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LTC2379IDE-18TRPBF Datasheet(PDF) 11 Page - Linear Technology |
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LTC2379IDE-18TRPBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 24 page  LTC2379-18 11 237918p APPLICATIONS INFORMATION 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 source must be considered since they add to the ADC noise and distortion. Noisy input signals should be filtered prior to the buffer amplifier input with an appropriate filter to minimize noise. The simple 1-pole RC lowpass filter (LPF1) shown in Figure 4 is sufficient for many applications. Another filter network consisting of LPF2 should be used between the buffer and ADC input to both minimize the noise contribution of the buffer and to help minimize distur- 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. High quality capacitors and resistors should be used in the RC filters since these components can add distortion. 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 occur during soldering. Metal film surface mount resistors are much less susceptible to both problems. 20Ω 3300pF 6600pF 20Ω 500Ω LPF2 LPF1 BW = 800kHz BW = 48kHz SINGLE-ENDED- TO-DIFFERENTIAL DRIVER SINGLE-ENDED- INPUT SIGNAL LTC2379-18 IN+ IN– 237918 F04 3300pF 3300pF Single-Ended-to-Differential Conversion For single-ended input signals, a single-ended to differential 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 amplifier is configured as a unity gain buffer and the single- 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. Figure 4. Input Signal Chain LT6350 VCM = VREF/2 237918 F05a 0V 5V 0V 5V OUT1 RINT RINT OUT2 8 4 5 2 1 + – + – – + 0V 5V FREQUENCY (kHz) 0 100 200 300 400 800 700 600 500 –180 –60 –40 –20 –80 –100 –120 –140 –160 0 237918 F05b SNR = 101dB THD = –111.5dB SINAD = 100.8dB SFDR = 114.5dB Figure 5a. LT6350 Converting a 0V-5V Single-Ended Signal to a ±5V Differential Input Signal Figure 5b. 32k Point FFT Plot with fIN = 10kHz for Circuit Shown in Figure 5a |
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