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DAC0832LJ Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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DAC0832LJ Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 28 page DAC0830 Series Application Hints (Continued) Pulling WR 2 low will then update the analog output. A logic “1” on either of these lines will prevent the changing of the analog output. 1.2 Single-Buffered Operation In a microprocessor controlled system where maximum data throughput to the DAC is of primary concern, or when only one DAC of several needs to be updated at a time, a single-buffered configuration can be used. One of the two internal registers allows the data to flow through and the other register will serve as the data latch. Digital signal feedthrough (see Section 1.5) is minimized if the input register is used as the data latch. Timing for this mode is shown in Figure 4. Single-buffering in a “stand-alone” system is achieved by strobing WR 1 low to update the DAC with CS, WR2 and XFER grounded and ILE tied high. 1.3 Flow-Through Operation Though primarily designed to provide microprocessor inter- face compatibility, the MICRO-DAC’s can easily be config- ured to allow the analog output to continuously reflect the state of an applied digital input. This is most useful in appli- cations where the DAC is used in a continuous feedback control loop and is driven by a binary up-down counter, or in function generation circuits where a ROM is continuously providing DAC data. Simply grounding CS, WR 1,WR2, and XFER and tying ILE high allows both internal registers to follow the applied digital inputs (flow-through) and directly affect the DAC analog output. 1.4 Control Signal Timing When interfacing these MICRO-DAC to any microprocessor, there are two important time relationships that must be con- sidered to insure proper operation. The first is the minimum WR strobe pulse width which is specified as 900 ns for all valid operating conditions of supply voltage and ambient temperature, but typically a pulse width of only 180ns is adequate if V CC=15VDC. A second consideration is that the guaranteed minimum data hold time of 50ns should be met or erroneous data can be latched. This hold time is defined as the length of time data must be held valid on the digital inputs after a qualified (via CS) WR strobe makes a low to high transition to latch the applied data. If the controlling device or system does not inherently meet these timing specs the DAC can be treated as a slow memory or peripheral and utilize a technique to extend the write strobe. A simple extension of the write time, by adding a wait state, can simultaneously hold the write strobe active and data valid on the bus to satisfy the minimum WR pulse- width. If this does not provide a sufficient data hold time at the end of the write cycle, a negative edge triggered one-shot can be included between the system write strobe and the WR pin of the DAC. This is illustrated in Figure 5 for an exemplary system which provides a 250ns WR strobe time with a data hold time of less than 10ns. The proper data set-up time prior to the latching edge (LO to HI transition) of the WR strobe, is insured if the WR pulse- width is within spec and the data is valid on the bus for the duration of the DAC WR strobe. 1.5 Digital Signal Feedthrough When data is latched in the internal registers, but the digital inputs are changing state, a narrow spike of current may flow out of the current output terminals. This spike is caused by the rapid switching of internal logic gates that are responding to the input changes. There are several recommendations to minimize this effect. When latching data in the DAC, always use the input register as the latch. Second, reducing the V CC supply for the DAC from +15V to +5V offers a factor of 5 improvement in the magnitude of the feedthrough, but at the expense of internal logic switching speed. Finally, increasing C C (Figure 8)toa value consistent with the actual circuit bandwidth require- ments can provide a substantial damping effect on any output spikes. 00560807 ILE=LOGIC “1”; WR2 and XFER GROUNDED FIGURE 4. www.national.com 11 |
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