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AD9058KD Arkusz danych(PDF) 7 Page - Analog Devices |
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AD9058KD Arkusz danych(HTML) 7 Page - Analog Devices |
7 / 8 page REV. B AD9058 –7– t V t PD N tA N + 1 ANALOG INPUT ENCODE D – D 0 7 DATA CHANGING VALID DATA FOR N – 1 VALID DATA FOR N VALID DATA FOR N + 1 N + 2 tV = DATA DELAY OF PRECEDING ENCODE t PD = OUTPUT PROPAGATION DELAY = APERTURE TIME t A Figure 4. AD9058 Timing Diagram Figure 5 below 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. AD9058 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 in- terval 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 impor- tant 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 – LOG2 [Error (measured)/Error (ideal)] 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 0.1 1 10 100 INPUT FREQUENCY – MHz 8.0 7.2 6.4 5.5 55 50 45 40 35 30 +25°C AND +125°C –55°C Figure 7. AD9058 Dynamic Performance vs. Analog Input Frequency |
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