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AD8303 Fiches technique(PDF) 8 Page - Analog Devices |
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AD8303 Fiches technique(HTML) 8 Page - Analog Devices |
8 / 16 page AD8303 REV. 0 –8– THEORY OF OPERATION The AD8303 is a complete, ready-to-use, dual, 12-bit digital-to- analog converter. Only one +2.7 V to +5.5 V power supply is necessary for operation. It contains two voltage-switched, 12-bit, laser-trimmed digital-to-analog converters, a curvature- corrected bandgap reference, rail-to-rail output op amps, input shift register, and two DAC registers. The serial data interface consists of a serial data input (SDI), clock (CLK), chip select (CS) and two DAC load strobe pins (LDA and LDB). For battery operation and similar low power applications, a shutdown feature (SHDN) is available to reduce power supply current to less than 1 µA. In addition an asynchronous reset pin (RS) will set both DAC outputs to either zero volts or to midscale, depending on the logic value applied to the MSB pin. This function is useful for power-on reset or system failure recovery to a known state. D/A CONVERTER SECTION Each of the two DACs is a 12-bit device with an output that swings from GND potential to 0.4 V generated from the internal bandgap voltage (Figure 22). Each DAC uses a laser-trimmed segmented R-2R ladder that is switched by n-channel MOSFETs. The output voltage of the DAC has a constant resistance independent of digital input code. The DAC output is internally connected to the rail-to-rail output op amp. 12-BIT DAC 2.5k Ω 10k Ω VOUT 2.047V FS BANDGAP REF 1.0V 0.4V 0.4V FS VREF 1.0V 2k Ω 10k Ω Figure 22. AD8303 Equivalent Schematic of Analog Section AMPLIFIER SECTION The internal DAC’s output is buffered by a low power consumption, precision amplifier. This low power amplifier contains a differential PNP pair input stage that provides low offset voltage and low noise, as well as the ability to amplify the zero-scale DAC output voltages, The rail-to-rail amplifier is configured with a gain of approximately five in order to set the 2.0475 volt full-scale output (0.5 mV/LSB). An equivalent circuit schematic for the amplifier section is shown in Figure 22. The op amp has a 4 µs typical settling time to 0.1% of full scale. There are slight differences in settling time for negative slewing signals versus positive. Also, negative transition settling time to within the last 6 LSBs of zero volts has an extended settling time. See the oscilloscope photos in the typical performances section of this data sheet. OUTPUT SECTION The rail-to-rail output stage of this amplifier has been designed to provide precision performance while operating near either power supply. Figure 23 shows an equivalent output schematic of the rail-to-rail amplifier with its N-channel pull-down FETs that will pull an output load directly to GND. The output sourcing current is provided by a P-channel pull-up device that can source current to GND terminated loads. The rail-to-rail output stage permits operation at supply voltages down to +2.7 V. The N-channel output pull-down MOSFET shown in Figure 23 has a 35 Ω ON resistance which sets the sink current capability near ground. In addition to resistive load driving capability, the amplifier has also been carefully designed and characterized for up to 500 pF capacitive load driving capability. P-CH N-CH VDD VOUT AGND Figure 23. Equivalent Analog Output Circuit REFERENCE SECTION The internal curvature-corrected bandgap voltage reference is laser trimmed for both initial accuracy and low temperature coefficient. Figure 18 provides a histogram of total output performance of full-scale versus temperature, which is dominated by the reference performance. VREF Output The internal reference drives two resistor-divider networks. One divider provides a 0.4 V reference for the DAC. The second divider is trimmed to 1.0 V and is available at the VREF pin. The VREF output is useful for ratiometric applications, and also for generating a “false ground” or bipolar offset. See Figures 30 and Figure 31 for typical applications. Since VREF has a high output impedance, it must be buffered if it is required to deliver current to an external load. |
Numéro de pièce similaire - AD8303 |
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Description similaire - AD8303 |
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