Some Applications Just Can't Get Enough . . .
The insatiable demand for "more bits at more speed " continues to characterize a growing subset of digital-imaging and instrumentation applications. Medical imagers (MRI, CAT, ultrasound, etc.) want to observe more detail; astronomers want to peer deeper into space; seismic explorers for oil want to "hear" deeper; spectrum analyzers want more dynamic range . . . and everyone always wants it faster. Sophisticated DSP techniques have enabled many of these modern-day explorers to extract meaningful results from digital data gathered using ultra-high-speed, lower resolution (8-10 bits A/D converters . . . but in many applications . . . there is simply no substitute for "more bits."

You Can't Get More Bits Faster than This!
DATEL's new ADS-951 is the world's first, 18-bit, 1MHz sampling A/D converter. Competitive comparisons are meaningless since there are no competitors. The device is a fully functional, true sampling A/D. It utilizes a standard subranging (two-pass) architecture and, consequently, does not have the input-bandwidth limitations of high-resolution, sigma-delta A/D converters.

The ADS-951 sucessfully accommodates input-signal step changes. This means it functions well in multi-channel, multiplexed applications (in which there can be significant channel-to-channel variations in signal amplitude and frequency) and in electronic-imaging applications (in which there can be dramatic pixel-to-pixel changes in signal amplitude).

Small Package . . . Low Power . . . High Performance!
The ADS-951 is packaged in a standard, 32-pin, side-brazed, ceramic TDIP package, and though it requires 4 supply voltages (+/-5V and +/-15V), it consumes a mere 1.25 Watts.

I really enjoyed this press release; such enthusiasm is hard to find in the normal prose the industry seems to have come to expect! While the nightmare of working with 18 bits would drive many design engineers into another profession, the true analog designer will have the time of his/her life with this part from Datel. It is quite amazing to think that when Analog Avenue started product reviews 84 weeks ago the Datel ADS-931 (16-bit, 1-MHz ADC) was the first Product of the Week. My opinion of the company being on the leading end of this technology has not changed, and I believe the monolithic manufacturers will have a harder time catching up to this level than they have had with 16-bit products.

The use of the higher 15-V rails for most of the analog part of the ADS-951 is obviously essential to get the noise headroom necessary for an 18-bit part that allows for +/-5-V inputs; that will limit its use for portable products but the majority of the applications will not be hindered in that respect. There is a small difference between the press release and the data sheet over power consumption, the data sheet specifying 1.45 W instead of 1.25 W. The performance, of course, is spectacular with peak harmonics, THD and SNR all close to 90dB all the way up to Nyquist, and two-tone intermodulation distortion at about -85 dB with 100 kHz and 240 kHz tones. The aperture delay time is +20 ns while the sample-and-hold acquisition time is 260 ns.

Layout and filtering around this product are going to be absolutely critical, with both grounds and supply rail conditioning needing special attention. Datel provides no internal connections between analog and digital grounds, but the external pins for one analog and the digital ground are next to one another on the package, and the second analog ground "wraps" around the most sensitive analog pins to the first ground. Any inaccuracy ( for use tighter then the part's specifications) can be zeroed out using a gain adjust pin around the 5-V reference loop.