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LTC2381IMS-16PBF Datasheet(PDF) 10 Page - Linear Technology |
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LTC2381IMS-16PBF Datasheet(HTML) 10 Page - Linear Technology |
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10 / 24 page  LTC2381-16 10 238116f Figure 2. LTC2381-16 Transfer Function INPUT VOLTAGE (V) 0V –1 LSB 238116 F02 011...111 011...110 000...001 000...000 100...000 100...001 111...110 1 LSB BIPOLAR ZERO 111...111 FSR/2 – 1LSB –FSR/2 FSR = +FS – –FS 1LSB = FSR/65536 RON CIN RON REF REF CIN IN+ IN– BIAS VOLTAGE 238116 F03 Figure 3. The Equivalent Circuit for the Differential Analog Input of the LTC2381-16 circuit shown in Figure 3. The diodes at the input provide ESD protection. In the acquisition phase, each input sees approximately 45pF (CIN)fromthesamplingCDACinseries with 40Ω (RON) from the on-resistance of the sampling switch. Any unwanted signal that is common to both inputs will be reduced by the common mode rejection of the ADC. The inputs draw a current spike while charging the CIN capacitors during acquisition. When the LTC2381-16 is not acquiring the input, the analog inputs draw only a small leakage current. INPUT DRIVE CIRCUITS A low impedance source can directly drive the high impedance inputs of the LTC2381-16 without gain error. A high impedance source should be buffered to minimize settling time during acquisition and to optimize the distortion 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 LTC2381-16. The amplifier 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 and the 100Ω series input resistors should be used between the buffer and ADC inputs to both minimize the noise contribution of the buffer and to help minimize disturbances 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. With the 482kHz lowpass filter shown in Figure 4, the LT6350 provides the full data sheet performance of the LTC2381-16. 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. APPLICATIONS INFORMATION 50Ω 3300pF 6600pF 50Ω 500Ω 100Ω 100Ω LPF2 LPF1 BW = 482kHz BW = 48kHz SINGLE-ENDED- TO-DIFFERENTIAL DRIVER SINGLE-ENDED- INPUT SIGNAL LTC2381-16 IN+ IN– 238116 F04 LT6350 Figure 4. Input Signal Chain |
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