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AD711BH Datasheet(PDF) 10 Page - Analog Devices |
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AD711BH Datasheet(HTML) 10 Page - Analog Devices |
10 / 12 page AD711 REV. A –10– Figure 31. Unipolar Binary Operation Figure 32. Bipolar Operation R1 and R2 calibrate the zero offset and gain error of the DAC. Specific values for these resistors depend upon the grade of AD7545 and are shown below. Table I. Recommended Trim Resistor Values vs. Grades of the AD7545 for VDD = +5 V TRIM RESISTOR JN/AQ/SD KN/BQ/TD LN/CQ/UD GLN/GCQ/GUD R1 500 Ω 200 Ω 100 Ω 20 Ω R2 150 Ω 68 Ω 33 Ω 6.8 Ω Figures 33a and 33b show the settling time characteristics of the AD711 when used as a DAC output buffer for the AD7545. a. Full-Scale Positive b. Full-Scale Negative Transition Transition Figure 33. Settling Characteristics for AD711 with AD7545 NOISE CHARACTERISTICS The random nature of noise, particularly in the 1/f region, makes it difficult to specify in practical terms. At the same time, design- ers of precision instrumentation require certain guaranteed maxi- mum noise levels to realize the full accuracy of their equipment. The AD711C grade is specified at a maximum level of 4.0 µV p-p, in a 0.1 to 10 Hz bandwidth. Each AD711C receives a 100% noise test for two 10-second intervals; devices with any ex- cursion in excess of 4.0 µV are rejected. The screened lot is then submitted to Quality Control for verification on an AQL basis. All other grades of the AD711 are sample-tested on an AQL basis to a limit of 6 µV p-p, 0.1 to 10 Hz. DRIVING THE ANALOG INPUT OF AN A/D CONVERTER An op amp driving the analog input of an A/D converter, such as that shown in Figure 34, must be capable of maintaining a con- stant output voltage under dynamically changing load condi- tions. In successive-approximation converters, the input current is compared to a series of switched trial currents. The compari- son point is diode clamped but may deviate several hundred mil- livolts resulting in high frequency modulation of A/D input current. The output impedance of a feedback amplifier is made artificially low by the loop gain. At high frequencies, where the loop gain is low, the amplifier output impedance can approach its open loop value. Most IC amplifiers exhibit a minimum open loop output impedance of 25 Ω due to current limiting resistors. A few hundred microamps reflected from the change in converter Figure 34. AD711 as ADC Unity Gain Buffer a. Full-Scale Positive Transition b. Full-Scale Negative Transition Figure 35. ADC Input Unity Gain Buffer Recovery Times a. Source Current = 2 mA b. Sink Current = 1 mA |
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