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AD7303BRZ 데이터시트(PDF) 8 Page - Analog Devices |
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AD7303BRZ 데이터시트(HTML) 8 Page - Analog Devices |
8 / 16 page AD7303 –8– REV. 0 GENERAL DESCRIPTION D/A Section The AD7303 is a dual 8-bit voltage output digital-to-analog converter. The architecture consists of a reference amplifier and a current source DAC, followed by a current-to-voltage con- verter capable of generating rail-to-rail voltages on the output of the DAC. Figure 20 shows a block diagram of the basic DAC architecture. REFERENCE AMPLIFIER OUTPUT AMPLIFIER VO A/B VDD REF AD7303 CURRENT DAC 11.7k Ω 30k Ω 11.7k Ω 30k Ω Figure 20. DAC Architecture Both DAC A and DAC B outputs are internally buffered and these output buffer amplifiers have rail-to-rail output character- istics. The output amplifier is capable of driving a load of 10 k Ω to both VDD and ground and 100 pF to ground. The reference selection for the DAC can be either internally generated from VDD or externally applied through the REF pin. Reference selection is via a bit in the control register. The range on the external reference input is from 1.0 V to VDD/2. The output voltage from either DAC is given by: VO A/B = 2 × V REF × (N/256) where: VREF is the voltage applied to the external REF pin or VDD/2 when the internal reference is selected. N is the decimal equivalent of the code loaded to the DAC register and ranges from 0 to 255. Reference The AD7303 has the facility to use either an external reference applied through the REF pin or an internal reference generated from VDD. Figure 21 shows the reference input arrangement where the internal VDD/2 has been selected. 30k Ω 30k Ω REFERENCE AMPLIFIER AD7303 REF 0.1µF VDD INT/EXT Figure 21. Reference Input When the internal reference is selected during the write to the DAC, both switches are closed and VDD/2 is generated and applied to the reference amplifier. This internal VDD/2 reference appears at the reference pin as an output voltage for decoupling purposes. When using the internal reference, external references should not be connected to the REF Pin. This internal VDD/2 reference appears at the reference pin as an output voltage for decoupling purposes. When using the internal reference, external references should not be connected to the REF pin. If external ref- erence is selected, both switches are open and the externally applied voltage to the REF pin is applied to the reference amplifier. Decoupling capacitors applied to the REF pin decouple both the internal reference and external reference. In noisy environ- ments it is recommended that a 0.1 µF capacitor be connected to the REF pin to provide added decoupling even when the in- ternal reference is selected. Analog Outputs The AD7303 contains two independent voltage output DACs with 8-bit resolution and rail-to-rail operation. The output buffer provides a gain of two at the output. Figures 3 to 5 show the sink and source capabilities of the output amplifier. The slew rate of the output amplifier is typically 8 V/ µs and has a full-scale settling to 8 bits with a 100 pF capacitive load in typically 1.2 µs. The input coding to the DAC is straight binary. Table I shows the binary transfer function for the AD7303. Figure 22 shows the DAC transfer function for binary coding. Any DAC output voltage can ideally be expressed as: VOUT = 2 × VREF (N/256) where: N is the decimal equivalent of the binary input code. N ranges from 0 to 255. VREF is the voltage applied to the external REF pin when the external reference is selected and is VDD/2 if the internal reference is used. Table I. Binary Code Table for AD7303 DAC Digital Input MSB . . . LSB Analog Output 1111 1111 2 × 255/256 × V REF V 1111 1110 2 × 254/256 × V REF V 1000 0001 2 × 129/256 × V REF V 1000 0000 VREF V 0111 1111 2 × 127/256 × V REF V 0000 0001 2 × V REF/256 V 0000 0000 0 V 2.VREF VREF 0 00 01 DAC INPUT CODE FF 80 81 FE 7F Figure 22. DAC Transfer Function |
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