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AD9271BSVZ-40 Datasheet(PDF) 35 Page - Analog Devices |
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AD9271BSVZ-40 Datasheet(HTML) 35 Page - Analog Devices |
35 / 61 page AD9271 Rev. B | Page 34 of 60 External Reference Operation Internal Reference Operation The use of an external reference may be necessary to enhance the gain accuracy of the ADC or to improve thermal drift charac- teristics. Figure 69 shows the typical drift characteristics of the internal reference in 1 V mode. A comparator within the AD9271 detects the potential at the SENSE pin and configures the reference. If SENSE is grounded, the reference amplifier switch is connected to the internal resistor divider (see Figure 66), setting VREF to 1 V. When the SENSE pin is tied to AVDD, the internal reference is disabled, allowing the use of an external reference. The external reference is loaded with an equivalent 6 kΩ load. An internal reference buffer generates the positive and negative full-scale references, REFT and REFB, for the ADC core. Therefore, the external reference must be limited to a nominal voltage of 1.0 V. The REFT and REFB pins establish their input span of the ADC core from the reference configuration. The analog input full- scale range of the ADC equals twice the voltage at the reference pin for either an internal or an external reference configuration. 1µF 0.1µF VREF SENSE 0.5V REFT 0.1µF 0.1µF 4.7µF 0.1µF REFB SELECT LOGIC ADC CORE + VIN– VIN+ .5 5 0 –5 –10 –15 –20 –25 03 3.0 2.5 2.0 1.5 1.0 0.5 CURRENT LOAD (mA) Figure 66. Internal Reference Configuration Figure 68. VREF Accuracy vs. Load, AD9271-50 0.02 0 –0.02 –0.04 –0.06 –0.08 –0.10 –0.12 –0.14 –0.16 –0.18 –0.20 –40 80 60 40 20 0 –20 TEMPERATURE (°C) VIN– VIN+ 1µF* 0.1µF* VREF SENSE AVDD 0.5V REFT 0.1µF 0.1µF 4.7µF 0.1µF REFB SELECT LOGIC ADC CORE + EXTERNAL REFERENCE *OPTIONAL. Figure 69. Typical VREF Drift, AD9271-50 Figure 67. External Reference Operation |
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