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ADS804U bảng dữ liệu(PDF) 9 Page - Texas Instruments |
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ADS804U bảng dữ liệu(HTML) 9 Page - Texas Instruments |
9 / 18 page ADS804 9 SBAS068B www.ti.com DC voltage differences between the IN and IN inputs of the ADS804 effectively will 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-establish 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 ADS804, 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 ADS804 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 ADS804 is a bandgap reference circuit including logic that provides either a +1V or +2.5V reference output, by simply selecting the corresponding pin-strap con- figuration. Different reference voltages can be generated by the use of two external resistors, which will set a different FIGURE 2. DC-Coupled, Single-Ended Input Configuration with DC-Level Shift. gain for the internal reference buffer. For more design flexibil- ity, 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. FIGURE 3. Transformer-Coupled Input. V IN IN IN CM 22 Ω 22 Ω 100pF R T 100pF + 4.7µF 0.1µF ADS804 1:n 0.1µF R G INPUT FULL-SCALE REQUIRED MODE RANGE VREF CONNECT TO Internal 2Vp-p +1V SEL VREF Internal 5Vp-p +2.5V SEL GND Internal 2V ≤ FSR < 5V 1V < VREF < 2.5V R1 VREF and SEL FSR = 2 • VREF VREF = 1 + (R1/R2)R2 SEL and Gnd External 1V < FSR < 5V 0.5V < VREF < 2.5V SEL +VS VREF Ext. VREF TABLE I. Selected Reference Configuration Examples. V IN 2Vp-p 0 +1V –1V R F R IN NOTE: R F = RIN, G = –1 +V S ADS804 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 OPA691 +V S |
Số phần tương tự - ADS804U |
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Mô tả tương tự - ADS804U |
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