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LTC2356 数据表(PDF) 13 Page - Linear Technology |
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LTC2356 数据表(HTML) 13 Page - Linear Technology |
13 / 20 page LTC2315-12 13 231512f For more information www.linear.com/2315-12 applicaTions inForMaTion Choosing an Input Amplifier Choosing an input amplifier is easy if a few requirements are taken into consideration. First, to limit the magnitude of the voltage spike seen by the amplifier from charging the sampling capacitor, choose an amplifier that has a low output impedance (<50Ω) at the closed-loop bandwidth frequency. For example, if an amplifier is used in a gain of 1 and has a unity-gain bandwidth of 100MHz, then the output impedance at 100MHz must be less than 50Ω. The second requirement is that the closed-loop bandwidth must be greater than 100MHz to ensure adequate small signal settling for full throughput rate. If slower op amps are used, more time for settling can be provided by in- creasing the time between conversions. The best choice for an op amp to drive the LTC2315-12 will depend on the application.Generally,applicationsfallintotwocategories: AC applications where dynamic specifications are most critical and time domain applications where DC accuracy and settling time are most critical. The following list is a summary of the op amps that are suitable for driving the LTC2315-12. (More detailed information is available on the Linear Technology website at www.linear.com.) LT6230: 215MHz GBWP, –80dBc Distortion at 1MHz, Unity-Gain Stable, Rail-to-Rail Input and Output, 3.5mA/ Amplifier, 1.1nV/√Hz. LT6200:165MHzGBWP,–85dBcDistortionat1MHz,Unity- Gain Stable, R-R In and Out, 15mA/Amplifier, 0.95nV/√Hz. LT1818/LT1819: 400MHz GBWP, –85dBc Distortion at 5MHz, Unity-Gain Stable, 9mA/Amplifier, Single/Dual Voltage Mode Operational Amplifier. Input Drive Circuits TheanaloginputoftheLTC2315-12isdesignedtobedriven single-ended with respect to GND. A low impedance source can directly drive the high impedance analog input of the LTC2315-12 without gain error. A high impedance source shouldbebufferedtominimizesettlingtimeduringacquisi- tion and to optimize the distortion performance of the ADC. For best performance, a buffer amplifier should be used to drive the analog input of the LTC2315-12. The amplifier provides low output impedance to allow for fast settling of the analog signal during the acquisition phase. It also provides isolation between the signal source and the ADC inputswhichdrawasmallcurrentspikeduringacquisition. Input Filtering The noise and distortion of the buffer amplifier and other circuitry must be considered since they add to the ADC noiseanddistortion.Noisyinputcircuitryshouldbefiltered prior to the analog inputs to minimize noise. A simple 1-pole RC filter is sufficient for many applications. Large filter RC time constants slow down the settling at the analog inputs. It is important that the overall RC time constants be short enough to allow the analog inputs to completelysettleto>12-bitresolutionwithintheminimum acquisition time (tACQ-MIN) of 40ns. 47pF 50 2315112 F10 AIN LTC2315-12 LT1818 GND ANALOG IN + – Figure 10. RC Input Filter A simple 1-pole RC filter is sufficient for many applica- tions. For example, Figure 10 shows a recommended single-ended buffered drive circuit using the LT1818 in unity gain mode. The 47pF capacitor from AIN to ground and 50Ω source resistor limits the input bandwidth to 68MHz. The 47pF capacitor also acts as a charge reservoir for the input sample-and-hold and isolates the LT1818 from sampling glitch kick-back. The 50Ω source resistor is used to help stabilize the settling response of the drive amplifier. When choosing values of source resistance and shunt capacitance, the drive amplifier data sheet should be consulted and followed for optimum settling response. If lower input bandwidths are desired, care should be taken to optimize the settling response of the driver amplifier with higher values of shunt capacitance or series resistance. High quality capacitors and resistors should be used in the RC filter since these components canadddistortion.NP0/C0Gandsilvermicatypedielectric capacitors have excellent linearity. Carbon surface mount resistors can generate distortion from self heating and from damage that may occur during soldering. Metal film surface mount resistors are much less susceptible to both |
类似零件编号 - LTC2356 |
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类似说明 - LTC2356 |
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