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ADS850 bảng dữ liệu(PDF) 11 Page - Texas Instruments |
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ADS850 bảng dữ liệu(HTML) 11 Page - Texas Instruments |
11 / 15 page ADS850 SBAS154C 11 www.ti.com V IN IN IN REFB 22 Ω 22 Ω 100pF 0.1 µF 0.1 µF R T 100pF 0.1 µF REFT ADS850 2k Ω 2k Ω 1:n 0.1 µF R G FIGURE 3. Transformer-Coupled Input. SINGLE-ENDED-TO-DIFFERENTIAL CONFIGURATION (TRANSFORMER COUPLED) In order to select the best suited interface circuit for the ADS850, 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 ADS850 differentially, the even-order harmonics will be reduced. Figure 3 shows the schematic for 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). REFERENCE OPERATION Integrated into the ADS850 is a bandgap reference circuit including logic that provides either a +1V or +2V reference output, by simply selecting the corresponding pin-strap con- figuration. For more design flexibility, the internal reference can be shut off and an external reference voltage used. Table I provides an overview of the possible reference options and pin configurations. INPUT MODE RANGE SEL VREF REFB REFT Internal 2Vp-p VREF SEL NC NC Internal 4Vp-p GND NC NC NC External 2V < FSR < 4V +VS 1V < FSR < 2V NC NC External (REFB – REFT) • 2+VS GND 1.5V < REFB < 2V 2V < REFT <3.5V TABLE I. Selected Reference Configuration Examples. 800 Ω 800 Ω REFB CM Reference Driver Bandgap and Logic REFT V REF SEL 1V DC Resistor Network and Switches Disable Switch to ADC to ADC ADS850 FIGURE 4. Equivalent Reference Circuit. A simple model of the internal reference circuit is shown in Figure 4. The internal blocks are a 1V bandgap voltage reference, buffer, the resistive reference ladder, and the drivers for the top and bottom reference which supply the necessary current to the internal nodes. As shown, the output of the buffer appears at the VREF pin. The full-scale input span of the ADS850 is determined by the voltage at VREF, according to Equation 1: Full-Scale Input Span = 2 • VREF (1) Note that the current drive capability of this amplifier is limited to about 1mA and should not be used to drive low loads. The programmable reference circuit is controlled by the voltage applied to the select pin (SEL). Refer to Table I for an overview. |
Số phần tương tự - ADS850 |
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Mô tả tương tự - ADS850 |
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