InP gives boost to track/hold amps

Design engineers are being challenged to seek alternatives to expensive, bulky analog/RF components, as demand continues to grow for higher-performance, lower-cost, smaller components for digital sampling scopes, automatic test equipment (ATE) and digital receivers.

From higher-input analog bandwidth and sampling rates to requirements for performance at dc to well over 10 GHz, changing market needs are leading manufacturers to adopt more-innovative technologies and processes. One of the latest technologies being exploited to meet these demands is indium phosphide (InP).

InP devices exhibit the highest cutoff frequency (a figure of merit that measures the speed of a transistor) of all semiconductor technologies in production today. As a result, circuits made in InP--even those with relaxed geometry of 1.0 micrometers--outperform similar circuits manufactured in traditional gallium arsenide (GaAs) and silicon germanium (SiGe) with much smaller geometries (e.g., 0.18 µm).

High-speed apps
For high-speed applications, InP technology has been proved to be a highly cost-effective solution, competing well with GaAs and SiGe technologies.

Inphi Corp., for example, has shipped InP circuits in high volume since 2002 and continues to develop advanced InP products to meet the ever-increasing demand for high-performance integrated circuits.

A new class of very-high-input analog bandwidth track-and-hold amplifiers (THAs) has been developed by Inphi for test and measurement, ATE and digital receivers. A THA is typically used as the front end of an analog-to-digital converter (ADC) in high-speed digital sampling applications. The THA's primary function is to track the input signal and hold its voltage constant during the interval required for the ADC to perform the conversion.

Today, most commercially available ADCs are designed to have an input analog bandwidth of less than 250 MHz. This eases the conflicting requirements of high dynamic range and fast sampling rate, and ensures delivery of high-resolution components at low cost.

By using the THA as the "front end" of a low-cost, commercially available ADC, system designers can extend the input analog bandwidth of the ADC from approximately 100 MHz to well over 12 GHz.

The resulting circuit offers a significant cost advantage over alternative approaches and is readily available for a digital sampling signal analysis test instrument.

Recent advances in ADC technology, however, offer an alternative solution. By using a broadband ADC, it is now possible to eliminate multiple mixers, LO drivers, gain blocks, filters and narrowband ADCs. The role of the THA is to extend the input analog bandwidth of the ADC, as well as to improve the system dynamic range and linearity.

Significant improvements--5 dB to 10 dB--were obtained with the InP track-and-hold amplifier front end over the full frequency range from 100 MHz to 3 GHz for the single-tone total harmonic distortion of a National Semiconductor broadband ADC (the Model ADC081500) with and without the InP THA at a 1.5-GHz sampling clock.

Loi Nguyen (lnguyen@inphi-corp.com) is vice president of technology at Inphi Corp. He holds a BS and PhD in electrical engineering from Cornell and an MBA from the Anderson School at UCLA.