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LTC2379IDE-18PBF Datasheet(PDF) 11 Page - Linear Technology |
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LTC2379IDE-18PBF Datasheet(HTML) 11 Page - Linear Technology |
11 / 24 page LTC2379-18 11 237918f APPLICATIONS INFORMATION INPUT DRIVE CIRCUITS A low impedance source can directly drive the high im- pedance inputs of the LTC2379-18 without gain error. A high impedance source should be buffered to minimize settling time during acquisition and to optimize the dis- tortion performance of the ADC. Minimizing settling time is important even for DC inputs, because the ADC inputs draw a current spike when entering acquisition. For best performance, a buffer amplifier should be used to drive the analog inputs of the LTC2379-18. The ampli- fier provides low output impedance, which produces fast settling of the analog signal during the acquisition phase. It also provides isolation between the signal source and the current spike the ADC inputs draw. Input Filtering The noise and distortion of the buffer amplifier and signal sourcemustbeconsideredsincetheyaddtotheADCnoise and distortion. Noisy input signals should be filtered prior to the buffer amplifier input with an appropriate filter to minimizenoise.Thesimple1-poleRClowpassfilter(LPF1) shown in Figure 4 is sufficient for many applications. 20 3300pF 6600pF 20 500 LPF2 LPF1 BW = 800kHz BW = 48kHz SINGLE-ENDED- TO-DIFFERENTIAL DRIVER SINGLE-ENDED- INPUT SIGNAL LTC2379-18 IN+ IN– 237918 F04 3300pF 3300pF High quality capacitors and resistors should be used in the RCfilterssincethesecomponentscanadddistortion.NPO and silver mica type dielectric capacitors have excellent linearity. Carbon surface mount resistors can generate distortion from self heating and from damage that may occurduringsoldering.Metalfilmsurfacemountresistors are much less susceptible to both problems. Single-Ended-to-Differential Conversion Forsingle-endedinputsignals,asingle-endedtodifferential conversion circuit must be used to produce a differential signal at the inputs of the LTC2379-18. The LT6350 ADC driver is recommended for performing single-ended-to- differential conversions. The LT6350 is flexible and may be configured to convert single-ended signals of various amplitudes to the ±5V differential input range of the LTC2379-18. The LT6350 is also available in H-grade to complement the extended temperature operation of the LTC2379-18 up to 125°C. Figure 5a shows the LT6350 being used to convert a 0V to 5V single-ended input signal. In this case, the first amplifierisconfiguredasaunitygainbufferandthesingle- ended input signal directly drives the high-impedance input of the amplifier. As shown in the FFT of Figure 5b, the LT6350 drives the LTC2379-18 to near full datasheet performance. The LT6350 can also be used to buffer and convert large true bipolar signals which swing below ground to the ±5V differential input range of the LTC2379-18 in order to maximize the signal swing that can be digitized. Fig- ure 6a shows the LT6350 being used to convert a ±10V true bipolar signal for use by the LTC2379-18. In this case, the first amplifier in the LT6350 is configured as an inverting amplifier stage, which acts to attenuate and level shift the input signal to the 0V to 5V input range of the LTC2379-18. In the inverting amplifier configuration, the single-ended input signal source no longer directly drives a high impedance input of the first amplifier. The input impedance is instead set by resistor RIN. RIN must be chosen carefully based on the source impedance of the signal source. Higher values of RIN tend to degrade both the noise and distortion of the LT6350 and LTC2379-18 as a system. Figure 4. Input Signal Chain Another filter network consisting of LPF2 should be used between the buffer and ADC input to both minimize the noisecontributionofthebufferandtohelpminimizedistur- bances reflected into the buffer from sampling transients. Long RC time constants at the analog inputs will slow down the settling of the analog inputs. Therefore, LPF2 requires a wider bandwidth than LPF1. A buffer amplifier with a low noise density must be selected to minimize degradation of the SNR. |
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