Datasheet AD7711 (Analog Devices)

LC2MOS Signal Conditioning ADC with RTD Excitation Currents AD7711 FEATURES FUNCTIONAL BLOCK DIAGRAM Charge-Balancing ADC REF REF AVDD DVDD IN (–) IN (+) VBIAS REF OUT 24 Bits, No Missing Codes
ⴞ
0.0015% Nonlinearity AVDD 2-Channel Programmable Gain Front End 2.5V REFERENCE Gains from 1 to 128 4.5
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A CHARGE-BALANCING A/D 1 Differential Input CONVERTER 1 Single-Ended Input AIN1(+) AUTO-ZEROED SYNC Low-Pass Filter with Programmable Filter Cutoffs M PGA
⌺
DIGITAL -
⌬
AIN1(–) FILTER Ability to Read/Write Calibration Coefficients U MODULATOR X RTD Excitation Current Sources AIN2 A = 1–128 MCLK Bidirectional Microcontroller Serial Interface CLOCK IN 200
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A AV Internal/External Reference Option DD GENERATION MCLK RTD1 OUT Single- or Dual-Supply Operation SERIAL INTERFACE Low Power (25 mW typ) with Power-Down Mode 200
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A CONTROL OUTPUT (7 mW typ) RTD2 REGISTER REGISTER APPLICATIONS AD7711 RTD Transducers Process Control AGND DGND VSS RFS TFS MODE SDATA SCLK DRDY A0 Smart Transmitters Portable Industrial Instruments
The AD7711 is ideal for use in smart, microcontroller based systems. Gain settings, signal polarity, input channel selection,
GENERAL DESCRIPTION
and RTD current control can be configured in software using The AD7711 is a complete analog front end for low frequency the bidirectional serial port. The AD7711 contains self- measurement applications. The device accepts low level signals calibration, system calibration, and background calibration directly from a transducer and outputs a serial digital word. It options, and also allows the user to read and write the on-chip employs a ⌺-⌬ conversion technique to realize up to 24 bits of calibration registers. no missing codes performance. The input signal is applied to a proprietary programmable gain front end based around an ana- CMOS construction ensures low power dissipation, and a software log modulator. The modulator output is processed by an on-chip programmable power-down mode reduces the standby power digital filter. The first notch of this digital filter can be pro- consumption to only 7 mW typical. The part is available in a grammed via the on-chip control register, allowing adjustment 24-lead, 0.3-inch-wide, plastic and hermetic dual-in-line pack- of the filter cutoff and settling time. age (DIP) as well as a 24-lead small outline (SOIC) package. The part features one differential analog input and one single-
PRODUCT HIGHLIGHTS
ended analog input as well as a differential reference input. 1. The programmable gain front end allows the AD7711 to Normally, one of the input channels will be used as the main accept input signals directly from an RTD transducer, channel with the second channel used as an auxiliary input to removing a considerable amount of signal conditioning. periodically measure a second voltage. It can be operated from a On-chip current sources provide excitation for 3-wire and single supply (by tying the VSS pin to AGND), provided that the 4-wire RTD configurations. input signals on the analog inputs are more positive than –30 mV. By taking the V 2. No missing codes ensure true, usable, 23-bit dynamic range SS pin negative, the part can convert signals down to –V coupled with excellent ± 0.0015% accuracy. The effects of REF on its inputs. The part provides two current sources that can be used to provide excitation in 3-wire and 4-wire temperature drift are eliminated by on-chip self-calibration, RTD configurations. The AD7711 thus performs all signal which removes zero-scale and full-scale errors. conditioning and conversion for a single- or dual-channel system. 3. The AD7711 is ideal for microcontroller or DSP processor applications with an on-chip control register that allows control over filter cutoff, input gain, channel selection, signal polarity, RTD current control, and calibration modes. 4. The AD7711 allows the user to read and to write the on-chip REV. G calibration registers. This means that the microcontroller has much greater control over the calibration procedure. Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. No license is granted by implication or otherwise
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Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM PRODUCT HIGHLIGHTS SPECIFICATIONS TIMING CHARACTERISTICS PIN FUNCTION DESCRIPTIONS TERMINOLOGY Intergral Nonlinearity Positive Full-Scale Error Unipolar Offset Error Bipolar Zero Error Bipolar Negative Full-Scale Error Positive Full-Scale Overrange Negative Full-Scale Overrange Offset Calibration Range Full-Scale Calibration Range Input Span CONTROL REGISTER (24 BITS) FILTER SELECTION (FS11–FS0) CIRCUIT DESCRIPTION THEORY OF OPERATION Input Sample Rate DIGITAL FILTERING Filter Characteristics Post Filtering Antialias Considerations ANALOG INPUT FUNCTIONS Analog Input Ranges Burnout Current RTD Excitation Current Bipolar/Unipolar Inputs REFERENCE INPUT/OUTPUT VBIAS Input USING THE AD7711 SYSTEM DESIGN CONSIDERATIONS Clocking System Synchronization Accuracy Autocalibration Self-Calibration System Calibration System Offset Calibration Background Calibration Span and Offset Limits POWER-UP AND CALIBRATION Drift Considerations POWER SUPPLIES AND GROUNDING DIGITAL INTERFACE Self-Clocking Mode Read Operation Write Operation External Clocking Mode Read Operation Write Operation SIMPLIFYING THE EXTERNAL CLOCKING MODE INTERFACE MICROCOMPUTER/MICROPROCESSOR INTERFACING AD7711 to 8051 Interface AD7711 to 68HC11 Interface APPLICATIONS 4-Wire RTD Configurations 3-Wire RTD Configurations OUTLINE DIMENSIONS Revision History