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HI1-674AJD-5 Datasheet(PDF) 9 Page - Intersil Corporation |
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HI1-674AJD-5 Datasheet(HTML) 9 Page - Intersil Corporation |
9 / 17 page 9 change from the initial (25oC) valuetothe valueat TMIN or TMAX. Power Supply Rejection The standard specifications for the HI-X74A assume use of +5.00V and ±15.00V or ±12.00V supplies. The only effect of power supply error on the performance of the device will be a small change in the full scale calibration. This will result in a linear change in all lower order codes. The specifications show the maximum change in calibration from the initial value with the supplies at the various limits. Code Width A fundamental quantity for A/D converter specifications is the code width. This is defined as the range of analog input values for which a given digital output code will occur. The nominal value of a code width is equivalent to 1 least significant bit (LSB) of the full scale range or 2.44mV out of 10V for a 12-bit ADC. Quantization Uncertainty Analog-to-digital converters exhibit an inherent quantization uncertainty of ±1/2 LSB. This uncertainty is a fundamental characteristic of the quantization process and cannot be reduced for a converter of given resolution. Left-Justified Data The data format used in the HI-X74A is left-justified. This means that the data represents the analog input as a fraction of full-scale, ranging from 0 to . This implies a binary point to the left of the MSB. Applying the HI-X74A For each application of this converter, the ground connections, power supply bypassing, analog signal source, digital timing and signal routing on the circuit board must be optimized to assure maximum performance. These areas are reviewed in the following sections, along with basic operating modes and calibration requirements. Physical Mounting and Layout Considerations LAYOUT Unwanted, parasitic circuit components, (L, R, and C) can make 12-bit accuracy impossible, even with a perfect A/D converter. The best policy is to eliminate or minimize these parasitics through proper circuit layout, rather than try to quantify their effects. The recommended construction is a double-sided printed circuit board with a ground plane on the component side. Other techniques, such as wire-wrapping or point-to-point wiring on vector board, will have an unpredictable effect on accuracy. In general, sensitive analog signals should be routed between ground traces and kept well away from digital lines. If analog and digital lines must cross, they should do so at right angles. Power Supplies Supply voltages to the HI-X74A (+15V, -15V and +5V) must be “quiet” and well regulated. Voltage spikes on these lines can affect the converter’s accuracy, causing several LSBs to flicker when a constant input is applied. Digital noise and spikes from a switching power supply are especially troublesome. If switching supplies must be used, outputs should be carefully filtered to assure “quiet” DC voltage at the converter terminals. Further, a bypass capacitor pair on each supply voltage terminal is necessary to counter the effect of variations in supply current. Connect one pair from pin 1 to 15 (VLOGIC supply), one from pin 7 to 9 (VCC to Analog Common) and onefrom pin 11 to 9(VEE to Analog Common). For each capacitor pair, a 10 µF tantalum type in parallel with a 0.1µF ceramic type is recommended. Ground Connections Pins 9 and 15 should be tied together at the package to guarantee specified performance for the converter. In addition, a wide PC trace should run directly from pin 9 to (usually) +15V common, and from pin 15 to (usually) the +5V Logic Common. If the converter is located some distance from the system’s “single point” ground, make only these connections to pins 9 and 15: Tie them together at the package, and back to the system ground with a single path. This path should have low resistance. (Code dependent currents flow in the VCC,VEE and VLOGIC terminals, but not through the HI-X74A’s Analog Common or Digital Common). Analog Signal Source HI-574A and HI-674A The device chosen to drive the HI-X74A analog input will see a nominal load of 5k Ω (10V range) or 10kΩ (20V range). However, the other end of these input resistors may change ±400mV with each bit decision, creating abrupt changes in current at the analog input. Thus, the signal source must maintain its output voltage while furnishing these step changes in load current, which occur at 1.6 µs and 950ns intervals for the HI-574A and HI-674A, respectively. This requires low output impedance and fast settling by the signal source. The output impedance of an op amp, for example, has an open loop value which, in a closed loop, is divided by the loop gain available at a frequency of interest. The amplifier should have acceptable loop gain at 600kHz for use with the HI-X74A. To check whether the output properties of a signal source are suitable, monitor the HI-X74A’s input (pin 13 or 14) with an oscilloscope while a conversion is in progress. Each of the twelve disturbances should subside in 1 µsor less for the HI-574A and 500ns or less for the HI-674A. (The comparator decision is made about 1.5 µs and 850ns after each code change from the SAR for the HI-574A and HI-674A, respectively.) If the application calls for a Sample/Hold to precede the converter, it should be noted that not all Sample/Holds are compatible with the HI-574A in the manner described above. 4095 4096 HI-574A, HI-674A |
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