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AD7877 データシート(PDF) 28 Page - Analog Devices |
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AD7877 データシート(HTML) 28 Page - Analog Devices |
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28 / 46 page  Data Sheet AD7877 When the DAC output voltage is zero, it sinks the maximum current through R1. The feedback current and, therefore, VOUT are at their maximum. As the DAC output voltage increases, the sink current and, thus, the feedback current decrease, and VOUT falls. If the DAC output exceeds VREF, it starts to source current, and VOUT has to further decrease to compensate. When the DAC output is at full scale, VOUT is at its minimum. Note that the effect of the DAC on VOUT is opposite in voltage mode to that in current mode. In current mode, increasing DAC code increases the sink current, so VOUT increases with increasing DAC code. In voltage mode, increasing DAC code increases the DAC output voltage, reducing the sink current. Calculate the resistor values as follows: 1. Decide on the feedback current as before. 2. Calculate the parallel combination of R1 and R3 when the DAC output is zero RP = VREF/IFB 3. Calculate R2 as before, but use RP and VOUT(MAX) R2 = RP(VOUT(MAX) − VREF)/VREF 4. Calculate the change in feedback current between minimum and maximum output voltages as before using ∆I = VR2(MAX)/R2 − VR2(MIN)/R2 This is equal to the change in current through R1 between zero output and full scale, which is also given by ∆I = current at zero − current at full scale = V/R1 − (VREF − V)/R1 = V/R1 5. R1 = VFS/∆. 6. Calculate R3 from R1 and R using R3 = (R1 × RP)/(R1 − RP) Example: 1. VCC = 5 V and VFS = VCC. VOUT(MIN) is 20 V and VOUT(MAX) is 25 V. VREF is 1.25 V. Allow 100 µA around the feedback loop. 2. RP = 1.25 V/100 µA = 12.5 kΩ. 3. R2 = 12.5 kΩ × (25 Ω − 1.25 Ω)/1.25 Ω = 237 kΩ. Use the nearest preferred value of 240 kΩ. 4. ∆I = 25 V/240 kΩ − 20 V/240 kΩ = 21 µA. 5. R1 = 5 V/21 µA = 238 kΩ. Use the nearest preferred value of 250 kΩ. 6. R3 = (180 kΩ × 12.5 kΩ)/(180 kΩ − 12.5 kΩ) = 13.4 kΩ. Use nearest preferred value of 13 kΩ. The actual adjustment range using these values is 21 V to 26 V. Rev. E | Page 27 of 45 |
同様の部品番号 - AD7877_17 |
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同様の説明 - AD7877_17 |
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