LTC2368-18 APPLICATIONS INFORMATIONOVERVIEW 1LSB = FS/262144 111...111 The LTC2368-18 is a low noise, low power, high speed 18-bit 111...110 successive approximation register (SAR) ADC. Operating 111...101 from a single 2.5V supply, the LTC2368-18 supports a 111...100 0V to VREF pseudo-differential unipolar input range with VREF ranging from 2.5V to 5.1V, making it ideal for high OUTPUT CODE UNIPOLAR performance applications which require a wide dynamic ZERO 000...011 range. The LTC2368-18 achieves ±2.5LSB INL max, no 000...010 missing codes at 18 bits and 97dB SNR. 000...001 000...000 Fast 1Msps throughput with no cycle latency makes the 0V 1 FS – 1LSB LSB LTC2368-18 ideally suited for a wide variety of high speed INPUT VOLTAGE (V) 236818 F02 applications. An internal oscillator sets the conversion time, Figure 2. LTC2368-18 Transfer Function easing external timing considerations. The LTC2368-18 dis- sipates only 13.5mW at 1Msps, while an auto power-down ANALOG INPUT feature is provided to further reduce power dissipation The analog inputs of the LTC2368-18 are pseudo-differential during inactive periods. in order to reduce any unwanted signal that is common to both inputs. The analog inputs can be modeled by the CONVERTER OPERATION equivalent circuit shown in Figure 3. The diodes at the input provide ESD protection. In the acquisition phase, each The LTC2368-18 operates in two phases. During the ac- input sees approximately 45pF (C quisition phase, the charge redistribution capacitor D/A IN) from the sampling CDAC in series with 40Ω (R converter (CDAC) is connected to the IN+ and IN– pins to ON) from the on-resistance of the sampling switch. The IN+ input draws a current sample the pseudo-differential analog input voltage. A ris- spike while charging the CIN capacitor during acquisition. ing edge on the CNV pin initiates a conversion. During the During conversion, the analog inputs draw only a small conversion phase, the 18-bit CDAC is sequenced through a leakage current. successive approximation algorithm, effectively comparing the sampled input with binary-weighted fractions of the REF reference voltage (e.g. V CIN REF/2, VREF/4 … VREF/262144) RON 45pF using the differential comparator. At the end of conver- 40Ω IN+ sion, the CDAC output approximates the sampled analog input. The ADC control logic then prepares the 18-bit BIAS digital output code for serial transfer. REF VOLTAGE C R IN ON 45pF 40Ω IN– 236818 F03 TRANSFER FUNCTION The LTC2368-18 digitizes the full-scale voltage of REF into 218 levels, resulting in an LSB size of 19µV with Figure 3. The Equivalent Circuit for the REF = 5V. The ideal transfer function is shown in Figure 2. Differential Analog Input of the LTC2368-18 The output data is in straight binary format. 236818f 10 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Converter Characteristics Dynamic Accuracy Reference Input Digital Inputs And Digital Outputs Power Requirements ADC Timing Characteristics Typical Performance Characteristics Pin Functions Functional Block Diagram Timing Diagram Applications Information Timing Diagrams Board Layout Package Description Typical Application Related Parts
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