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DAC08ED Folha de dados(PDF) 9 Page - NXP Semiconductors |
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DAC08ED Folha de dados(HTML) 9 Page - NXP Semiconductors |
9 / 12 page Philips Semiconductors Linear Products Product specification DAC08 Series 8-Bit high-speed multiplying D/A converter August 31, 1994 724 TYPICAL APPLICATION NOTES: REQ = RIN || RP Typical Values RIN = 5kΩ +VIN = 10V Pulsed Referenced Operation OPTIONAL RESISTOR FOR OFFSET INPUTS NO CAP 14 15 16 2 4 +VREF RREF REQ =200 Ω RP RIN 0V FUNCTIONAL DESCRIPTION Reference Amplifier Drive and Compensation The reference amplifier input current must always flow into Pin 14 regardless of the setup method or reference supply voltage polarity. Connections for a positive reference voltage are shown in Figure 1. The reference voltage source supplies the full reference current. For bipolar reference signals, as in the multiplying mode, R15 can be tied to a negative voltage corresponding to the minimum input level. R15 may be eliminated with only a small sacrifice in accuracy and temperature drift. The compensation capacitor value must be increased as R14 value is increased. This is in order to maintain proper phase margin. For R14 values of 1.0, 2.5, and 5.0kΩ, minimum capacitor values are 15, 37, and 75pF, respectively. The capacitor may be tied to either VEE or ground, but using VEE increases negative supply rejection. (Fluctuations in the negative supply have more effect on accuracy than do any changes in the positive supply.) A negative reference voltage may be used if R14 is grounded and the reference voltage is applied to R15 as shown. A high input impedance is the main advantage of this method. The negative reference voltage must be at least 3.0V above the VEE supply. Bipolar input signals may be handled by connecting R14 to a positive reference voltage equal to the peak positive input level at Pin 15. When using a DC reference voltage, capacitive bypass to ground is recommended. The 5.0V logic supply is not recommended as a reference voltage, but if a well regulated 5.0V supply which drives logic is to be used as the reference, R14 should be formed of two series resistors with the junction of the two resistors bypassed with 0.1 µF to ground. For reference voltages greater than 5.0V, a clamp diode is recommended between Pin 14 and ground. If Pin 14 is driven by a high impedance such as a transistor current source, none of the above compensation methods applies and the amplifier must be heavily compensated, decreasing the overall bandwidth. Output Voltage Range The voltage at Pin 4 must always be at least 4.5V more positive than the voltage of the negative supply (Pin 3) when the reference current is 2mA or less, and at least 8V more positive than the negative supply when the reference current is between 2mA and 4mA. This is necessary to avoid saturation of the output transistors, which would cause serious accuracy degradation. Output Current Range Any time the full-scale current exceeds 2mA, the negative supply must be at least 8V more negative than the output voltage. This is due to the increased internal voltage drops between the negative supply and the outputs with higher reference currents. Accuracy Absolute accuracy is the measure of each output current level with respect to its intended value, and is dependent upon relative accuracy, full-scale accuracy and full-scale current drift. Relative accuracy is the measure of each output current level as a fraction of the full-scale current after zero-scale current has been nulled out. The relative accuracy of the DAC08 series is essentially constant over the operating temperature range due to the excellent temperature tracking of the monolithic resistor ladder. The reference current may drift with temperature, causing a change in the absolute accuracy of output current. However, the DAC08 series has a very low full-scale current drift over the operating temperature range. The DAC08 series is guaranteed accurate to within ± LSB at +25°C at a full-scale output current of 1.992mA. The relative accuracy test circuit is shown in Figure 1. The 12-bit converter is calibrated to a full-scale output current of 1.99219mA, then the DAC08 full-scale current is trimmed to the same value with R14 so that a zero value appears at the error amplifier output. The counter is activated and the error band may be displayed on the oscilloscope, detected by comparators, or stored in a peak detector. Two 8-bit D-to-A converters may not be used to construct a 16-bit accurate D-to-A converter. 16-bit accuracy implies a total of ± part in 65,536, or ±0.00076%, which is much more accurate than the ±0.19% specification of the DAC08 series. Monotonicity A monotonic converter is one which always provides analog output greater than or equal to the preceding value for a corresponding increment in the digital input code. The DAC08 series is monotonic for all values of reference current above 0.5mA. The recommended range for operation is a DC reference current between 0.5mA and 4.0mA. Settling Time The worst-case switching condition occurs when all bits are switched on, which corresponds to a low-to-high transition for all input bits. This time is typically 70ns for settling to within LSB for 8-bit accuracy. This time applies when RL<500Ω and CO<25pF. The slowest single switch is the least significant bit, which typically turns on and settles in 65ns. In applications where the DAC functions in a positive-going ramp mode, the worst-case condition does not occur and settling times less than 70ns may be realized. Extra care must be taken in board layout since this usually is the dominant factor in satisfactory test results when measuring settling time. Short leads, 100 µF supply bypassing for low frequencies, minimum scope lead length, and avoidance of ground loops are all mandatory. |
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