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DAC16FS 数据表(PDF) 8 Page - Analog Devices |
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DAC16FS 数据表(HTML) 8 Page - Analog Devices |
8 / 12 page DAC16 REV. B –8– reference amplifier. Completely removing the compensation network would introduce large linearity errors, reference amplifier instability, wideband reference amplifier noise, and poor settling time. Because the DAC exhibits an internal current scaling factor of eight times (8 ×), the reference amplifier requires only 500 µA input current from the user-supplied precision reference for a 4 mA full-scale output current. In applications that do not re- quire such high output currents, good accuracy can be achieved with input reference currents in the range of 350 µA ≤ I REF ≤ 625 µA. The best signal-to-noise ratios, of course, will be achieved with a 625 µA reference current which yields a maxi- mum 5 mA output current. Figure 22 illustrates how to form the reference input current with a REF02 and a 10 k Ω precision resistor. REF GND RREF 10k IREF +15V 0.1 F DAC16 IOUT REF02 IREF = VREF RREF Figure 22. Generating the DAC16’s Reference Input Current Reducing Voltage Reference Noise In data converters of 16-bit and greater resolution, noise is of critical importance. Surprisingly, the integrated voltage refer- ence circuit used may contribute the dominant share of a system’s noise floor, thereby degrading system dynamic range and signal-to-noise ratio. To maximize system dynamic range and SNR, all external noise contributions should be effectively much less than 1/2 LSB. For example, in a 5 V DAC16 applica- tion, one LSB is equivalent to 76 µV. This means that the total wideband noise contribution due to a voltage reference and all other sources should be less than 38 µV rms. These noise levels are not easy targets to hit with standard off-the-shelf reference devices. For example, commercially available references might exhibit 5 µV rms noise from 0.1 Hz to 10 Hz: but, over a 100 kHz bandwidth, its 300 µV rms of noise can easily swamp out a 16-bit system. Such noisy behavior can degrade a DAC’s effec- tive resolution by increasing its differential nonlinearity which, in turn, can lead to nonmonotonic behavior or analog errors. The easiest way to reduce noise in the reference circuit is to band-limit its noise before feeding it to the converter. In the case of the DAC16, the reference is not a voltage, but a current. Illustrated in Figure 23 is a simple way of hand-limiting REF GND R1 5k IREF +15V 0.1 F DAC16 REF02 C1 22 F R2 5k AGND Figure 23. Filtering a Reference’s Wideband Noise voltage reference noise by splitting RREF into two equal resistors and bypassing the common node with a capacitor. To minimize thermally induced errors, R1 and R2 must be electrically and thermally well-matched. Thin-film resistor networks work well here. In this circuit, the parallel combination of R1 and R2 forms a 3 Hz low-pass filter with C1. The only noise source that remains is the thermal noise of R2 which can be a significantly lower noise generator than the voltage reference. Input Coding The unipolar digital input coding of the DAC16 employs nega- tive logic to control the output current; that is, an all zero input code (0000H) yields an output current 1 LSB below full scale. Conversely, an all 1s input code (FFFFH) yields a zero analog current output. An expression for the DAC16’s transfer equa- tion can be expressed by: I OUT = 8 × IREF × 65,535 – Digital Code 65,536 Table II provides the relationship between the digital input codes and the output current of the DAC16. Table II. Unipolar Code Table Digital Input DAC16 Output Word (Hex) Current IOUT Comment 0000 8 × (216 – 1)/216 × I REF Full Scale 7FFE 8 × (215 + 1)/216 × I REF Midscale + 1 LSB 7FFF 8 × (215/216) × I REF Midscale 8000 8 × (215– 1)/216 × I REF Midscale – 1 LSB FFFF 0 Zero Scale Since the DAC16 exhibits a small output voltage compliance on the order of a few millivolts, a high accuracy operational ampli- fier must be used to convert the DAC’s output current to a volt- age. Refer to the section on selecting operation amplifiers for the DAC16. The circuit shown in Figure 24 illustrates a unipolar output configuration. In symbolic form, the transfer equation for this circuit can be expressed by: V O = R3 × 8 × IREF 65,535 – Digital Code 65,536 In this example, the reference input current was set to 500 µA which produces a full-scale output current of 4 mA – 1 LSB. The DAC’s output current was scaled by R3, a 1.25 k Ω resistor, to produce a 5 V full-scale output voltage. Bear in mind that to ensure the highest possible accuracy, matched thin-film resistor networks are almost a necessity, not an option. The resistors used in the circuit must have close tolerance and tight thermal tracking. Table III illustrates the relationship between the input digital code and the circuit’s output voltage for the component values shown. Table III. Unipolar Output Voltage vs. Digital Input Code Digital Input Word Decimal Number in Analog Output (Hex) in DAC Decoder Voltage (V) 0000 65,535 4.999924 7FFE 32,769 2.500076 7FFF 32,768 2.500000 8000 32,767 2.499924 FFFF 0 0 |
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类似说明 - DAC16FS |
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