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LTC2351CUH-14 数据表(PDF) 14 Page - Linear Technology |
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LTC2351CUH-14 数据表(HTML) 14 Page - Linear Technology |
14 / 20 page 14 LTC2351-14 235114f APPLICATIO S I FOR ATIO INPUT FILTERING AND SOURCE IMPEDANCE The noise and the distortion of the input amplifier and other circuitry must be considered since they will add to the LTC2351-14 noise and distortion. The small-signal bandwidth of the sample-and-hold circuit is 50MHz. 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. A simple 1-pole RC filter is sufficient for many applica- tions. For example, Figure 1 shows a 47pF capacitor from CHO+ to ground and a 51 Ω source resistor to limit the net input bandwidth to 30MHz. The 47pF capacitor also acts as a charge reservoir for the input sample-and-hold and isolates the ADC input from sampling-glitch sensitive cir- cuitry. High quality capacitors and resistors should be used since these components can add distortion. NPO and silvermica type dielectric capacitors have excellent linear- ity. Carbon surface mount resistors can generate distor- tion from self heating and from damage that may occur during soldering. Metal film surface mount resistors are much less susceptible to both problems. When high am- plitude unwanted signals are close in frequency to the desired signal frequency a multiple pole filter is required. High external source resistance, combined with 13pF of input capacitance, will reduce the rated 50MHz input band- width and increase acquisition time beyond 39ns. INPUT RANGE The analog inputs of the LTC2351-14 may be driven fully differentially with a single supply. Either input may swing up to VCC, provided the differential swing is no greater than 2.5V with BIP (Pin 29) Low, or ±1.25V with (BIP Pin 29) High. The 0V to 2.5V range is also ideally suited for single- ended input use with single supply applications. The common mode range of the inputs extend from ground to the supply voltage VCC. If the difference between the CH+ and CH– at any input pair exceeds 2.5V (unipolar) or 1.25V (bipolar), the output code will stay fixed at positive full- scale, and if this difference goes below 0V (unipolar) or –1.25V (bipolar), the output code will stay fixed at negative full-scale. INTERNAL REFERENCE The LTC2351-14 has an on-chip, temperature compen- sated, bandgap reference that is factory trimmed to 2.5V to obtain a precise 2.5V input span. The reference ampli- fier output VREF, (Pin 23) must be bypassed with a capaci- tor to ground. The reference amplifier is stable with capaci- tors of 1 µF or greater. For the best noise performance, a 10 µF ceramic or a 10µF tantalum in parallel with a 0.1µF ceramic is recommended. The VREF pin can be overdriven with an external reference as shown in Figure 2. The volt- age of the external reference must be higher than the 2.5V of the open-drain P-channel output of the internal refer- ence. The recommended range for an external reference is 2.55V to VDD. An external reference at 2.55V will see a DC quiescent load of 0.75mA and as much as 3mA during conversion. Figure 1. RC Input Filter LTC2351-14 CH0+ CH0– VREF GND 235114 F01 1 2 11 3 10 µF 47pF* 51 Ω* CH1+ CH1– 4 5 47pF* *TIGHT TOLERANCE REQUIRED TO AVOID APERTURE SKEW DEGRADATION 51 Ω* ANALOG INPUT ANALOG INPUT Figure 2. External Reference LTC2351-14 VREF GND 235114 F02 23 22 10 µF LT1790-3 3.5V to 18V |
类似零件编号 - LTC2351CUH-14 |
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类似说明 - LTC2351CUH-14 |
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