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MAX154BEAG Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX154BEAG Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 12 page Input Current The converters’ analog inputs behave somewhat differ- ently from conventional ADCs. The sampled data com- parators take varying amounts of current from the input, depending on the cycle they are in. The equivalent cir- cuit of the converter is shown in Figure 9a. When the conversion starts, AIN(n) is connected to the MS and LS comparators. Thus, AIN(n) is connected to thirty-one 1pF capacitors. To acquire the input signal in approximately 1µs, the input capacitors must charge to the input voltage through the on-resistance of the multiplexer (about 600 Ω) and the comparator’s analog switches (2k Ω to 5kΩ per compara- tor). In addition, about 12pF of stray capacitance must be charged. The input can be modeled as an equivalent RC network shown in Figure 9b. As RS (source impedance) increases, the capacitors take longer to charge. Since the length of the input acquisition time is internal- ly set, large source resistances (greater than 100 Ω) will cause settling errors. The output impedance of an op- amp is its open-loop output impedance divided by the loop gain at the frequency of interest. It is important that the amplifier driving the converter input have suffi- cient loop gain at approximately 1MHz to maintain low output impedance. Input Filtering The transients in the analog input caused by the sam- pled data comparators do not degrade the converter’s performance, since the ADC does not “look” at the input when these transients occur. The comparator’s outputs track the input during the first 1µs of the con- version, and are then latched. Therefore, at least 1µs will be provided to charge the ADC’s input capaci- tance. It is not necessary to filter these transients with an external capacitor on the AIN terminals. Sinusoidal Inputs The MAX154/MAX158 can measure input signals with slew rates as high as 157mV/µs to the rated specifications. This means that the analog input frequency can be as high as 10kHz without the aid of an external track/hold. The maximum sampling rate is limited by the conversion time (typical tCRD = 2µs) plus the time required between conversions (tp = 500ns). It is calculated as: fMAX = 1 = 1 = 400kHz tCRD + tp (2.0 + 0.5) µs fMAX permits a maximum sampling rate of 50kHz per channel when using the MAX158 and 100kHz per channel when using the MAX154. These rates are well above the Nyquist requirement of 20kHz sampling rate for a 10kHz input bandwidth. CMOS High-Speed 8-Bit ADCs with Multiplexer and Reference _______________________________________________________________________________________ 9 MAX154 MAX158 VIN GND VDD REFOUT VREF+ AINx(+) AINx(-) +5V 0.1 µF47µF 0.01 µF VREF- MAX154 MAX158 VIN GND VDD VREF+ AINx(+) AINx(-) +5V 0.1 µF 47 µF VREF- MAX154 MAX158 VIN GND * Current path must still exist from VIN(-) to Ground VDD VREF+ AINx(+) AINx(-) * +5V 2.5V 0.1 µF 47 µF VREF- Figure 8a. Internal Reference Figure 8b. Power Supply as Reference Figure 8c. Inputs Not Referenced to GND |
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