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ADS5421 Datasheet(PDF) 9 Page - Texas Instruments |
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ADS5421 Datasheet(HTML) 9 Page - Texas Instruments |
9 / 26 page ADS5421 9 SBAS237E www.ti.com ANALOG INPUTS TYPES OF APPLICATIONS The analog input of the ADS5421 can be configured in various ways and driven with different circuits, depending on the appli- cation and the desired level of performance. Offering an ex- tremely high dynamic range at high input frequencies, the ADS5421 is particularly well suited for communication systems that digitize wideband signals. Features on the ADS5421, like the input range selector, or the option of an external reference, provide the needed flexibility to accommodate a wide range of applications. In any case, the analog interface/driver require- ments should be carefully examined before selecting the appro- priate circuit configuration. The circuit definition should include considerations on the input frequency spectrum and amplitude, as well as the available power supplies. DIFFERENTIAL INPUTS The ADS5421 input structure is designed to accept the applied signal in differential format. Differential operation of the ADS5421 requires an input signal that consists of an in-phase and a 180 ° out-of-phase component simultaneously applied to the inputs (IN, IN). Differential signals offer a number of advantages, which in many applications will be instrumental in achieving the best harmonic performance of the ADS5421: • The signal amplitude is half of 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, a wider selection of the best suitable driver amplifier. • Even-order harmonics are minimized. • Improves the noise immunity based on the common- mode input rejection of the converter. Both inputs are identical in terms of their impedance and performance with the exception that by applying the signal to the complementary input (IN) instead of the IN input will invert the orientation of the input signal relative to the output code. INPUT FULL-SCALE RANGE VERSUS PERFORMANCE Employing dual-supply amplifiers and AC-coupling will usually yield the best results. DC-coupling and/or single-supply ampli- fiers impose additional design constraints due to their head- room requirements, especially when selecting the 4VPP input range. The full-scale input range of the ADS5421 is defined either by the settings of the reference select pins (SEL1, SEL2) or by an external reference voltage (see Table I). By choosing between the different signal input ranges, trade-offs can be made between noise and distortion performance. For maximizing the SNR—important for time- domain applications—the 4VPP range may be selected. This range may also be used with low-level (–6dBFS to –40dBFS) but high-frequency inputs (multi-tone). The 3VPP range may be considered for achieving a combination of both low-noise and distortion performance. Here, the SNR number is typically 3dB down compared to the 4VPP range, while an improvement in the distortion performance of the driver amplifier may be realized due to the reduced output power level required. INPUT BIASING (VCM) The ADS5421 operates from a single +5V supply, and requires each of the analog inputs to be externally biased to a common-mode voltage of typically +2.5V. This allows a symmetrical signal swing while maintaining sufficient head- room to either supply rail. Communication systems are usu- ally AC-coupled in between signal processing stages, mak- ing it convenient to set individual common-mode voltages and allow optimizing the DC operating point for each stage. Other applications, such as imaging, process mainly unipolar or DC-restored signals. In this case, the common-mode voltage may be shifted such that the full input range of the converter is utilized. It should be noted that the CM pin is not internally buffered, but ties directly to the reference ladder. Therefore, it is recommended to keep loading of this pin to a minimum (< 100 µA) to avoid an increase in the nonlinearity of the converter. Additionally, the DC voltage at the CM pin is not precisely +2.5V, but is subject to the tolerance of the top and bottom references, as well as the resistor ladder. Further- more, the common-mode voltage typically declines with an increase in sampling frequency. This, however, does not affect the performance. INPUT IMPEDANCE The input of the ADS5421 is capacitive, and the driving source needs to provide the slew current to charge or discharge the input sampling capacitor while the track-and-hold amplifier is in track mode (see Figure 1). This effectively results in a dynamic input impedance that is a function of the sampling frequency. Figure 2 depicts the differential input impedance of the ADS5421 as a function of the input frequency. FIGURE 2. Differential Input Impedance vs Input Frequency. 1000 100 10 1 0.1 0.01 0.1 1 10 100 1000 f IN (MHz) |
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