THE MAGAZINE FOR COMPUTER APPLICATIONS
Circuit Cellar Online offers articles illustrating creative solutions
and unique applications through complete projects, practical
tutorials, and useful design techniques.

ANALOG BIT BOMB

by Tom Cantrell

Start ý Whatýs a nV Between Friends? ý All Greek To Me ý Bank Heist ý More For Less ý Sources and PDF

WHATýS A nV BETWEEN FRIENDS?

Never sell silicon short. Taking note of a press release from analog specialist Burr-Brown (recently acquired by Texas Instruments), I found that the resolution race is still in high gear with the ADS1216 (see Figure 1), the latest entry in their lineup of A/D converters that features a whopping 24 bits of resolution.

(Click to enlarge)

Figure 1ýWith 24-bit resolution and eight channels, the TI ADS1216 proves A/D converters have come a long way.

Like most high-end converters these days, the ý1216 is a complete data acquisition sub-system, which includes a front-end multiplexer, programmable gain amp (PGA), filtering, and a buried controller. Thereýs also a bunch of other handy stuff, including an internal offset DAC, two external 8-bit DACs, a temp sensor, loopback (burnout) self-test current source, voltage reference, and eight general-purpose CMOS digital I/Os.

An SPI-clocked serial port makes the host connection. Itýs not a bottleneck because the ý1216 is designed to deliver precise, not speedy, conversions. Furthermore, in the industrial and scientific apps targeted, it may well be that the converter must be electrically isolated from the rest of the system, something much easier and cheaper to do with a serial rather than a parallel interface.

Letýs ponder that 24-bit spec a moment. Each count is about 0.06 ý 10^ý6 of a 0- to 5-V input range, which means weýre talking about 0.30 ýV per LSB.

Yes, admittedly there is a whiff of "specsmanship" in the air, not unheard of in the arcane world of analog. The 24-bit "no missing code" spec (like MIPS for processors) is subject to a reality check. For a converter, the signal-to-noise ratio achievable across the entire on-chip signal chain must be considered in order to determine effective resolution. Put another way, bits that only measure noise arenýt useful.

For instance, setting the onboard PGA to maximum gain (128) cuts the effective resolution to 19 bits (see Figure 2). Nevertheless, although dynamic range is reduced, the overall result is even higher resolution (19 bits + 7 bits gain = 26 bits), a mere 75 nV.

Figure 2ýLike MIPS, your mileage may vary, and in this case, it depends on the gain setting.

Um, let me check it out with my scopeýnot! I have no idea how you test these things to make sure there arenýt any missing codes or to confirm the other fractions-of-PPM-type specs. Iýll take their word for it, because I figure the combined TI and Burr-Brown crew knows a thing or two about analog.

PREVIOUS • NEXT

Circuit Cellar provides up-to-date information for engineers. Visit www.circuitcellar.com for more information and additional articles.
For subscription information, call (860) 875-2199, subscribe@circuitcellar.com or subscribe online. ýCircuit Cellar, the Magazine for Computer Applications. Posted with permission.