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Moteur de recherche de fiches techniques de composants électroniques |
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Moteur de recherche de fiches techniques de composants électroniques |
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LTC6404 Fiches technique(PDF) 21 Page - Linear Technology |
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LTC6404 Fiches technique(HTML) 21 Page - Linear Technology |
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21 / 28 page  LTC6404 21 6404f APPLICATIONS INFORMATION VINCM is defined as the average of the two input voltages VINP, and VINM (also called the source-referred input com- mon mode voltage): VV V INCM INP INM =+ () 1 2 • and VINDIFF is defined as the difference of the input voltages: VINDIFF = VINP – VINM When the feedback ratios mismatch ( Δβ), common mode to differential conversion occurs. Setting the differential input to zero (VINDIFF = 0), the de- gree of common mode to differential conversion is given by the equation: VV V VV V OUTDIFF OUT OUT INCM OCM AVG I = ≈ () Δ + – –• – β β ⏐ N NDIFF = 0 In general, the degree of feedback pair mismatch is a source of common mode to differential conversion of both signals and noise. Using 1% resistors or better will mitigate most problems, and will provide about 34dB worst-case of common mode rejection. Using 0.1% resistors will provide about 54dB of common mode rejection. A low impedance ground plane should be used as a reference for both the input signal source, and the VOCM pin. A direct short of VOCM to this ground or bypassing the VOCM with a high quality 0.1μF ceramic capacitor to this ground plane, will further prevent common mode signals from being con- verted to differential. There may be concern on how feedback ratio mismatch affects distortion. Distortion caused by feedback ratio mis- match using 1% resistors or better is negligible. However, in single supply level shifting applications where there is a voltage difference between the input common mode voltage and the output common mode voltage, resistor mismatch can make the apparent voltage offset of the amplifier appear higher than specified. The apparent input referred offset induced by feedback ratio mismatch is derived from the following equation: VOSDIFF(APPARENT) ≈ (VICM – VOCM) • Δβ Using the LTC6404-1 in a single supply application on a single 5V supply with 1% resistors, and the input common mode grounded, with the VOCM pin biased at mid-supply, the worst-case DC offset can induce 25mV of apparent offset voltage. With 0.1% resistors, the worst case appar- ent offset reduces to 2.5mV. Input Impedance and Loading Effects The input impedance looking into the VINP or VINM input of Figure 1 depends on whether the sources VINP and VINM are fully differential. For balanced input sources (VINP = –VINM), the input impedance seen at either input is simply: RINP = RINM = RI For single ended inputs, because of the signal imbalance at the input, the input impedance increases over the bal- anced differential case. The input impedance looking into either input is: RR R R RR INP INM I F IF == + ⎛ ⎝⎜ ⎞ ⎠⎟ ⎛ ⎝⎜ ⎞ ⎠⎟ 1 1 2 –• Input signal sources with non-zero output impedances can also cause feedback imbalance between the pair of feedback networks. For the best performance, it is recommended that the source’s output impedance be compensated for. If input impedance matching is required by the source, R1 should be chosen (see Figure 4): R RR RR INM S INM S 1 = • – Figure 4. Optimal Compensation for Signal Source Impedance VS + – – + RF RF RI RINM RS RI R2 = RS || R1 R1 CHOSEN SO THAT R1 || RINM = RS R2 CHOSEN TO BALANCE R1 || RS R1 6404 F04 |
Numéro de pièce similaire - LTC6404 |
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Description similaire - LTC6404 |
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