acquisitionZONE Products for the week of September 8, 2003
Texas Instruments (TI) Incorporated introduced the industry's highest performance analog-to-digital converters (ADCs) for industrial applications from the company's Burr-Brown product line. Featuring best-in-class combination of noise-free resolution, data rate and functionality, these 24-bit delta-sigma ADCs provide designers with complete, high-resolution measurement solutions for the most demanding industrial applications, including scientific instrumentation, process control, medical equipment and weighing scales.
"When talking with instrumentation designers, we identified the need for a converter that offers high conversion rates while preserving the effective resolution of low-speed designs in order to enhance overall system performance. Designed to meet this demand, the ADS1255 and ADS1256 offer an unmatched combination of noise performance, speed and features which allows users to reach new levels of measurement accuracy with extended bandwidth," said Rich Wegner, strategic marketing manager for TI's oversampling data converter products.
Wegner added: "The flexibility and high conversion rate also allows
customers to reconfigure an ADS1256 design to meet the speed/resolution
requirements of multiple applications with a simple change in programming."
analogZONE Says . . .
I very much approve of any, and every, attempt to simplify the ability to optimize the operation of a precision ADC when the majority of designers are not trained to do so themselves. These 24-bit ADCs incorporate a smart MUX, an on-chip buffer (which can be bypassed and powered down), and a PGA to optimize the input span to the modulator. The 24-bit 4th-order delta-sigma modulator is followed with a programmable digital filter, allowing a flexible trade-off between resolution and speed. The maximum resolution is 23 bits while the data rate of the filter is programmable from 2.5 ksample/s to 30 ksample/s with a 7.68 MHz clock. The clock can be generated on-chip with an external crystal or an external source can be used.
The ADS1255 has a single differential input (or two single-ended inputs) while the ADS1256 adds a further three differential (or six single-ended inputs) to the input MUX. The MUX is flexible enough that single-ended and differential inputs can be mixed. All the inputs are ESD protected and the diodes will not conduct until the pins go 100 mV below ground or 100 mV above the analog rail. Sensor detection is provided with a current source that can set a current through an external sensor of about 0.5 µA, or 2 µA, or 10 µA through the MUX. When the source current is turned on the buffer is automatically turned on -- regardless of any other setting -- so that the lower input impedance of a direct connection does not load down the current.
The buffer is a low-drift chopper with a large input impedance that varies with the data rate. The input impedance can be modeled as a resistance across the differential input plus resistance from the positive input up to the rail and from the negative input down to ground. Those latter resistances are fixed at greater than 200 MOhm while the differential resistance varies from 6 MOhm at a 30 ksample/s data rate up to 80 MOhm at less than 1 ksample/s. If a higher impedance is required the clock frequency can be reduced -- halving the clock frequency, for example, doubles the impedance. With the buffer bypassed the input impedance is 150 divided by the PGA setting in kOhm for gain settings of 1, 2, 4, 8 and 16, and is 4.7 kOhm for PGA settings of 32 and 64 (the specifications in the data sheet differ a little from the numbers given in a later table.)
The PGA settings allow full-scale input voltage to be controlled. With a 2.5-V reference (not on-chip) a gain setting of 1 will allow for a ±5 V FS input, and directly scaled down to a gain setting of 64 to allow a FS input of ±78.125 mV. The modulator input circuitry will also have slightly changing sampling times at the two higher gains, when it halves to 260 ns from 521 ns (with a 7.68 MHz clock.)
During self-calibration of the modulator input all the input switches are opened and the reference connected directly to the inputs. It is obviously of extreme importance for maximum performance for the reference being used to be as accurate and as noise- and drift-free as possible. The programmable digital filter after the modulator provides a first notch at a frequency matching the sampling rate -- 30 kHz for 30 ksample/s, for example -- while the 3-dB bandwidth varies from 6.106 kHz, at 30 ksample/s, to 1.1 Hz, at 2.5 sample/s.
Two general purpose I/Os are provided on the ADS1255, with four on the ADS1256 while communications are handled over an SPI-compatible interface that can be operated on two wires. Registers are on-chip for the various settings of the MUX, current sources, buffer, PGA, and data rate.
The noise performance of the ICs is outstanding and may be optimized by data rate/PGA setting. Tables in the data sheet show the trade offs between input referred noise, ENOB and noise-free resolution with the buffer both on and off.
The voltage rail for the analog sections of the parts is optimized for 5 V (range 4.75 V to 5.25 V, while the digital sections can be powered from 1.8 V to 3.6 V. In the analog sections the lowest power drain is with PGA =1 and the buffer off, for a typical 7 mA. The highest drain is with PGA = 64 and the buffer on, for a typical 36 mA. Power down in either case is a typical 20 µA. Power down in the digital sections offers a maximum drain of 2 µA, while the worst case quiescent is 0.9 mA. PSRR is a typical 70 dB in analog, and 100 dB in digital, while CMRR is a typical 110 dB with a common-mode input frequency of 60 Hz and 30 ksample/s data rate.
Non-linearity numbers are in the 4th decimal place of % with gain errors down in the 3rd decimal place.
These are superb parts which greatly simplify the way a designer can interface to a sensor system. They are also priced to take sockets in any high-resolution/low-noise applications in medical equipment, instrumentation, scales, etc.
The ADS1255 and ADS1256 are both in production with the ADS1255 in SSOP-20 priced at $7.95, and the ADS1256 in SSOP-28 priced at $8.95, both in 1000-piece lots.