ADC Moves DSPs Closer To Software-Defined Radio Antennas

Every passing year sees radio transceivers get nearer to the ideal software-defined radio (SDR). The "ideal" will occur when the antenna is connected directly to a DSP baseband processor, which performs all radio functions like filtering, frequency conversion, modulation, and demodulation.

Yet this requires a really fast analog-to-digital converter (ADC) between the antenna and the DSP. ADCs are, in fact, the weak link in digital SDRs. But progress is being made in the form of Texas Instruments' ADS5500, a product of TI's Burr Brown division.

The ADS5500 is a 14-bit pipeline architecture ADC with a speed of 125 Msamples/s, giving it a 62.5-MHz Nyquist bandwidth. Of course, it can handle higher analog inputs using undersampling methods. With a 100-MHz input, the device exhibits a 70-dB signal-to-noise ratio (SNR) and an 82-dB spurious-free dynamic range (SFDR). This ADC has a differential input of the S/H amplifier and built-in digital error correction at the output of the pipelined core. The reference is internal.

The chip operates from a single 3.3-V supply and consumes a maximum of 750 mW while sampling at 125 Msamples/s. It comes housed in a 64-pin TQFP PowerPad.

The ADS5500 should become popular among wireless designers, especially those addressing multiband receiver applications like multimode, multiband cell-phone basestation receivers that may handle many types of signals simultaneously. The device provides higher performance for receivers by enabling them to more accurately receive weaker, more distant signals while reducing the number of analog-signal chain components needed per basestation. Combined with TI's GC5016 digital up/downconverter chip, the ADS5500 permits oversampling and undersampling of the input IF signal to improve signal quality and reduce circuit complexity.

Medical systems and test and measurement equipment could benefit from the high-end SNR and fast sampling. MRI machines using the device would produce higher image quality scans with lower magnetic fields. Production test equipment and laboratory instruments would provide better measurements over a wider range of frequencies. And, many digital video applications would also boast higher quality video.

The ADS5500 is sampling now, with production scheduled for the first quarter of 2004. Pricing is $95 in 1000-unit quantities. Evaluation modules are also available.