A High Speed Family of Commercial Track-and-Hold Devices for
InP technology is the fastest semiconductor technology in production today. Because of this inherent advantage, circuits made in InP typically outperform those made in traditional gallium arsenide (GaAs) and silicon germanium (SiGe) for high speed applications. InP technology is also a cost-effective solution for circuits with reasonable complexity up to about 5000 transistors, competing very well with GaAs and SiGe technologies for high speed front-end applications. 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 demands for high performance integrated circuit solutions. Fast sampling oscilloscopes and direct conversion receivers in radios, radar or electronic warfare systems all demand wider-bandwidth and higher sample rate analog-to-digital conversion. With the GigaTrack THAs, engineers can, for the first time, replace numerous components in traditional heterodyne receiver architectures with a track-and-hold and a high sample rate analog-to-digital converter (ADC). The resulting receivers are lower power and more compact than traditional heterodyne receivers and provide far more flexibility. Signal processing (that is, down-conversion or channelizing) that was “hard-wired” in heterodyne receivers, can now be performed digitally and can be “software defined.” A direct conversion receiver can serve multiple applications with system differentiation occurring in software or firmware.
Track-and-hold amplifiers are often used as the high speed front-end of an ADC. 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 analog-to-digital conversion. By using a high performance THA as the front-end of a low cost commercially available ADC, system designers can extend the input analog bandwidth of the ADC from megahertz to gigahertz frequencies. The resulting circuit offers a significant cost advantage over alternative approaches such as diode bridges and mixers. High input analog bandwidth, high sampling rate and low harmonic distortion are key parameters for THAs. Existing THAs are made in GaAs and more recently SiGe technology. These devices typically have an input analog bandwidth of 4 to 6 GHz. Table 1 compares the performance of an InP THA (Inphi model 1821TH) against competing products in GaAs and SiGe. The InP THA offers a 12 GHz input analog bandwidth at full swing, 1 Vpp, which is an exceptionally high value for any commercial-off-the-shelf THAs today.
Because of the high input analog bandwidth of the InP THA, system designers now can extend the input analog bandwidth of the ADC from around 100 MHz to well over 12 GHz. Figure 1 depicts the block diagram of such a design, in which an InP THA is driving a commercial-off-the-shelf ADC with 100 MHz input analog bandwidth. The resulting circuit offers a significant cost advantage over alternative approaches and is now in mass production for high speed digital sampling scope and signal analyzer applications.
Another popular application for high input analog bandwidth THAs is for automatic test equipment. At speed testing, it is critical that the high speed signal be captured and digitized in real time. This application requires a very high speed ADC operating at multi-giga samples per second. Such an ADC has recently become available commercially, but its input analog bandwidth is usually not high enough to capture the signal faithfully above 1 or 2 GHz. A high input analog bandwidth THA alleviates this issue, extending the bandwidth of the ADC while improving the overall performance of the system.
As an example, Figure 2 compares the performance of a National Semiconductor high speed ADC (model ADC08D1500) with and without the Inphi THA at a 1.5 GHz sampling clock.
Without the Inphi THA, the performance of the ADC, as expected, begins to degrade at input frequencies above 1 GHz, whereas with the Inphi THA, the performance of the combined THA/ADC continues to be excellent up to approximately 3 GHz before experiencing distortion. Five to 10 dB improvements in single-tone total harmonic distortion were obtained with the Inphi THA “front-end” over the entire frequency range from 100 MHz to 3 GHz.
The GigaTrack family consists of four track-and-hold amplifiers with 2 GS/s sample rates. The ball-grid-array versions offer 18 GHz (small signal) and 15 GHz (0.5 Vpp) input analog bandwidths with very fast settling times (60 ps) and low power consumption (1.3 W). Plastic QFN versions provide 13 GHz analog bandwidth (100 mVpp).
To deliver a wider hold time window for the downstream ADC, a master/slave (dual) track-and-hold architecture was developed. This provides higher accuracy in the digitization process by increasing the hold time window to almost one full cycle of the THA. For users who want to sub-sample the output of the master track-and-hold with the slave track-and-hold, the 1821TH and lower performance 1321TH devices provide a flexible clock mode select pin which, in one mode, allows the user to provide different clocks to the master and slave track-and-holds.
The GigaTrack family’s best-in-class settling time (< 60 ps) maximizes timing margin to improve accuracy and performance, while the best-in-class total harmonic distortion (–70 dB typical at 1 GHz and 500 mVpp input) and aperture jitter (< 50 fs) support improved ADC signal-to-noise-and-distortion ratios leading to more sensitive and accurate acquisition systems. Also, by eliminating the requirement for two separate power supply voltages, these track-and-hold devices simplify board layout, lower system cost and help reduce power consumption by up to 20 percent.
The GigaTrack family supports all popular, broadband analog-to-digital ADC devices including National Semiconductor ADC08100/81500, Atmel AT84AS003/008, ADI 9480, Maxim/Dallas MAX104/108 and others. In summary, a new class of high input analog bandwidth, high sampling rate GigaTrack THAs is now available for test and measurement, automatic test equipment, digital receivers and radar systems applications. These THAs offer system designers attractive solutions to directly capture and digitize high bandwidth signals at gigahertz frequencies, which result in higher performance, lower cost, smaller size and lower weight systems. Additional information may be obtained from the Inphi GigaTrack web site at www.inphi-corp.com/products/ 1821th.shtml.
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