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ADS803U1K Datasheet(PDF) 8 Page - Texas Instruments |
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ADS803U1K Datasheet(HTML) 8 Page - Texas Instruments |
8 / 18 page ADS803 8 SBAS074B www.ti.com FIGURE 2. DC-Coupled, Single-Ended Input Configuration with DC-Level Shift. 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 in- duced errors of the amplifier. The addition of a small series resistor (RS) between the output of the op amp and the input of the ADS803 will be beneficial in almost all interface configu- rations. 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 performance, the resistor value should be kept below 50 Ω. Furthermore, the series resistor together with the 100pF capacitor, establish a passive low-pass filter, limiting the bandwidth for the wideband noise thus help improving the SNR 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 ADS803. 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. For those applications requiring the driving amplifier to provide a signal amplification (with a gain ≥ 3), consider using the decompensated voltage-feed- back op amp OPA643. DC-COUPLED WITH LEVEL SHIFT Several applications may require that the bandwidth of the signal path includes DC, in which case the signal has to be DC-coupled to the A/D converter. In order to accomplish this, the interface circuit has to provide a DC-level shift. The circuit presented in Figure 2 employs an op amp, A1, to sum the ground centered input signal with a required DC offset. OPA691 V IN +V S 2Vp-p 0 +1V –1V R F R IN NOTE: R F = RIN, G = –1 +V S ADS803 R S 24.9 Ω 2k Ω 100pF 10 µF + +2.5V R 2 R 1 2k Ω IN IN REFB (+1V) V REF SEL REFT 0.1 µF 0.1 µF The ADS803 typically operates with a +2.5V common-mode voltage, which is established at the center tap of the ladder 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. Due to 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 ADS803 will effectively produce an offset, which can be corrected for by adjusting the values of resistors R1 and R2. The bias current of the op amp may also result in an undesired offset. The selection criteria of the appropriate op amp should include the input bias current, output voltage swing, distortion, and noise specification. Note that in this example the overall signal phase is inverted. To re-estab- lish the original signal polarity it is always possible to interchange the IN and IN connections. SINGLE-ENDED-TO-DIFFERENTIAL CONFIGURATION (TRANSFORMER COUPLED) In order to select the best suited interface circuit for the ADS803, the performance requirements must be known. If an ac-coupled input is needed for a particular application, the next step is to determine the method of applying the signal; either single-ended or differentially. The differential input configuration may provide a noticeable advantage of achiev- ing good SFDR performance based on the fact that in the differential mode, the signal swing can be reduced to half of the swing required for single-ended drive. Secondly, by driving the ADS803 differentially, the even-order harmonics will be reduced. See Figure 3 for the schematic of the suggested transformer coupled interface circuit. The resistor across the secondary side (RT) should be set to get an input impedance match (e.g., RT = n2 • RG). |
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