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DAC0808LCM Folha de dados(PDF) 8 Page - National Semiconductor (TI) |
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DAC0808LCM Folha de dados(HTML) 8 Page - National Semiconductor (TI) |
8 / 10 page Application Hints (Continued) A negative reference voltage may be used if R14 is ground- ed and the reference voltage is applied to R15 as shown in Figure 8 A high input impedance is the main advantage of this method Compensation involves a capacitor to VEE on pin 16 using the values of the previous paragraph The neg- ative reference voltage must be at least 4V above the VEE supply Bipolar input signals may be handled by connecting R14 to a positive reference voltage equal to the peak posi- tive input level at pin 15 When a DC reference voltage is used capacitive bypass to ground is recommended The 5V logic supply is not recom- mended as a reference voltage If a well regulated 5V sup- ply which drives logic is to be used as the reference R14 should be decoupled by connecting it to 5V through another resistor and bypassing the junction of the 2 resistors with 01 mF to ground For reference voltages greater than 5V 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 apply and the amplifier must be heavily compensated de- creasing the overall bandwidth OUTPUT VOLTAGE RANGE The voltage on pin 4 is restricted to a range of b055 to 04V when VEE eb5V due to the current switching meth- ods employed in the DAC0808 The negative output voltage compliance of the DAC0808 is extended to b5V where the negative supply voltage is more negative than b10V Using a full-scale current of 1992 mA and load resistor of 25 kX between pin 4 and ground will yield a voltage output of 256 levels between 0 and b 4980V Floating pin 1 does not affect the converter speed or power dissipation However the value of the load resistor determines the switching time due to increased volt- age swing Values of RL up to 500X do not significantly affect performance but a 25 kX load increases worst-case settling time to 12 ms (when all bits are switched ON) Refer to the subsequent text section on Settling Time for more details on output loading OUTPUT CURRENT RANGE The output current maximum rating of 42 mA may be used only for negative supply voltages more negative than b8V due to the increased voltage drop across the resistors in the reference current amplifier ACCURACY Absolute accuracy is the measure of each output current level with respect to its intended value and is dependent upon relative accuracy and full-scale current drift Relative accuracy is the measure of each output current level as a fraction of the full-scale current The relative accuracy of the DAC0808 is essentially constant with temperature 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 cur- rent However the DAC0808 has a very low full-scale cur- rent drift with temperature The DAC0808 series is guaranteed accurate to within g LSB at a full-scale output current of 1992 mA This corre- sponds to a reference amplifier output current drive to the ladder network of 2 mA with the loss of 1 LSB (8 mA) which is the ladder remainder shunted to ground The input current to pin 14 has a guaranteed value of between 19 and 21 mA allowing some mismatch in the NPN current source pair The accuracy test circuit is shown in Figure 4 The 12- bit converter is calibrated for a full-scale output current of 1992 mA This is an optional step since the DAC0808 accu- racy is essentially the same between 15 and 25 mA Then the DAC0808 circuits’ 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 an 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 accuracy D-to-A converter 16-bit accuracy implies a total error of g of one part in 65536 or g000076% which is much more accurate than the g0019% specifica- tion provided by the DAC0808 MULTIPLYING ACCURACY The DAC0808 may be used in the multiplying mode with 8-bit accuracy when the reference current is varied over a range of 2561 If the reference current in the multiplying mode ranges from 16 mA to 4 mA the additional error con- tributions are less than 16 mA This is well within 8-bit accu- racy when referred to full-scale A monotonic converter is one which supplies an increase in current for each increment in the binary word Typically the DAC0808 is monotonic for all values of reference current above 05 mA The recommended range for operation with a DC reference current is 05 to 4 mA SETTLING TIME The worst-case switching condition occurs when all bits are switched ON which corresponds to a low-to-high transition for all bits This time is typically 150 ns for settling to within g LSB for 8-bit accuracy and 100 ns to LSB for 7 and 6-bit accuracy The turn OFF is typically under 100 ns These times apply when RL s 500X and CO s 25 pF Extra care must be taken in board layout since this is usually the dominant factor in satisfactory test results when mea- suring settling time Short leads 100 mF supply bypassing for low frequencies and minimum scope lead length are all mandatory 8 |
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