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ADS805U1K Datasheet(PDF) 8 Page - Texas Instruments |
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ADS805U1K Datasheet(HTML) 8 Page - Texas Instruments |
8 / 20 page ADS805 8 SBAS073B www.ti.com APPLICATION INFORMATION DRIVING THE ANALOG INPUT The ADS805 allows its analog inputs to be driven either single-ended or differentially. The focus of the following discussion is on the single-ended configuration. Typically, its implementation is easier to achieve and the rated specifica- tions for the ADS805 are characterized using the single- ended mode of operation. AC-COUPLED INPUT CONFIGURATION Given in Figure 1 is the circuit example of the most common interface configuration for the ADS805. With the VREF pin connected to the SEL pin, the full-scale input range is defined to be 2Vp-p. This signal is ac-coupled in single-ended form to the ADS805 using the low distortion voltage-feedback amplifier OPA642. As is generally necessary for single- supply components, operating the ADS805 with a full-scale input signal swing requires a level-shift of the amplifier’s zero centered analog signal to comply with the ADC’s input range requirements. Using a DC-blocking capacitor between the output of the driving amplifier and the converter’s input, a simple level-shifting scheme can be implemented. In this configuration, the top and bottom references (REFT, REFB) provide an output voltage of +3V and +2V, respectively. Here, two resistor pairs (2 • 2k Ω) are used to create a common-mode voltage of approximately +2.5V to bias the inputs of the ADS805 (IN, IN) to the required DC voltage. An advantage of ac-coupling is that the driving amplifier still operates with a ground-based signal swing. This will keep the distortion performance at its optimum since the signal swing stays within the linear region of the op amp and sufficient headroom to the supply rails can be maintained. Consider using the inverting gain configuration to eliminate CMR induced errors of the amplifier. The addition of a small series resistor (RS) between the output of the op amp and the input of the ADS805 will be beneficial in almost all interface configurations. This will decouple the op amp’s output from the capacitive load and avoid gain peaking, which can result in increased noise. For best spurious and distortion perfor- mance, the resistor value should be kept below 100 Ω. Furthermore, the series resistor, together with the 100pF capacitor, establish a passive low-pass filter, limiting the bandwidth for the wideband noise, thus helping improve the signal-to-noise performance. DC-COUPLED WITHOUT LEVEL SHIFT In some applications the analog input signal may already be biased at a level which complies with the selected input range and reference level of the ADS805. In this case, it is only necessary to provide an adequately low source imped- ance to the selected input, IN or IN. Always consider wideband op amps since their output impedance will stay low over a wide range of frequencies. DC-COUPLED WITH LEVEL SHIFT Several applications may require that the bandwidth of the signal path include DC, in which case the signal has to be DC- coupled to the ADC. In order to accomplish this, the interface circuit has to provide a DC-level shift. The circuit presented in Figure 2 utilizes the single-supply, current-feedback op amp OPA681 (A1), to sum the ground-centered input signal with a required DC offset. The ADS805 typically operates with a +2.5V common-mode voltage, which is established with resis- tors R3 and R4 and connected to the IN input of the converter. Amplifier A1 operates in inverting configuration. Here, resistors R1 and R2 set the DC-bias level for A1. Because of the op amp’s noise gain of +2V/V, assuming RF = RIN, the DC offset voltage applied to its noninverting input has to be divided down to +1.25V, resulting in a DC output voltage of +2.5V. DC voltage differences between the IN and IN inputs of the ADS805 effectively will produce an offset, which can be cor- rected for by adjusting the values of resistors R1 and R2. The bias current of the op amp may also result in an undesired FIGURE 1. AC-Coupled Input Configuration for 2Vp-p Input Swing and Common-Mode Voltage at +2.5V Derived from Internal Top and Bottom Reference. OPA642 V IN +V IN 0V –V IN R F 402 Ω R G 402 Ω ADS805 R S 24.9 Ω 2k Ω 2k Ω 2k Ω 2k Ω +2.5V 100pF 0.1 µF 0.1 µF 2Vp-p +5V –5V IN IN (+2V) REFB (+1V) V REF SEL REFT (+3V) |
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