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ADS5221 Datasheet(PDF) 10 Page - Texas Instruments |
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ADS5221 Datasheet(HTML) 10 Page - Texas Instruments |
10 / 19 page ADS5221 10 SBAS262A www.ti.com INPUT COMMON-MODE VOLTAGE The ADS5221 operates from a single +3.3V supply, and requires a external common-mode voltage. This allows a symmetrical signal swing while maintaining sufficient head- room to the supply rail. The common-mode voltage can be from an external DC voltage source, for example, an analog +3.3V supply with a simple resister divider, or from the input signal source with DC-coupling. When it is a single-ended input configuration, the common-mode voltage is typically +1.25V. When the input configuration is differential, the common-mode voltage is +1.5V. INPUT FULL SCALE RANGE The input full-scale range (FSR) of the ADS5221 is select- able from +1V to +2V and any value within this range, by setting the reference select pin RSEL and reference voltage pin VREF (see Table I). The input FSR (differential) is always twice VREF (the voltage at the VREF pin) for all reference modes. By choosing different signal input ranges, trade-offs can be made between noise and distortion performance. For maxi- mizing SNR, the maximum signal input 2VPP range may be selected. This range may also be selected for low-level (–6dBFS to –40dBFS) but high-frequency multi-tone input. The signal distortion at the output of the device could be sensitive to the input signal with large full-scale. Reducing signal amplitude will improve the distortion performance. The large input signal amplitude of the converter will impose additional design constraints to the op amp due to its head- room requirements when the op amp is from single-supply and DC-coupling. DIFFERENTIAL INPUTS The ADS5221 input structure is designed to accept both a single-end or differential analog signal. Distortion perfor- mance, however, can be improved by utilizing the differential input configuration. Differential operation of the ADS5221 requires that an input signal at the inputs (IN, IN) has the same amplitude and is 180 degrees out-of-phase. Differen- tial signals offer a number of advantages: • The signal amplitude is half that required for the single- ended operation, and is therefore less demanding to achieve, while maintaining good linearity performance from the signal source. • The reduced signal swing allows for more headroom of the interface circuitry, and therefore also allows a wider selec- tion of the best suitable driver amplifier. • Minimization of even-order harmonics. • Improved noise immunity based on the common-mode input rejection of the converter. ANALOG INPUT DRIVEN BY TRANSFORMER The ADS5221 can be driven by a transformer, which pro- vides signal AC-coupling and allows a signal conversion from single-ended input to differential output, or from single-ended input to single-ended output. Using a transformer offers a number of advantages. As a passive component, it does not add to the total noise and has better harmonics in wide frequency bands, compared to an op amp driver. By using a step-up transformer, further signal amplification can be real- ized; as a result, the signal swing from the source can be reduced. For transformer selection, it is important to carefully examine the application requirements and determine the correct model, the desired impedance ratio, and frequency characteristics. Furthermore, the appropriate model must support the targeted distortion level and should not exhibit any core saturation at full-scale voltage levels. A variety of miniature RF transformers from different manufacturers, (such as, Mini-Circuits, Coilcraft, or Trak) can be selected. Figure 3 shows a transformer-coupled input configuration of the ADS5221. The ADS5221 receives a differential AC signal from the output of the transformer and common-mode volt- age of +1.5V from the center tap. A source termination resistor, RT , is required, which can be placed at the input or output of the transformer to satisfy the termination require- ments of the source impedance, RS. The circuit also shows the use of an additional RC low-pass filter placed in series with each converter input to attenuate some of the wideband noise. The resistor values are typically in the range of 10 Ω to 50 Ω, and capacitors are in the range of 10pF to 100pF for individual application requirements. FIGURE 3. Transformer-Coupled Differential Input Configura- tion of ADS5221. ADS5221 24.9 Ω 24.9 Ω 22pF IN IN 0.1 µF R S R T T4-6T 1.7k Ω 1.5k Ω 1.5V AV DD 3.3V |
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