acquisitionZONE Products for the week of November 8, 2004
Providing a breakthrough combination of DC accuracy and AC response, Texas Instruments Incorporated (TI) announced the industry's widest bandwidth 24-bit industrial analog-to-digital converter (ADC). The ADS1271 offers 50kHz bandwidth, 1.8uV/C offset drift and up to 109dB signal-to-noise ratio (SNR), enabling a new level of precision measurements in industrial, medical and automotive applications.
Traditionally, industrial delta-sigma ADCs offering good drift performance used digital filters with large pass-band droop, resulting in limited signal bandwidth mostly suited for DC measurements. High-resolution ADCs for audio applications offer larger usable bandwidths, but the offset and drift specifications are significantly worse (and often unspecified) than their industrial counterparts. The ADS1271 combines these two converter types, allowing high-precision measurements with excellent DC and AC specifications ensured over a -40C to +105C temperature range.
"Texas Instruments has utilized its expertise in precision industrial and high-performance audio data conversion to develop a new class of data converters with unmatched precision and speed," said Gregg Lowe, senior vice president of TI's high-performance analog business. "This breakthrough data converter provides our customers with the best of both worlds, allowing them to achieve uncompromised performance in their applications."
Utilizing proprietary design techniques, the high-order, chopper-stabilized
modulator achieves very low drift (less than 1.8µV°C) with low in-band noise (6.5uV rms). The
onboard linear phase decimation filter suppresses modulator and signal out-of-band
noise, providing a signal pass-band of 90% of the Nyquist rate with less
than 0.005dB of ripple.
analogZONE Says . . .
A product like the ADS1271 does not just pop out from a designer's first attempt; it requires serious attention and iterations and we have been seeing some of those coming along in the last couple of years from TI/Burr-Brown. The first indication to me was with the multi-channel ADS1255/56, and then earlier in 2004 we saw the 41 ksample/s ADS1252 inching towards the goals achieved here. Getting the speed would be easier with a SAR architecture but delta-sigma is more and more desirable for its better ac performance. The design here also reminds us of the terrific performance achieved earlier by the 16-bit >5 Msample/s ADS1605.
The ADS1271 delta-sigma ADC not only offers a higher than 100 ksample/s data rate but also breaks assumed digital filter rules by achieving an extremely smooth bandwidth out to 0.9 of Nyquist. At the same time the dc accuracy is very high. It gives us a picture that says the part look like an industrial ADC that has had a bandwidth extension, or an audio ADC that has had a front-end job! Either way it's a winning combination.
This is a 24-bit, no missing code, delta-sigma ADC with three different modes available. High-speed mode allows for 105,469 sample/s operation with a very reasonable 92 mW typical total dissipation. High-resolution mode (at 52,734 sample/s) reduces the power a fraction to 90 mW but improves both SNR as the serial clock rate is doubled. At regular serial clock with 52,734 sample/s the current used in the analog portion of the IC is cut to nearly one-third reducing overall consumption to a typical 35 mW. A power-down mode is also available for a typical 15 µW. The supply rail for the analog is a nominal 5 V while the digital portion can be powered from between 1.65 V and 3.6 V.
The full-scale input is determined by the reference and would be typically ±2.5 V. The offset error with calibration is a typical 25 µV with a drift of 1.8 µV°C. Equally nice the gain error is typically 0.1% with a drift of 2 ppm/°C. CMRR is 95 dB at 60 Hz and PSRR is 80 dB. Typical INL with a 2.5 V common-mode and using the inputs differentially is ±0.0007%FS.
Ac performance is not stinted either. With a 1 kHz input at a level of -0.5 dBFS the SNR in high-res mode is 109 dB (106 dB otherwise) while the SFDR is -108 dB and THD is -105 dB. The passband response in the high-speed and the low-power modes has a gentle two-cycle ripple over about a 0.01 dB spread out to about 0.45 normalised frequency (input to data) and then falls off to -3 dB at 0.49. The frequency response repeats at 64x multiples of the modulator frequency (the oversampling ratio) but the stop band attenuation is better than 100 dB so any anti-aliasing filter in front of the part can be very simple. In high-res mode oversamplng is 128x and the stop band is down at nearly -140 dB. The passband ripple in this case is a gentle single cycle, again with an about 0.01 dB spread.
The data output from the ADS1271 is in two's complement format with output codes of 7FFFFFh at positive full-scale input and 800000h at negative full-scale input. The output clips at those codes. The output is serial with either SPI-compatible read-only, or as a slave-only serial frame sync (as is often used in audio ADCs).
Parts can be daisy-chained to achieve multiple channels using a common clock with a maximum number determined by the clock frequency. The master clock can be between 1 MHz and 27 MHz and with a clock at 27 MHz, using the SPI and in high-res mode, the maximum number of parts that can be daisy-chained is 21.
The ADS1271 is a remarkable achievement that will change the thinking in industrial sensing and analysis. It may even end up in some audio applications and it will certainly be widely adopted by T&M manufacturers. I do think that TI may be leaving some money on the table with their pricing.
The ADS1271 is sampling in TSSOP-16 with production scheduled for Q4 2004. It is being priced at $5.75 in 1000-piece lots. Evaluation modules are available.