High-Speed ADC Offers 16 b At 200 MSamples/s

Nov. 12, 2008
This high-speed analog-to-digital converter combines 16-b resolution and a wide analog input bandwidth with sampling capability to 200 MSamples/s to capture a wide range of signals.

Analog-to-digital conversion plays a major role in many electronics systems, especially when signal processing is performed in the digital realm. The audio world has been accustomed to 16-b resolution at the audio standard sampling rate of 44.1 kSamples/s. But for radio-frequency (RF) signals, the road to high-resolution sampling has represented a much longer journey because of the sampling speeds needed to achieve such high resolution for RF signals. That wait is over, however, since Texas Instruments has taken the wraps off its model ADS5485 analogto- digital converter (ADC), which delivers 16-b resolution at maximum sampling rate of 200 MSamples/s. With its speed and resolution, the ADS5485 is well suited for applications in general data acquisition, medical imaging, radar systems, software-defined radios (SDRs), and in test equipment.

The ADS5485 (see figure) will aid applications in a wide range of fields, including in industrial, commercial, and military areas. It provides as much as 100-MHz differential input bandwidth for capturing input analog signals with 16-b fidelity. It has a 3-dB analog input bandwidth of 730 MHz with a differential input voltage range of 3 V peak to peak. The signal-to-noise ratio (SNR) is 75 dB full scale (FS) for a 70-MHz analog input signal and 74.4 dB FS for a 230-MHz analog input signal. The spurious-free dynamic range (SFDR) is -87 dBc for a 70-MHz analog input signal and -73 dBc for a 230-MHz analog input signal. Second harmonics are -95 dBc for the 70-MHz input signal and -73 dBc for the 230-MHz input s ignal . (Comparing this performance to other commercially available ADCs can be a challenge, but there is help in understanding differences among manufacturers as pointed out in an application note from Texas Instruments, detailed on p. 109.)

The low-noise data converter, which is based on the firm's BiCom3 complementary bipolar process, is designed for use with a +3.3- and +5-VDC supplies, typically drawing 310 and 126 mA, respectively. It includes a power-down mode to cut power consumption to only 70 mW. The ADC provides LVDS-compatible digital output signals following conversion of input analog signals. An on-board differential input buffer amplifier provides isolation between a source and the internal track-andhold (T/H) amplifier, while an internal reference generator helps simplify system design. An internal dither circuit can be switched on or off as needed to help improve SFDR performance. The ADS5485 ADC provides outstanding accuracy, with typical differential linearity error (DNL) of only 0.5 least significant bits (LSB) and typical integral linearity error (INL) of 3 LSB.

In the words of a user, Robert Sgandurra, Senior Product Manager with modular DSP and SDR developer Pentek (www.pentek.com), "TI continues to push the performance envelope with high-speed ADCs. The ADS5485 was a clear choice for our model 7150 Quad A/D Software Radio Module. The higher sample rate means that users will be able to directly digitize nearly 100 MHz of bandwidth, which is invaluable for our customers working on wideband radar and wideband communication systems."

The ADS5485 is housed in a 9 x 9 mm QFN-64 PowerPAD package and is specified for use at temperatures from -40 to +85C. It is actually part of a family of 16-b ADCs that also includes the 170-MSamples/s model ADS54, the 135-MSamples/s model ADS5483, the 105-MSamples/s model ADS5482, and the 80-MSamples/s model ADS5481. All are pin compatible, supplied in the compact 9 x 9 mm 64-pin QFN package. Samples of all five 16-b ADC family members are available.

The company is also offering its model ADS5485EVM evaluation module to aid in testing the ADS5485 ADC. The evaluation board simplifies probing the ADC's LVDS output signals with either a model E5405A touchless probe from Agilent Technologies (www.agilent.com) or a model P6980 touchless probe from Tektronix (www. tektronix.com). The ADS5485EVM evaluation board is also compatible to the firm's model TSW1200EVM High Speed LVDS evaluation and capture system, allowing samples to be captured and passed to a personal computer (PC) for analysis and evaluation. The ADS5485EVM also includes the company's new 10-output low jitter clock synchronizer and jitter cleaner device, the model CDCE72010, which can be used to drive the clocking input to the ADS5485 ADC. Open sockets are provided for an external voltage-controlled crystal oscillator (VCXO) and crystal bandpass filter for rapid evaluation of a combined high-performance ADC and clocking circuit equivalent to the reference-frequency circuitry in a final system-level solution. Texas Instruments, Inc., 13532 N. Central Expressway, M/S 3807, Dallas, TX 75243-1108; (972) 644-5580, Fax: (972) 927-6377, Internet: www.ti.com.

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