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ADS2807Y250G4 Arkusz danych(PDF) 11 Page - Texas Instruments |
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ADS2807Y250G4 Arkusz danych(HTML) 11 Page - Texas Instruments |
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11 / 24 page  ADS2807 11 SBAS169B www.ti.com Figure 2 shows an example of such an interface circuit specifically designed to maximize the dynamic performance. The voltage feedback amplifier, OPA642, maintains an ex- cellent distortion performance for input frequencies of up to 15MHz. The two amplifiers (A1, A2) are configured as an inverting and noninverting gain stage to convert the input signal from single-ended to differential. The nominal gain for this stage is set to +2V/V. The outputs of the OPA642s are AC-coupled to the converter’s differential inputs. This will keep the distortion performance at its best since the signal range stays within the linear region of the op amp and sufficient headroom to the supply rails can be maintained. Four resistors located between the top (REFT) and bottom (REFB) reference shift the input signal to a common-mode voltage of approximately +2.5V. The interface circuit of Figure 2 can be modified to extend the bandwidth to approximately 25MHz, by replacing the OPA642 with its decompensated version, the OPA643. The OPA643 provides the necessary slew rate for a low distortion front end to the ADS2807. With a minimum gain stability of +3, the gain resistors have to be modified, as well as optimizing the series resistor and shunt capacitance at each of the converter inputs. AC-Coupled, Single-Ended-to-Differential Interface for Single-Supply Operation The previously discussed interface circuit can be modified if the system only allows for a single-supply operation, e.g., VS = +5V. Single-supply operation requires the driver ampli- fier to be biased as well in order to process a bipolar input signal. Typically, single-supply amplifiers do not achieve distortion performance as well as dual-supply op amps. The driver amplifier’s output swing must exceed the full-scale input range of the converter. In addition, dual op amps, such as the current-feedback OPA2681, should be considered since they provide the closest open-loop gain and phase matching between the two channels. Shown in Figure 3 is a single-supply interface circuit for an AC-coupled input signal. With the ADS2807 set to the 2Vp-p input range, the top and bottom references (REFT, REFB) provide an output voltage FIGURE 2. AC-Coupled Differential Driver Interface with OPA642. FIGURE 3. AC-Coupled, Differential Interface for Single-Supply Operation. V IN 1/2 ADS2807Y 100pF IN IN 402 Ω 1.82k Ω 1.82k Ω 16.5 Ω 200 Ω 402 Ω 402 Ω REFB 100pF 0.1 µF 0.1 µF A1 OPA642 A2 OPA642 REFT 1.82k Ω 1.82k Ω 16.5 Ω One Channel of Two V IN 1/2 ADS2807Y 68pF IN CM IN +5V 68pF 0.1 µF 0.1 µF 0.1 µF 1/2 OPA2681 1/2 OPA2681 R G 249 Ω R F 499 Ω 499 Ω R S 24.9 Ω V CM = +2.5V R P 499 Ω R P 499 Ω R F 499 Ω R IN 249 Ω R S 24.9 Ω One Channel of Two |
Podobny numer części - ADS2807Y250G4 |
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Podobny opis - ADS2807Y250G4 |
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