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ADC08062 数据表(PDF) 13 Page - National Semiconductor (TI) |
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ADC08062 数据表(HTML) 13 Page - National Semiconductor (TI) |
13 / 16 page Application Information (Continued) minimum output voltages above GND can also be compen- sated by connecting V REF− to a voltage that is equal to this minimum voltage. By reducing V REF (VREF = VREF+ −VREF−) to less than 5V, the sensitivity of the converter can be in- creased (i.e., if V REF = 2.5V, then 1 LSB = 9.8 mV). The ADC08061/2’s reference arrangement also facilitates ratio- metric operation and in many cases the ADC08061/2’s power supply can be used for transducer power as well as the V REF source. Ratiometric operation is achieved by con- necting V REF− to GND and connecting VREF+ and a trans- ducer’s power supply input to V +. The ADC08061/2’s linear- ity degrades when V REF+ −|VREF−| is less than 2.0V. The voltage at V REF− sets the input level that produces a digital output of all zeros. Though V IN is not itself differential, the reference design affords nearly differential-input capabil- ity for some measurement applications. Figure 7 shows one possible differential configuration. It should be noted that, while the two V REF inputs are fully differential, the digital output will be zero for any analog input voltage if V REF− ≥ VREF+. 4.0 ANALOG INPUT AND SOURCE IMPEDANCE The ADC08061/2’s analog input circuitry includes an analog switch with an “on” resistance of 70 Ω and capacitance of 1.4 pF and 12 pF (see Figure 7). The switch is closed during the A/D’s input signal acquisition time (while WR is low when using the WR -RD Mode). A small transient current flows into the input pin each time the switch closes. A transient voltage, whose magnitude can increase as the source impedance in- creases, may be present at the input. So long as the source impedance is less than 500 Ω, the input voltage transient will not cause errors and need not be filtered. Large source impedances can slow the charging of the sam- pling capacitors and degrade conversion accuracy. There- fore, only signal sources with output impedances less than 500 Ω should be used if rated accuracy is to be achieved at the minimum sample time (100 ns maximum). A signal source with a high output impedance should have its output buffered with an operational amplifier. Any ringing or voltage shifts at the op amp’s output during the sampling period can result in conversion errors. Correct conversion results will be obtained for input voltages greater than GND − 100 mV and less than V+ + 100 mV. Do not allow the signal source to drive the analog input pin more than 300 mV higher than V +, or more than 300 mV lower than GND. The current flowing through any analog input pin should be limited to 5 mA or less to avoid permanent dam- age to the IC if an analog input pin is forced beyond these voltages. The sum of all the overdrive currents into all pins must be less than 20 mA. Some sort of protection scheme should be used when the input signal is expected to extend more than 300 mV beyond the power supply limits. A simple protection network using resistors and diodes is shown in Figure 9. 6.0 INHERENT SAMPLE-AND-HOLD An important benefit of the ADC08061/2’s input architecture is the inherent sample-and-hold (S/H) and its ability to mea- sure relatively high speed signals without the help of an ex- ternal S/H. In a non-sampling converter, regardless of its speed, the input must remain stable to at least 1⁄2 LSB throughout the conversion process if full accuracy is to be maintained. Consequently, for many high speed signals, this signal must be externally sampled and held stationary during the conversion. The ADC08061 and ADC08062 are suitable for DSP-based systems because of the direct control of the S/H through the WR signal. The WR input signal allows the A/D to be syn- chronized to a DSP system’s sampling rate or to other ADC08061 and ADC08062s. DS011086-19 *Represents a multiplexer channel in the ADC08062. FIGURE 7. ADC08061 and ADC08062 Equivalent Input Circuit Model www.national.com 13 |
类似零件编号 - ADC08062 |
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类似说明 - ADC08062 |
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