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AD7112 데이터시트(PDF) 6 Page - Analog Devices |
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AD7112 데이터시트(HTML) 6 Page - Analog Devices |
6 / 12 page AD7112 REV. 0 –6– INTERFACE LOGIC INFORMATION DAC Selection Both DAC latches share a common 8-bit port. The control in- put DAC A/DAC B selects which DAC can accept data from the input port. Mode Selection Inputs CS and WR control the operating mode of the selected DAC. See the Mode Selection Table below. Write Mode When CS and WR are both low the DAC is in the write mode. The input data latches of the selected DAC are transparent and its analog output responds to activity on DB0–DB7. Hold Mode The selected DAC latch retains the data which was present on DB0–DB7 just prior to CS and WR assuming a high state. Both analog outputs remain at the values corresponding to the data in their respective latches. Mode Selection Table DACA / DAC B CS WR DAC A DAC B L L L WRITE HOLD H L L HOLD WRITE X H X HOLD HOLD X X H HOLD HOLD L = Low State, VIL; H = High State, VIH; X = Don’t Care. t AS DAC A/DAC B t AH t WR t DS VIH VIL t DH t CS t CH DB0–DB7 CS WR NOTES 1. ALL INPUT SIGNAL RISE AND FALL TIMES MEASURED FROM 10% TO 90% OF VDD. tR = tF = 20ns. 2. CONTROL TIMING MEASUREMENT REFERENCE LEVEL = (VIH + VIL) / 2 Figure 3. Write Cycle Timing Diagram DYNAMIC PERFORMANCE The dynamic performance of the AD7112 will depend on the gain and phase characteristics of the output amplifier, together with the optimum choice of PC board layout and decoupling components. Circuit layout is most important if the optimum performance of the AD7112 is to be achieved. Most application problems stem from either poor layout, grounding errors, or in- appropriate choice of amplifier. Ensure that the layout of the printed circuit board has the digital and analog lines separated as much as possible. Take care not to run any digital track alongside an analog signal track. Establish a single point analog ground (star ground) separate from the logic system ground. Place this ground as close as possible to the AD7112. Connect all analog grounds to this star ground, and also connect the AD7112 DGND to this ground. Do not connect any other digi- tal grounds to this analog ground point. Low impedance analog and digital power supply common returns are essential for low noise and high performance of these converters, therefore the foil width of these tracks should be as wide as possible. The use of ground planes is recommended as this minimizes impedance paths and also guards the analog circuitry from digital noise. It is recommended that when using the AD7112 with a high speed amplifier, a capacitor (C1) be connected in the feedback path as shown in Figure 2. This capacitor which should be be- tween 5 pF and 15 pF, compensates for the phase lag intro- duced by the output capacitance of the D/A converter. Figures 4 and 5 show the performance of the AD7112 using the AD712, a high speed, low cost BiFET amplifier, and the OP275, a dual bipolar/JFET amplifier suitable for audio appli- cations. The performance with and without the compensation capacitor is shown in both cases. For operation beyond 250 kHz, capacitor C1 may be reduced in value. This gives an increase in bandwidth at the expense of a poorer transient re- sponse as shown in Figure 7. In circuits where C1 is not in- cluded, the high frequency roll-off point is primarily determined by the characteristics of the output amplifier and not the AD7112. Feedthrough and accuracy are sensitive to output leakage cur- rents effects. For this reason it is recommended that the operat- ing temperature of the AD7112 be kept as close to +25 °C as is practically possible, particularly where the devices performance at high attenuation levels is important. A typical plot of leakage current vs. temperature is shown in Figure 11. Some solder fluxes and cleaning materials can form slightly conductive films which cause leakage effects between analog in- put and output. The user is cautioned to ensure that the manu- facturing process for circuits using the AD7112 does not allow such films to form. Otherwise the feedthrough, accuracy and maximum usable range will be affected. STATIC ACCURACY PERFORMANCE The D/A converter section of the AD7112 consists of a 17-bit R–2R type converter. To obtain optimum static performance at this level of resolution it is necessary to pay great attention to amplifier selection, circuit grounding, etc. Amplifier input bias current results in a dc offset at the output of the amplifier due to current flowing in the feedback resistor RFB. It is recommended that amplifiers with input bias currents of less than 10 nA be used (e.g., AD712) to minimize this offset. |
유사한 부품 번호 - AD7112_15 |
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유사한 설명 - AD7112_15 |
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