3 billion by 2009, according to Databeans Inc. (Reno, Nev.) China represents the fastest growing region with high-speed amplifier sales expected to reach nearly $474 million in 2009.

Other players in this market include Analog Devices Inc., Intersil Corp., Linear Technology Corp., Maxim Integrated Products Inc., National Semiconductor Corp. and Texas Instruments Inc.

Last month, National Semi (Santa Clara, Calif.) also announced a high-speed amplifier, dubbed the LMH6550. With a fully differential VFB (voltage feedback) topology, it allows balanced inputs to A/D converters and can be used as single-ended-to-differential or used as differential-to-differential. Harmonic distortion of -88dBc @20 MHz and high-speed (400 MHz bandwidth and 3000 V/μs sampling rate) supports driving a variety of 10-14 bit A/D converters. It is available now in SOIC-8 and MSOP-8 packaging and is priced at $2.29 in 1,000-unit quantities.

In terms of performance though, Karki said TI's new device comes closest to Analog Devices Inc.'s (Norwood, Mass.) AD8351, which came out in September 2002, and allows users to adjust common-mode voltage and set gain from 0 dB to 26 dB (voltage gains from 1 to 20) through a single external resistor. The device provides a 2.2-GHz bandwidth (-3 dB) with a 12-dB gain; low-noise spectral density of 2.2 nV/rt Hz; second- and third-order harmonic distortion of -79 and -81 dBc, respectively, at 70 MHz; an 11,000-V/millisecond slew rate; and fast settling/overdrive recovery.

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