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ADS574KP Arkusz danych(PDF) 6 Page - Texas Instruments |
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ADS574KP Arkusz danych(HTML) 6 Page - Texas Instruments |
6 / 17 page ® ADS574 6 THEORY OF OPERATION In the ADS574, the advantages of advanced CMOS technol- ogy—high logic density, stable capacitors, precision analog switches—and Burr-Brown’s state of the art laser trimming techniques are combined to produce a fast, low power analog-to-digital converter with internal sample/hold. The charge-redistribution successive-approximation circuitry converts analog input voltages into digital words. A simple example of a charge-redistribution A/D converter with only 3 bits is shown in Figure 1. approximation is made by connecting S2 to the reference and S 3 to GND, and latching S2 according to the output of the comparator. After three successive approximation steps have been made the voltage level at the comparator will be within 1/2LSB of GND, and a digital word which represents the analog input can be determined from the positions of S 1, S2 and S3. OPERATION BASIC OPERATION Figure 2 shows the minimum circuit required to operate the ADS574 in a basic ±10V range in the Control Mode (dis- cussed in detail in a later section.) The falling edge of a Convert Command (a pulse taking pin 5 LOW for a mini- mum of 25ns) both switches the ADS574 input to the hold state and initiates the conversion. Pin 28 (STATUS) will output a HIGH during the conversion, and falls only after the conversion is completed and the data has been latched on the data output pins (pins 16 to 27.) Thus, the falling edge of STATUS on pin 28 can be used to read the data from the conversion. Also, during conversion, the STATUS signal puts the data output pins in a High-Z state and inhibits the input lines. This means that pulses on pin 5 are ignored, so that new conversions cannot be initiated during the conver- sion, either as a result of spurious signals or to short-cycle the ADS574. The ADS574 will begin acquiring a new sample as soon as the conversion is completed, even before the STATUS output falls, and will track the input signal until the next conversion is started. The ADS574 is designed to complete a conversion and accurately acquire a new signal in 25 µs max over the full operating temperature range, so that conversions can take place at a full 40kHz. CONTROLLING THE ADS574 The Burr-Brown ADS574 can be easily interfaced to most microprocessor systems and other digital systems. The microprocessor may take full control of each conversion, or the converter may operate in a stand-alone mode, controlled only by the R/C input. Full control consists of selecting an 8- or 12-bit conversion cycle, initiating the conversion, and reading the output data when ready—choosing either 12 bits all at once, or the 8 MSB bits followed by the 4 LSB bits in a left-justified format. The five control inputs (12/8, CS, A0, R/C, and CE) are all TTL/CMOS-compatible. The functions of the control inputs are described in Table II. The control function truth table is shown in Table III. STAND-ALONE OPERATION For stand-alone operation, control of the converter is accom- plished by a single control line connected to R/C. In this mode CS and A0 are connected to digital common and CE and 12/8 are connected to +5V. The output data are pre- sented as 12-bit words. The stand-alone mode is used in systems containing dedicated input ports which do not require full bus interface capability. FIGURE 1. 3-Bit Charge Redistribution A/D. INPUT SCALING Precision laser-trimmed scaling resistors at the input divide standard input ranges (0V to +10V, 0V to +20V, ±5V or ±10V) into levels compatible with the CMOS characteristics of the internal capacitor array. SAMPLING While sampling, the capacitor array switch for the MSB capacitor (S1) is in position “S”, so that the charge on the MSB capacitor is proportional to the voltage level of the analog input signal. The remaining array switches (S2 and S3) are set to position “G”. Switch S C is closed, setting the comparator input offset to zero. CONVERSION When a conversion command is received, switch S1 is opened to trap a charge on the MSB capacitor proportional to the analog input level at the time of the sampling command, and switch S C is opened to float the comparator input. The charge trapped in the capacitor array can now be moved between the three capacitors in the array by connecting switches S1, S2, and S3 to positions “R” (to connect to the reference) or “G” (to connect to GND), thus changing the voltage generated at the comparator input. During the first approximation, the MSB capacitor is con- nected through switch S 1 to the reference, while switches S2 and S3 are connected to GND. Depending on whether the comparator output is HIGH or LOW, the logic will then latch S1 in position “R” or “G”. Similarly, the second C R G 2C R G 4C R G + – S 3 S 2 S 1 S C Comparator L o g i c Reference Input S Signal Analog Input Out |
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Podobny opis - ADS574KP |
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