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LT1567 Datasheet(PDF) 10 Page - Linear Technology |
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LT1567 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  LT1567 10 1567fa APPLICATIO S I FOR ATIO output from the INV block (Pin 7). These two outputs maintain equal gain and 180º phase shift over a wide frequency range. This feature permits choosing the signal polarity in single ended applications, and also performs single ended to differential conversion. The latter property is useful as an antialiasing filter to drive standard mono- lithic A/D converters having differential inputs, as illus- trated on the first page of this data sheet. Dealing with High Source Impedances The voltage VIN in Figure 1, on the left side of R1, is the signal voltage that the filter sees. If a voltage source with significant internal impedance drives the VIN node in Figure 1, then the filter input VIN may differ from the source’s open-circuit output, and the difference can be complex, because the filter presents a complex imped- ance to VIN. A rule of thumb is that a source impedance is negligibly “low” if it is much smaller than R1 at frequencies of interest. Otherwise, the source impedance (resistive or reactive) effectively adds to R1 and may change the signal frequency response compared to that with a low source impedance. If the source is resistive and predictable, then it may be possible to design for it by reducing R1. Unpredictable or nonresistive source impedances that are not much less than R1 should be buffered. Construction and Instrumentation Cautions Electrically clean construction is important in applica- tions seeking the full dynamic range and bandwidth of the LT1567. Using the shortest possible wiring or printed- circuit paths will minimize parasitic capacitance and inductance. High quality supply bypass capacitors of 0.1 µF near the chip, connected to a ground plane, provide good decoupling from a clean, low inductance power source. But several inches of wire (i.e., a few microhenrys of inductance) from the power supplies, unless decoupled by substantial capacitance ( ≥10µF) near the chip, can cause a high Q LC resonance in the hundreds of kHz in the chip’s supplies or ground reference. This may impair filter performance at those frequencies. In stringent filter applications, a compact, carefully laid out printed circuit board with good ground plane makes a difference in both stopband rejection and distortion. Finally, equipment to measure filter performance can itself introduce distortion or noise. Checking for these limits with a wire in place of the filter is a prudent routine procedure. Low Noise Differential Circuits The LT1567 is an optimum analog building for designing single supply differential circuits to process low level signals. Figure 3 shows a single ended to differential amplifier driving a 1st order differential RC filter. The differential output of Figure 3 is a function of input (VIN) and the VREF voltage on Pin 5. (The range of the VREF voltage on Pin 5 in Figures 3, 4 and 5 is the common mode input voltage range parameter under Electrical Characteristics.)The graph of Figure 3 shows the differen- tial signal-to-noise ratio for a gain of 2 and a gain of 10. Increasing the differential gain increases the differential signal-to-noise ratio. The equivalent input noise is equal to the output noise divided by the gain. For example, with a gain equal to 2 (R2 = R1 = 200 Ω) and a gain equal to 10 (R2 = 1k, R1 = 200 Ω),theequivalentinputnoiseis4.59nV/ √Hz and 2.04nV/√Hz respectively. The VREF voltage on Pin 5 can be set by a voltage divider or a reference voltage source. To maximize the unclipped LT1567 output swing, the DC output voltage should be set at V+/2. However, if VINDC (the input DC voltage) is within the range of VREF, then VREF can be equal to VINDC. The input signal can also be AC coupled to the input resistor, R1, and VREF set to the DC voltage of the circuit following the amplifier. For example, VREF might be set to 1.2V to bias the input of an I and Q modulator used in broadband communication systems. |
Similar Part No. - LT1567_15 |
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Similar Description - LT1567_15 |
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