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ML2259CCP Folha de dados(PDF) 9 Page - Micro Linear Corporation |
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ML2259CCP Folha de dados(HTML) 9 Page - Micro Linear Corporation |
9 / 13 page ML2252, ML2259 9 1.4 REFERENCE The voltage applied to the +VREF and –VREF inputs defines the voltage span of the analog input (the difference between VINMAX and VINMIN) over which the 256 possible output codes apply. The devices can be used in either ratiometric applications or in systems requiring absolute accuracy. The reference pins must be connected to a voltage source capable of driving the reference input resistance, typically 20k. In a ratiometric system, the analog input voltage is proportional to the voltage used for the A/D reference. This voltage is typically the system power supply, so the +VREF pin can be tied to VCC and –VREF tied to GND. This technique relaxes the stability requirements of the system reference as the analog input and A/D reference move together maintaining the same output code for a given input condition. For absolute accuracy, where the analog input varies between specific voltage limits, the reference pins can be biased with a time and temperature stable voltage source. +VREF and –VREF can be at any voltage between VCC and GND. In addition, the difference between +VREF and –VREF can be set to small values for conversions over smaller voltage ranges. Particular care must be taken with regard to noise pickup, circuit layout ond system error voltage sources when operating with a reduced span due to the increased sensitivity converter. 1.5 POWER SUPPLY AND REFERENCE DECOUPLING A 10µF electrolytic capacitor is recommended to bypass VCC to GND, using as short a lead length as possible. In addition, with clock frequencies above 1MHz, a 0.1µF ceramic disc capacitor should be used to bypass VCC to GND. If REF+ and REF– inputs are driven by long lines, they should be bypassed by 0.1µF ceramic disc capacitors at the reference input pins (pins 12, 16). 1.6 DYNAMIC PERFORMANCE Signal-to-Noise Ratio Signal-to-noise ratio (SNR) is the measured signal to noise at the output of the converter. The signal is the rms magnitude of the fundamental. Noise is the rms sum of all the nonfundamental signals up to half the sampling frequency. SNR is dependent on the number of quantization levels used in the digitization process; the more levels, the smaller the quantization noise. The theoretical SNR for a sine wave is given by SNR = (6.02N + 1.76)dB where N is the number of bits. Thus for ideal 8-bit converter, SNR = 49.92dB. Harmonic Distortion Harmonic distortion is the ratio of the rms sum of harmonics to the fundamental. Total harmonic distortion (THD) of the ML2252 and ML2259 are defined as 20 2 2 3 2 4 2 5 2 12 1 log () / VVV V V ++ + where V1 is the rms amplitude of the fundamental and V2, V3, V4, V5 are the rms amplitudes of the individual harmonics. Figure 6. Output Spectrum 37.5 75 0 –10 –20 –30 –40 –50 –60 –70 –80 –90 –100 –110 FREQUENCY (kHz) |
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