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ADS5220 bảng dữ liệu(PDF) 10 Page - Burr-Brown (TI) |
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ADS5220 bảng dữ liệu(HTML) 10 Page - Burr-Brown (TI) |
10 / 20 page ADS5220 10 SBAS261A www.ti.com INPUT COMMON-MODE VOLTAGE The ADS5220 operates from a single +3.3V supply, and requires an external common-mode voltage. This allows a symmetrical signal swing while maintaining sufficient head- room to the supply rails. The common-mode voltage can be generated from an external DC voltage source (for example, an analog +3.3V supply with a simple resistor divider), or from the input signal source with DC-coupling. For 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 ADS5220 is select- able from 1VPP to 2VPP and any value within this range, by the configuration of 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 ex- ample, applications requiring the maximum signal-to-noise performance (SNR) will benefit from the 2VPP input range while lower distortion may be obtain with the reduced input range of 1VPP. Depending on the input driver configuration the 1VPP range may also relax the requirements for the driver, particularly for single-ended, single supply applica- tions. DIFFERENTIAL INPUTS The ADS5220 input structure is designed to accept both a single-ended or differential analog signal. However, the ADS5220 will achieve its optimum performance when the analog inputs are driven differentially. Differential operation of the ADS5220 requires that an input signal at the inputs (IN, IN) has the same amplitude and is 180 degrees out-of-phase. Differential 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 most suitable driver amplifier. • Minimization of even-order harmonics. • Improved noise immunity based on the common-mode input rejection of the converter. ANALOG INPUT Depending on the application and the desired level of perfor- mance, the analog input of the ADS5220 can be configured in various ways and driven with different circuits. In any case, the analog interface requirements should be carefully exam- ined before selecting the appropriate circuit configuration. The circuit definition should include considerations on the input frequency band and amplitude, as well as the available power supplies. INPUT IMPEDANCE The input impedance of the ADS5220 is capacitive due to the input stray and sampling capacitors. These capacitors effec- tively result in a dynamic input impedance that is a function of the sampling and input frequency. Figure 2 depicts the differential input impedance of the ADS5220 as a function of the signal input frequency. For applications that use op amps to drive the ADC, it is recommended that a series resistor be added between the amplifier output and the converter inputs. This will isolate the capacitive input of the converter from the driving source and avoid gain peaking, or instability; further- more, it will create a 1st-order, low-pass filter (LPF) in conjunction with the specified input capacitance of the ADS5220. The cutoff frequency of this LPF can be further adjusted by adding an external shunt capacitor. In any case, the use of the RC network is optional, but optimizing the values to adapt to the specific application is encouraged. DIFFERENTIAL INPUT IMPEDANCE Input Frequency (Hz) 1M 10M 100M 50k 40k 30k 20k 10k 100 FIGURE 2. Differential Input Impedance vs Input Frequency. |
Số phần tương tự - ADS5220 |
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Mô tả tương tự - ADS5220 |
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