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DAC08 데이터시트(PDF) 11 Page - Analog Devices |
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DAC08 데이터시트(HTML) 11 Page - Analog Devices |
11 / 12 page DAC08 –11– REV. A MULTIPLYING OPERATION The DAC08 provides excellent multiplying performance with an extremely linear relationship between IFS and IREF over a range of 4 mA to 4 mA. Monotonic operation is maintained over a typical range of IREF from 100 µA to 4.0 mA. SETTLING TIME The DAC08 is capable of extremely fast settling times, typically 85 ns at IREF = 2.0 mA. Judicious circuit design and careful board layout must be employed to obtain full performance po- tential during testing and application. The logic switch design enables propagation delays of only 35 ns for each of the 8 bits. Settling time to within 1/2 LSB of the LSB is therefore 35 ns, with each progressively larger bit taking successively longer. The MSB settles in 85 ns, thus determining the overall settling time of 85 ns. Settling to 6-bit accuracy requires about 65 ns to 70 ns. The output capacitance of the DAC08 including the package is approximately 15 pF, therefore the output RC time constant dominates settling time if RL > 500 Ω. Settling time and propagation delay are relatively insensitive to logic input amplitude and rise and fall times, due to the high gain of the logic switches. Settling time also remains essentially constant for IREF values. The principal advantage of higher IREF values lies in the ability to attain a given output level with lower load resistors, thus reducing the output RC time constant. Measurement of settling time requires the ability to accurately resolve ±4 µA, therefore a 1 kΩ load is needed to provide ad- equate drive for most oscilloscopes. The settling time fixture shown in schematic labelled “Settling Time Measurement” uses a cascode design to permit driving a 1 k Ω load with less than 5 pF of parasitic capacitance at the measurement node. At IREF values of less than 1.0 mA, excessive RC damping of the output is difficult to prevent while maintaining adequate sensitivity. However, the major carry from 01111111 to 10000000 provides an accurate indicator of settling time. This code change does not require the normal 6.2 time constants to settle to within ±0.2% of the final value, and thus settling times may be ob- served at lower values of IREF. DAC08 switching transients or “glitches” are very low and may be further reduced by small capacitive loads at the output at a minor sacrifice in settling time. Fastest operation can be obtained by using short leads, minimiz- ing output capacitance and load resistor values, and by adequate bypassing at the supply, reference and VLC terminals. Supplies do not require large electrolytic bypass capacitors as the supply current drain is independent of input logic states; 0.1 µF capaci- tors at the supply pins provide full transient protection. Figure 30. Settling Time Measurement |
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