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AD774BBD Datasheet(PDF) 5 Page - Analog Devices |
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AD774BBD Datasheet(HTML) 5 Page - Analog Devices |
5 / 12 page REV. C –5– DEFINITION OF SPECIFICATIONS Linearity Error Linearity error refers to the deviation of each individual code from a line drawn from “zero” through “full scale.” The point used as “zero” occurs 1/2 LSB (1.22 mV for 10 V span) before the first code transition (all zeroes to only the LSB “on”). “Full scale” is defined as a level 1 1/2 LSB beyond the last code tran- sition (to all ones). The deviation of a code from the true straight line is measured from the middle of each particular code. The K, B, and T grades are guaranteed for maximum nonlinear- ity of ± 1/2 LSB. For these grades, this means that an analog value that falls exactly in the center of a given code width will result in the correct digital output code. Values nearer the upper or lower transition of the code width may produce the next upper or lower digital output code. The J and A grades are guaranteed to ± 1 LSB max error. For these grades, an analog value that falls within a given code width will result in either the correct code for that region or either adjacent one. Note that the linearity error is not user adjustable. Differential Linearity Error (No Missing Codes) A specification that guarantees no missing codes requires that every code combination appear in a monotonic increasing sequence as the analog input level is increased. Thus every code must have a finite width. The AD674B and AD774B guarantee no missing codes to 12-bit resolution, requiring that all 4096 codes must be present over the entire operating temperature ranges. Unipolar Offset The first transition should occur at a level 1/2 LSB above analog common. Unipolar offset is defined as the deviation of the actual transition from that point. This offset can be adjusted as discussed later. The unipolar offset temperature coefficient specifies the maximum change of the transition point over temperature, with or without external adjustment. Bipolar Offset In the bipolar mode the major carry transition (0111 1111 1111 to 1000 0000 0000) should occur for an analog value 1/2 LSB below analog common. The bipolar offset error and temperature coefficient specify the initial deviation and maximum change in the error over temperature. Quantization Uncertainty Analog-to-digital converters exhibit an inherent quantization uncertainty of ±1/2 LSB. This uncertainty is a fundamental characteristic of the quantization process and cannot be reduced for a converter of given resolution. Left-Justified Data The output data format is left-justified. This means that the data represents the analog input as a fraction of full scale, rang- ing from 0 to 4095/4096. This implies a binary point 4095 to the left of the MSB. Full-Scale Calibration Error The last transition (from 1111 1111 1110 to 1111 1111 1111) should occur for an analog value 1 1/2 LSB below the nominal full scale (9.9963 V for 10.000 V full scale). The full-scale cali- bration error is the deviation of the actual level at the last transi- tion from the ideal level. This error, which is typically 0.05% to 0.1% of full scale, can be trimmed out as shown in Figures 7 and 8. The full-scale calibration error over temperature is given with and without the initial error trimmed out. The temperature coefficients for each grade indicate the maximum change in the full-scale gain from the initial value using the internal 10 V reference. Temperature Drift The temperature drift for full-scale calibration, unipolar offset, and bipolar offset specifies the maximum change from the initial (25 °C) value to the value at TMIN or TMAX. Power Supply Rejection The standard specifications assume use of +5.00 V and ±15.00 V or ±12.00 V supplies. The only effect of power supply error on the performance of the device will be a small change in the full-scale calibration. This will result in a linear change in all low-order codes. The specifications show the maximum full- scale change from the initial value with the supplies at the various limits. Code Width A fundamental quantity for A/D converter specifications is the code width. This is defined as the range of analog input values for which a given digital output code will occur. The nominal value of a code width is equivalent to 1 least significant bit (LSB) of the full-scale range or 2.44 mV out of 10 V for a 12-bit ADC. AD674B/AD774B |
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