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ADS825 bảng dữ liệu(PDF) 10 Page - Texas Instruments |
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ADS825 bảng dữ liệu(HTML) 10 Page - Texas Instruments |
10 / 18 page ADS822, ADS825 10 SBAS069B FIGURE 4. DC-Coupled Interface Circuit with Dual Current-Feedback Amplifier OPA2681. FIGURE 5. Transformer Coupled Input. FIGURE 6. Equivalent Reference Circuit with Recommended Reference Bypassing. SINGLE-ENDED-TO-DIFFERENTIAL CONFIGURATION (Transformer Coupled) If the application requires a signal conversion from a single- ended source to feed the ADS822 and ADS825 differentially, a RF transformer might be a good solution. The selected transformer must have a center tap in order to apply the common-mode DC voltage necessary to bias the converter inputs. AC-grounding the center tap will generate the differ- ential signal swing across the secondary winding. Consider a step-up transformer to take advantage of a signal amplifi- cation without the introduction of another noise source. Furthermore, the reduced signal swing from the source may lead to an improved distortion performance. The differential input configuration may provide a noticeable advantage of achieving good SFDR performance over a wide range of input frequencies. In this mode, both inputs of the ADS822 and ADS825 see matched impedances, and the differential signal swing can be reduced to half of the swing required for single-ended drive. Figure 5 shows the sche- matic for the suggested transformer-coupled interface circuit. The component values of the R-C low-pass may be opti- mized depending on the desired roll-off frequency. The resistor across the secondary side (RT) should be calculated using the equation RT = n2 • RG to match the source impedance (RG) for good power transfer and Voltage Standing Wave Ratio (VSWR). REFERENCE OPERATION Figure 6 depicts the simplified model of the internal reference circuit. The internal blocks are the bandgap voltage refer- ence, the drivers for the top and bottom references, and the 2Vp-p NOTE: R F = RIN, G = –1 V IN R 2 200 Ω R 1 1k Ω ADS822 ADS825 R S 50 Ω 10pF 0.1 µF IN IN CM (+2.5) INT/EXT R F 499 Ω R IN 499 Ω V CM = +1.25V REFB (+1.5V) REFT (+3.5V) 1/2 OPA2691 1/2 OPA2691 R F 1k Ω 50 Ω 0.1 µF 0.1 µF RSEL +V S +5V +5V A1 A2 V IN IN IN CM 22 Ω 22 Ω 47pF R T 47pF + 10 µF 0.1 µF INT/EXT RSEL +5V ADS822 ADS825 1:n 0.1 µF R G ADS822 REFT ByT CM ByB REFB Bandgap Reference and Logic V REF 400 Ω 400 Ω 400 Ω 400 Ω +1 +1 +V S 50k Ω 50k Ω INT/EXT RSEL Bypass Capacitors: 0.1 µF || 2.2µF each (optionally, 2.2µF tantalum capacitors maybe added to ByT and ByB pins for the best results). |
Số phần tương tự - ADS825 |
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Mô tả tương tự - ADS825 |
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