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AD7701AR Arkusz danych(PDF) 11 Page - Analog Devices |
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AD7701AR Arkusz danych(HTML) 11 Page - Analog Devices |
11 / 16 page –11– REV. D AD7701 VOLTAGE REFERENCE CONNECTIONS The voltage applied to the VREF pin defines the analog input range. The specified reference voltage is 2.5 V, but the AD7701 will operate with reference voltages from 1 V to 3 V with little degradation in performance. The reference input presents exactly the same dynamic load as the analog input, but in the case of the reference input, source resistance and long settling time introduce gain errors rather than offset errors. Fortunately, most precision references have sufficiently low output impedance and wide enough bandwidth to settle to 10 µV within 62 clock cycles. AGND AD7701 +5V AVDD VREF LT1019 Figure 14. Typical External Reference Connections The digital filter of the AD7701 removes noise from the reference input, just as it does with noise at the analog input, and the same limitations apply regarding lack of noise rejection at integer multiples of the sampling frequency. If reference noise is a problem, some voltage references offer noise reduction schemes using an external capacitor. Alternatively, a simple RC filter may be used, as shown in Figure 15. +5V AD580 AGND AD7701 AVDD VREF RF 13k Ω CF 100pF Figure 15. Filtered Reference Input The same considerations apply to this filter as to a filter at the analog input. In this case: [RF(CF +10 pF )]= 62 f CLKIN × ln 100 mV ×C IN (CIN + CF ) V FSE where: fCLKIN is the master clock frequency and VFSE is the maximum desired error in volts. GROUNDING AND SUPPLY DECOUPLING AGND is the ground reference voltage for the AD7701, and is completely independent of DGND. Any noise riding on the AGND input with respect to the system analog ground will cause conversion errors. AGND should therefore be used as the system ground and also as the ground for the analog input and the reference voltage. The analog and digital power supplies to the AD7701 are independent and separately pinned out, to minimize coupling between analog and digital sections of the device. The digital filter will provide rejections of broadband noise on the power supplies, except at integer multiples of the sampling frequency. Therefore, the two analog supplies should be decoupled to AGND using 100 nF ceramic capacitors to provide power supply noise rejections at these frequencies. The two digital supplies should similarly be decoupled to DGND. ACCURACY AND AUTOCALIBRATION Sigma-delta ADCs, like VFCs and other integrating ADCs, do not contain any source of nonmonotonicity and inherently offer no-missing-codes performance. The AD7701 achieves excellent linearity ( ±0.0007%) by the use of high quality, on-chip silicon dioxide capacitors, which have a very low capacitance/voltage coefficient. The AD7701 offers two self-calibration modes using the on-chip calibration microcontroller and SRAM. Table III is a truth table for the calibration control inputs SC1 and SC2. In the self-calibration mode, zero-scale is calibrated against the AGND pin and full scale is calibrated against the VREF pin, to remove internal errors. Note that in the bipolar mode the AD7701 calibrates positive full scale and midscale (bipolar zero). In the system-calibration mode, the AD7701 calibrates its zero and full scale to voltages present on the analog input pin in two sequential steps. This allows system offsets and/or gain errors to be nulled out. SYSTEM REF HI AIN SYSTEM REF LO ANALOG MUX A0 A1 SIGNAL CONDITIONING AD7701 MICRO COMPUTER SCLK SDATA CAL SC1 SC2 Figure 16. Typical Connections for System Calibration A typical system calibration scheme is shown in Figure 16. In normal operation the analog signal is fed to the AD7701 via an analog multiplexer. When the system is to be calibrated, AIN is first switched to the system REF LO via the multiplexer and CAL is strobed high, with SC1 and SC2 both high. AIN is then switched to the system REF HI and CAL is strobed, with SC1 low and SC2 high. In this way, the effect of all error sources |
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