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LTC1416IG Datasheet(PDF) 11 Page - Linear Technology |
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LTC1416IG Datasheet(HTML) 11 Page - Linear Technology |
11 / 20 page 11 LTC1416 APPLICATIONS INFORMATION Input Filtering The noise and the distortion of the input amplifier and other circuitry must be considered since they will add to the LTC1416 noise and distortion. The small-signal band- width of the sample-and-hold circuit is 15MHz. Any noise or distortion products that are present at the analog inputs will be summed over this entire bandwidth. Noisy input circuitry should be filtered prior to the analog inputs to minimize noise. A simple 1-pole RC filter is sufficient for many applications. For example, Figure 7 shows a 1000pF capacitor from AIN+ to ground and a 200Ω source resistor to limit the input bandwidth to 800kHz. The 1000pF capacitor also acts as a charge reservoir for the input sample-and-hold and isolates the ADC input from sam- pling glitch sensitive circuitry. High quality capacitors and resistors should be used since these components can add distortion. NPO and silver mica type dielectric capacitors have excellent linearity. Carbon surface mount resistors can also generate distortion from self-heating and from damage that may occur during soldering. Metal film surface mount resistors are much less susceptible to both problems. accommodate other input ranges often with little or no additional circuitry. The following sections describe the reference and input circuitry and how they affect the input range. Internal Reference The LTC1416 has an on-chip, temperature compensated, curvature corrected, bandgap reference that is factory trimmed to 2.500V. It is connected internally to a reference amplifier and is available at VREF (Pin 3). See Figure 8a. A 4k resistor is in series with the output so that it can be easily overdriven by an external reference or other cir- cuitry (see Figure 8b). The reference amplifier gains the voltage at the VREF pin by 1.625 to create the required internal reference voltage. This provides buffering be- tween the VREF pin and the high speed capacitive DAC. The VREF REFCOMP AGND 2.5V 4.0625V 3 4 5 22 µF 1416 F08a R1 4k R2 80k BANDGAP REFERENCE LTC1416 REF AMP R3 128k Figure 8a. LTC1416 Reference Circuit 1 2 3 22 µF ANALOG INPUT 1416 F08b AIN + AIN – VREF LTC1416 REFCOMP AGND 4 5 5V LT1019A-2.5 VOUT VIN Figure 8b. Using the LT1019-2.5 as an External Reference 1 2 3 1000pF 200 Ω 22 µF ANALOG INPUT 1416 F07 AIN + AIN – VREF LTC1416 REFCOMP AGND 4 5 Figure 7. RC Input Filter Input Range The ±2.5V input range of the LTC1416 is optimized for low noise and low distortion. Most op amps also perform best over this same range, allowing direct coupling to the analog inputs and eliminating the need for special transla- tion circuitry. Some applications may require other input ranges. The LTC1416 differential inputs and reference circuitry can |
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