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AD7394AN データシート(PDF) 11 Page - Analog Devices |
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AD7394AN データシート(HTML) 11 Page - Analog Devices |
11 / 12 page AD7394/AD7395 –11– REV. 0 generated by the microprocessor’s power-on RESET signal, by an output from the microprocessor, or by an external resistor and capacitor. RESET has a Schmitt trigger input which results in a clean reset function when using external resistor/capacitor generated pulses. See the Control-Logic Truth Table I. POWER SHUTDOWN ( SHDN) Maximum power savings can be achieved by using the power shutdown control function. This hardware activated feature is controlled by the active low input SHDN pin. This pin has a Schmitt trigger input which helps to desensitize it to slowly changing inputs. By placing a logic low on this pin the internal consumption of the device is reduced to nano amp levels, guar- anteed to 1.5 µA maximum over the operating temperature range. When the AD7394/AD7395 has been programmed into the power shutdown state, the present DAC register data is maintained as long as VDD remains greater than 2.7 V. Once a wake-up command SHDN = 1 is given, the DAC voltage out- puts will return to their previous values. It typically takes 80 microseconds for the output voltage to fully stabilize. In the shutdown state the DAC output amplifier exhibits an open- circuit with a nominal output resistance of 500 k Ω to ground. If the power shutdown feature is not needed, then the user should tie the SHDN pin to the V DD voltage thereby disabling this function. UNIPOLAR OUTPUT OPERATION This is the basic mode of operation for the AD7394. As shown in Figure 24, the AD7394 has been designed to drive loads as low as 5 k Ω in parallel with 100 pF. The code table for this operation is shown in Table V. 5 VDD DIGITAL INTERFACE CIRCUITRY OMITTED FOR CLARITY. DIGITAL VREF DGND AGND DAC A DAC B EXT REF +2.7V TO +5.5V R C 0.01 F 0.1 F10 F 75k 100pF 75k 100pF VOUTA VOUTB Figure 24. AD7394 Unipolar Output Operation Table IV. Typical Microcontroller Interface Formats MSB BYTE 1 LSB MSB BYTE 0 LSB B15 B14 B13 B12 B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0 X X X X D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0 X XXXXXD9 D8 D7 D6 D5 D4 D3 D2 D1 D0 D11–D0: 12-bit AD7394 DAC data; D9–D0: 10-bit AD7395 DAC data; X = Don’t Care; The MSB of byte 1 is the first bit that is loaded into the SDI input. Table V. Unipolar Code Table Hexadecimal Decimal Output Number Number Voltage (V) in DAC Register in DAC Register [VREF = 2.5 V] FFF 4095 2.4994 801 2049 1.2506 800 2048 1.2500 7FF 2047 1.2494 000 0 0 The circuit can be configured with an external reference plus power supply, or powered from a single dedicated regulator or reference depending on the application performance requirements. BIPOLAR OUTPUT OPERATION Although the AD7395 has been designed for single-supply op- eration, the output can easily be configured for bipolar opera- tion. A typical circuit is shown in Figure 25. This circuit uses a clean regulated +5 V supply for power, which also provides the circuit’s reference voltage. Since the AD7395 output span swings from ground to very near +5 V, it is necessary to choose an external amplifier with a common-mode input voltage range that extends to its positive supply rail. The micropower consumption OP196 has been designed just for this purpose and results in only 50 microamps of maximum current consumption. Connec- tion of the equally valued 470 k Ω resistors results in a differen- tial amplifier mode of operation with a voltage gain of two, which produces a circuit output span of ten volts, that is, –5 V to +5 V. As the DAC is programmed from zero code 000H to mid- scale 200H to full-scale 3FFH, the circuit output voltage VO is set at –5 V, 0 V and +5 V (–1 LSB). The output voltage VO is coded in offset binary according to Equation 4. VOUT = D 512 –1 ×5 (4) where D is the decimal code loaded in the AD7395 DAC regis- ter. Note that the LSB step size is 10/1024 = 10 mV. This cir- cuit has been optimized for micropower consumption including the 470 k Ω gain setting resistors, which should have low tem- perature coefficients to maintain accuracy and matching (prefer- ably the same resistor material, such as metal film). If better stability is required, the power supply could be substituted with a precision reference voltage such as the low dropout REF195, which can easily supply the circuit’s 262 microamps of current, and still provide additional power for the load connected to VOUT. The micropower REF195 is guaranteed to source 10 mA |
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同様の説明 - AD7394AN |
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