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DAC08Q Folha de dados(PDF) 10 Page - Analog Devices |
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DAC08Q Folha de dados(HTML) 10 Page - Analog Devices |
10 / 12 page REV. B DAC08 –10– APPLICATION INFORMATION REFERENCE AMPLIFIER SETUP The DAC08 is a multiplying D/A converter in which the output current is the product of a digital number and the input refer- ence current. The reference current may be fixed or may vary from nearly zero to 4.0 mA. The full-scale output current is a linear function of the reference current and is given by: II FR REF =× 255 256 , where IREF = I14 In positive reference applications, an external positive reference voltage forces current through R14 into the VREF(+) terminal (Pin 14) of the reference amplifier. Alternatively, a negative reference may be applied to VREF(–) at Pin 15; reference current flows from ground through R14 into VREF(+) as in the positive reference case. This negative reference connection has the advan- tage of a very high impedance presented at Pin 15. The voltage at Pin 14 is equal to and tracks the voltage at Pin 15 due to the high gain of the internal reference amplifier. R15 (nominally equal to R14) is used to cancel bias current errors; R15 may be elimi- nated with only a minor increase in error. Bipolar references may be accommodated by offsetting VREF or Pin 15. The negative common-mode range of the reference amplifier is given by: VCM– = V– plus (IREF × 1 kΩ) plus 2.5 V. The positive common-mode range is V+ less 1.5 V. When a dc reference is used, a reference bypass capacitor is recommended. A 5.0 V TTL logic supply is not recommended as a reference. If a regulated power supply is used as a reference, R14 should be split into two resistors with the junction bypassed to ground with a 0.1 µF capacitor. For most applications the tight relationship between IREF and IFS will eliminate the need for trimming IREF. If required, full-scale trimming may be accomplished by adjusting the value of R14, or by using a potentiometer for R14. An improved method of full-scale trimming which eliminates potentiometer T.C. effects is shown in the recommended full-scale adjustment circuit. Using lower values of reference current reduces negative power supply current and increases reference amplifier negative com- mon-mode range. The recommended range for operation with a dc reference current is 0.2 mA to 4.0 mA. REFERENCE AMPLIFIER COMPENSATION FOR MULTIPLYING APPLICATIONS AC reference applications will require the reference amplifier to be compensated using a capacitor from Pin 16 to V–. The value of this capacitor depends on the impedance presented to Pin 14: for R14 values of 1.0, 2.5 and 5.0 k Ω, minimum values of CC are 15, 37 and 75 pF. Larger values of R14 require proportion- ately increased values of CC for proper phase margin, so the ratio of CC (pF) to R14 (k Ω) = 15. For fastest response to a pulse, low values of R14 enabling small CC values should be used. If Pin 14 is driven by a high impedance such as a transistor current source, none of the above values will suffice and the amplifier must be heavily compensated which will decrease overall bandwidth and slew rate. For R14 = 1 k Ω and C C = 15 pF, the reference amplifier slews at 4 mA/ µs enabling a transition from IREF = 0 to IREF = 2 mA in 500 ns. Operation with pulse inputs to the reference amplifier may be accommodated by an alternate compensation scheme. This technique provides lowest full-scale transition times. An internal clamp allows quick recovery of the reference amplifier from a cutoff (IREF = 0) condition. Full-scale transition (0 mA to 2 mA) occurs in 120 ns when the equivalent impedance at Pin 14 is 200 Ω and CC = 0. This yields a reference slew rate of 16 mA/µs, which is relatively independent of RIN and VIN values. LOGIC INPUTS The DAC08 design incorporates a unique logic input circuit that enables direct interface to all popular logic families and provides maximum noise immunity. This feature is made pos- sible by the large input swing capability, 2 µA logic input current and completely adjustable logic threshold voltage. For V– = –15 V, the logic inputs may swing between –10 V and +18 V. This enables direct interface with 15 V CMOS logic, even when the DAC08 is powered from a 5 V supply. Minimum input logic swing and minimum logic threshold voltage are given by: V– plus (IREF × 1 kΩ) plus 2.5 V. The logic threshold may be adjusted over a wide range by placing an appropriate voltage at the logic threshold control pin (Pin 1, VLC). The appropriate graph shows the relationship between VLC and VTH over the temperature range, with VTH nominally 1.4 above VLC. For TTL and DTL interface, simply ground pin 1. When interfacing ECL, an IREF = 1 mA is recommended. For interfacing other logic families, see preceding page. For general set-up of the logic control circuit, it should be noted that Pin 1 will source 100 µA typical; external circuitry should be designed to accommodate this current. Fastest settling times are obtained when Pin 1 sees a low imped- ance. If Pin 1 is connected to a 1 k Ω divider, for example, it should be bypassed to ground by a 0.01 µF capacitor. ANALOG OUTPUT CURRENTS Both true and complemented output sink currents are provided where IO + IO = IFS. Current appears at the “true” (IO) output when a “1” (logic high) is applied to each logic input. As the binary count increases, the sink current at pin 4 increases pro- portionally, in the fashion of a “positive logic” D/A converter. When a “0” is applied to any input bit, that current is turned off at Pin 4 and turned on at Pin 2. A decreasing logic count increases IO as in a negative or inverted logic D/A converter. Both outputs may be used simultaneously. If one of the outputs is not required, it must be connected to ground or to a point capable of sourcing IFS; do not leave an unused output pin open. Both outputs have an extremely wide voltage compliance enabling fast direct current-to-voltage conversion through a resistor tied to ground or other voltage source. Positive compliance is 36 V above V– and is independent of the positive supply. Negative compliance is given by V– plus (IREF × 1 kΩ) plus 2.5 V. The dual outputs enable double the usual peak-to-peak load swing when driving loads in quasi-differential fashion. This feature is especially useful in cable driving, CRT deflection and in other balanced applications such as driving center-tapped coils and transformers. POWER SUPPLIES The DAC08 operates over a wide range of power supply voltages from a total supply of 9 V to 36 V. When operating at supplies of ±5 V or less, IREF ≤ 1 mA is recommended. Low reference current operation decreases power consumption and increases negative compliance, reference amplifier negative common-mode |
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