Datasheet LTC1418 (Analog Devices) - 10

LTC1418
APPLICATIONS INFORMATION
Referring to Figure 1, the A + – IN and AIN inputs are connected 0 to the sample-and-hold capacitors (C fSAMPLE = 200kHz SAMPLE) during the f –20 IN = 9.9609375kHz acquire phase and the comparator offset is nulled by the SFDR = 99.32 SINAD = 82.4 zeroing switches. In this acquire phase, a minimum delay –40 of 1µs will provide enough time for the sample-and-hold capacitors to acquire the analog signal. During the convert –60 phase, the comparator zeroing switches open, putting the AMPLITUDE (dB) –80 comparator into compare mode. The input switches the C –100 SAMPLE capacitors to ground, transferring the differential analog input charge onto the summing junction. This input –120 charge is successively compared with the binary weighted 0 10 20 30 40 50 60 70 80 90 100 FREQUENCY (kHz) charges supplied by the differential capacitive DAC. Bit 1418 F02a decisions are made by the high speed comparator. At the
Figure 2a. LTC1418 Nonaveraged, 4096 Point FFT,
end of a conversion, the differential DAC output balances
Input Frequency = 10kHz
the A + – IN and AIN input charges. The SAR contents (a 14-bit data word) which represent the difference of A + IN 0 and A – f IN are loaded into the 14-bit output latches. SAMPLE = 200kHz –20 fIN = 97.509765kHz SFDR = 94.29 SINAD = 81.4
DYNAMIC PERFORMANCE
–40 The LTC1418 has excellent high speed sampling capability. –60 FFT (Fast Fourier Transform) test techniques are used to test the ADC’s frequency response, distortion and noise AMPLITUDE (dB) –80 at the rated throughput. By applying a low distortion –100 sine wave and analyzing the digital output using an FFT algorithm, the ADC’s spectral content can be examined –120 0 10 20 30 40 50 60 70 80 90 100 for frequencies outside the fundamental. Figure 2a shows FREQUENCY (kHz) a typical LTC1418 FFT plot. 1418 F02b
Figure 2b. LTC1418 Nonaveraged, 4096 Point FFT, Signal-to-Noise Ratio Input Frequency = 97.5kHz
The signal-to-noise plus distortion ratio [S/(N + D)] is the
Effective Number of Bits
ratio between the RMS amplitude of the fundamental input The effective number of bits (ENOBs) is a measurement frequency to the RMS amplitude of all other frequency of the resolution of an ADC and is directly related to the components at the A/D output. The output is band limited S/(N + D) by the equation: to frequencies from above DC and below half the sampling frequency. Figure 2b shows a typical spectral content with N = [S/(N + D) – 1.76]/6.02 a 200kHz sampling rate and a 10kHz input. The dynamic where N is the effective number of bits of resolution performance is excellent for input frequencies up to and and S/(N + D) is expressed in dB. At the maximum beyond the Nyquist limit of 100kHz. sampling rate of 200kHz, the LTC1418 maintains near ideal ENOBs up to the Nyquist input frequency of 100kHz (refer to Figure 3). 1418fa 10 For more information www.linear.com/LTC1418 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Package/Order Information Converter Characteristics Analog Input Dynamic Accuracy Internal Reference Characteristics Digital Inputs and Outputs Power Requirements Timing Characteristics Typical Performance Characteristics Pin Functions Test Circuit Block Diagram Applications Information Package Description Package Description Revision History Typical Application Related Parts