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AD7845SQ883B Fiches technique(PDF) 9 Page - Analog Devices |
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AD7845SQ883B Fiches technique(HTML) 9 Page - Analog Devices |
9 / 12 page AD7845 REV. B –9– Figure 17. Programmable Current Sink 4–20 mA CURRENT LOOP The AD7845 provides an excellent way of making a 4-20 mA current loop circuit. This is basically a variation of the circuits in Figures 16 and 17 and is shown in Figure 18. The application resistor RA (Value 4R) produces the effective 4 mA offset. IL = I3 = I2 + I1 Since I2 > I1, IL = – V X 156 = 2.5 4R × R FB + 2.5 RDAC × D × R FB × 1 156 and since RDAC=RFB=R IL = 2.5 4 + D × 2.5 × 1000 156 mA = [4 + (16 × D)]mA, where D goes from 0 to 1 with Digital Code When D = 0 (Code of all 0s): IL = 4 mA When D = 1 (Code of all 1s): IL = 20 mA The above circuit succeeds in significantly reducing the circuit component count. Both the on-chip output amplifier and the application resistor RA contribute to this. Figure 18. 4–20 mA Current Loop APPLICATION HINTS General Ground Management: AC or transient voltages between AGND and DGND can cause noise injection into the analog output. The simplest method of ensuring that voltages at AGND and DGND are equal is to tie AGND and DGND together at the AD7845. In more complex systems where the AGND and DGND intertie is on the backplane, it is recom- mended that two diodes be connected in inverse parallel be- tween the AD7845 AGND and DGND pins (IN914 or equivalent). Digital Glitches: When a new digital word is written into the DAC, it results in a change of voltage applied to some of the DAC switch gates. This voltage change is coupled across the switch stray capacitance and appears as an impulse on the cur- rent output bus of the DAC. In the AD7845, impulses on this bus are converted to a voltage by RFB and the output amplifier. The output voltage glitch energy is specified as the area of the resulting spike in nV-seconds. It is measured with VREF con- nected to analog ground and for a zero to full-scale input code transition. Since microprocessor based systems generally have noisy grounds which couple into the power supplies, the AD7845 VDD and VSS terminals should be decoupled to signal ground. Temperature Coefficients: The gain temperature coefficient of the AD7845 has a maximum value of 5 ppm/ °C. This corre- sponds to worst case gain shift of 2 LSBs over a 100 °C tem- perature range. When trim resistors R1 and R2 in Figure 13 are used to adjust full-scale range, the temperature coefficient of R1 and R2 must be taken into account. The offset tempera- ture coefficient is 5 ppm of FSR/ °C maximum. This corre- sponds to a worst case offset shift of 2 LSBs over a 100 °C temperature range. The reader is referred to Analog Devices Application Note “Gain Error and Gain Temperature Coefficient of CMOS Mul- tiplying DACs,” Publication Number E630C-5-3/86. |
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Description similaire - AD7845SQ883B |
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