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AD9058AJJ-REEL Scheda tecnica(PDF) 8 Page - Analog Devices |
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AD9058AJJ-REEL Scheda tecnica(HTML) 8 Page - Analog Devices |
8 / 11 page REV. AD9058 –8– AD9058 in through-hole PCB designs, use the AD9058AJD/AKD with individual pin sockets (AMP Part No. 6-330808-0). Alterna- tively, surface-mount AD9058 units can be mounted in a through-hole socket (Circuit Assembly Corporation, Irvine, Cali- fornia Part No. CA-44SPC-T). AD9058 APPLICATIONS Combining two ADCs in a single package is an attractive alterna- tive in a variety of systems when cost, reliability, and space are important considerations. Different systems emphasize particular specifications, depending on how the part is used. In high density digital radio communications, a pair of high speed ADCs are used to digitize the in-phase (I) and quadrature (Q) components of a modulated signal. The signal presented to each ADC in this type of system consists of message-dependent amplitudes varying at the symbol rate, which is equal to the sample rates of the converters. t A = APERTURE TIME t V = DATA DELAY OF PRECEDING ENCODE t PD = OUTPUT PROPAGATION DELAY ANALOG INPUT ENCODE D0–D7 VALID DATA FOR N–1 VALID DATA FOR N VALID DATA FOR N+1 DATA CHANGING t V t A N N+1 N+2 t PD Figure 4. Timing Diagram Figure 5 shows what the analog input to the AD9058 would look like when observed relative to the sample clock. Signal-to- noise ratio (SNR), transient response, and sample rate are all critical specifications in digitizing this “eye pattern.” ANALOG INPUT SAMPLE CLOCK Figure 5. I and Q Input Signals Receiver sensitivity is limited by the SNR of the system. For the ADC, SNR is measured in the frequency domain and calculated with a Fast Fourier Transform (FFT). The signal-to-noise ratio equals the ratio of the fundamental component of the signal (rms amplitude) to the rms level of the noise. Noise is the sum of all other spectral components, including harmonic distortion, but excluding dc. Although the signal being sampled does not have a significant slew rate at the instant it is encoded, dynamic performance of the ADC and the system is still critical. Transient response is the time required for the AD9058 to achieve full accuracy when a step function input is applied. Overvoltage recovery time is the interval required for the AD9058 to recover to full accuracy after an overdriven analog input signal is reduced to its input range. Time domain performance of the ADC is also extremely important in digital oscilloscopes. When a track-/sample-and-hold is used ahead of the ADC, its operation becomes similar to that described above for receivers. The dynamic response to high frequency inputs can be described by the effective number of bits (ENOB). The effective number of bits is calculated with a sine wave curve fit and is expressed as: ENOB N LOG Error measured Error ideal =− () ( ) [] 2 where N is the resolution (number of bits) and measured error is actual rms error calculated from the converter’s outputs with a pure sine wave applied as the input. Maximum conversion rate is defined as the encode (sample) rate at which SNR of the lowest frequency analog test signal drops no more than 3 dB below the guaranteed limit. 60 55 50 45 40 35 INPUT FREQUENCY – MHz 0.1 1 10 100 30 +25 C –55 C +125 C Figure 6. Harmonic Distortion vs. Analog Input Frequency 55 50 45 40 35 INPUT FREQUENCY – MHz 0.1 1 10 100 30 +25 C AND +125 C –55 C 8.0 7.2 6.4 5.5 Figure 7. Dynamic Performance vs. Analog Input Frequency E |
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