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ADS828 bảng dữ liệu(PDF) 10 Page - Burr-Brown (TI) |
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ADS828 bảng dữ liệu(HTML) 10 Page - Burr-Brown (TI) |
10 / 12 page 10 ® ADS828 FIGURE 4. DC-Coupled Interface Circuit with Level Shifting Using 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 ADS828 differentially, a RF trans- former 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 differential signal swing across the secondary winding. Consider a step- up transformer to take advantage of a signal amplification without the introduction of another noise source. Further- more, the reduced signal swing from the source may lead to an improved distortion performance. The differential input configuration may provide a notice- able advantage of achieving good SFDR performance over a wide range of input frequencies. In this mode, both inputs of the ADS828 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 schematic for the suggested transformer-coupled interface circuit. The compo- nent values of the R-C low-pass may be optimized depend- ing on the desired roll-off frequency. The resistor across the secondary side (RT) should be calculated using the equation RT = n2 x 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 refer- ence circuit. The internal blocks are the bandgap voltage reference, the drivers for the top and bottom reference, and the resistive reference ladder. The bandgap reference circuit 2Vp-p NOTE: R F = RIN, G = –1 V IN R 2 200 Ω R 1 1k Ω ADS828 R S 50 Ω 10pF 0.1 µF IN IN CM (+2.5V) INT/EXT R F 499 Ω R IN 499 Ω V CM = +1.25V REFB (+1.5V) REFT (+3.5V) 1/2 OPA2681 1/2 OPA2681 R F 1k Ω 50 Ω 0.1 µF 0.1 µF RSEL +V S +5V +5V V IN IN IN CM 22 Ω 22 Ω 47pF R T 47pF + 10 µF 0.1 µF INT/EXT RSEL +5V ADS828 1:n 0.1 µF R G ADS828 REFT ByT CM ByB REFB Bypass Capacitors: 0.1 µF || 2.2µF each 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 |
Số phần tương tự - ADS828 |
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Mô tả tương tự - ADS828 |
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