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ADC10061 Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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ADC10061 Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 14 page Applications Information (Continued) 4.0 INHERENT SAMPLE-AND-HOLD Because the ADC10061, ADC10062, and ADC10064 sample the input signal once during each conversion, they are capable of measuring relatively fast input signals without the help of an external sample-hold. In a non-sampling successive-approximation A/D converter, regardless of speed, the input signal must be stable to better than ±1/2 LSB during each conversion cycle or significant errors will result. Consequently, even for many relatively slow input signals, the signals must be externally sampled and held constant during each conversion if a SAR with no internal sample-and-hold is used. Because they incorporate a direct sample/hold control input, the ADC10061, ADC10062, and ADC10064 are suitable for use in DSP-based systems. The S/H input allows synchro- nization of the A/D converter to the DSP system’s sampling rate and to other ADC10061s, ADC10062s, and ADC10064s. The ADC10061, ADC10062, and ADC10064 can perform accurate conversions of input signals with frequency compo- nents from DC to over 160 kHz. 5.0 POWER SUPPLY CONSIDERATIONS The ADC10061, ADC10062, and ADC10064 are designed to operate from a +5V (nominal) power supply. There are two supply pins, AV CC and DVCC. These pins allow separate external bypass capacitors for the analog and digital portions of the circuit. To guarantee accurate conversions, the two supply pins should be connected to the same voltage source, and each should be bypassed with a 0.1 µF ceramic capacitor in parallel with a 10 µF tantalum capacitor. De- pending on the circuit board layout and other system con- siderations, more bypassing may be necessary. The ADC10061 has a single ground pin, and the ADC10062 and ADC10064 each have separate analog and digital ground pins for separate bypassing of the analog and digital supplies. The devices with separate analog and digital ground pins should have their ground pins connected to the same potential, and all grounds should be “clean” and free of noise. In systems with multiple power supplies, careful attention to power supply sequencing may be necessary to avoid over- driving inputs. The A/D converter’s power supply pins should be at the proper voltage before digital or analog signals are applied to any of the other pins. 6.0 LAYOUT AND GROUNDING In order to ensure fast, accurate conversions from the ADC10061, ADC10062, and ADC10064, it is necessary to use appropriate circuit board layout techniques. The analog ground return path should be low-impedance and free of noise from other parts of the system. Noise from digital circuitry can be especially troublesome, so digital grounds should always be separate from analog grounds. For best performance, separate ground planes should be provided for the digital and analog parts of the system. All bypass capacitors should be located as close to the converter as possible and should connect to the converter and to ground with short traces. The analog input should be isolated from noisy signal traces to avoid having spurious signals couple to the input. Any external component (e.g., a filter capacitor) connected across the converter’s input should be connected to a very clean ground return point. Grounding the component at the wrong point will result in reduced conversion accuracy. 01102015 FIGURE 4. Typical Connection. Note the multiple bypass capacitors on the reference and power supply pins. If V REF− is not grounded, it should also be bypassed to analog ground using multiple capacitors (see 5.0 “Power Supply Considerations”). AGND and DGND should be at the same potential. V IN0 is shown with an input protection network. Pin 17 is normally left open, but optional “speedup” resistor R SA can be used to reduce the conversion time. www.national.com 11 |
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