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【正文】 half the sampling frequency. With a 10kHz sampling frequency, for example, the ADC is not able to distinguish between a 4kHz and a 6kHz input, because both are symmetrical about half the sampling frequency (5kHz). Though too detailed for the scope of this article, this effect (aliasing) and the theorem that describes it (Nyquist39。s R2R ladder. As the DAC39。s internal buffer eliminates this concern. For this design, the internal reference was overridden by an external, precision, lowdropout, lowdrift voltage reference (MAX873A). Although its use (vs. the internal reference) provided no noticable change in results, the MAX873A offers better initial accuracy and drift. Gaining an idea of the drift specification (in ppm/176。s operating temperature range. For this application, assume a normal lab temperature range of 30176。C. To normalise this drift value to ppm/176。C for a 14 bit system, 10ppm/176。C for a 10 bit system. These figures change according to the allowable temperature span and total reference drift. The MAX125 is driven by an Atmel AVR processor (AT90S4414), and both are clocked at the same frequency (10MHz) derived from an oscillator module. The program was written in assembly language, with a single instruction toggling each data line to reflect timing diagrams in the MAX125 datasheet. The program is divided into subroutines for easier understanding, and most lines are mented for the same At startup or poweronreset, the MAX125 is configured using the bidirectional data pins D3D0. Setting these pins to 0011 and strobing WR low sets the ADC39。s RS232 port. Each conversion includes 16 bits of data, so each result requires two RS232 write cycles. Because the ADC stores the conversions from channels 24, the time taken to transmit the RS232 data can be fairly leisurely. When the data from channel 1 has been sent, the process is repeated for Channels 24. Noise pickup was noted on unused data pins of the microprocessor bus (a 16bitwide bus with only 14 data lines). If these unused pins are configured as inputs, the noise faithfully masquerades as intermittent data. Fortunately, however, a simple software AND function filters out the induced noise and they say digital filtering is tough! As an alternative, you can configure the unused ports as outputs. The above code works perfectly, but establishing correct circuit performance can prove difficult. The interrupt pulse from the 181。s) that only a storage oscilloscope can detect it. If a suitable oscilloscope is not available, you can insert a delay routine immediately after the CONVST line has been set HIGH to enable a more leisurely viewing of the interrupt signal. The PC Interface The key ponent interfacing the microprocessor to the PC is the serially addressed UART (MAX3100). It takes input from the microprocessor via its 4wire SPI/Microwire bus3, and converts this data to the format used by most RS232 patible devices. An industrystandard, micropower 2TX/2RX transceiver (MAX202E) implements the level translation. This UART, which has hardware and software shutdown modes and es in a tiny QSOP package, is ideally suited for lowpower portable appliances. For hazardous environments, its interface also allows IrDA munication by attachment of suitable optical devices. Other features (not required in this application) include a receive buffer and an interrupt flag to give status information on the transmit and receive registers. A softwarebased SPI routine is implemented for exporting the code to microprocessors lacking a builtin SPI bus. The code shows that this is a trivial task, and ments guide the uninitiated through the clock cycles of the SPI bus. If a dedicated SPI port is used, it is worth noting that the MAX310039。 just the voltage levels, slew rate, and connector details. The MAX3100 and MAX202E, however, handle all the standards for making data pliant with RS232, and in a protocol expected by the PC. The Windows Interface The Windows program was written in Visual Basic Version 6. It takes readings from the RS232 port using the MSComm function, configured to accept text, and converts this data to ASCII. It also translates each 16bit data block back to the voltage and current measurements obtained from analogsensing circuitry that precedes the ADC input. From these results, the input power, output power, and efficiency are calculated and displayed on the PC screen, rounded to three decimal places. In addition, the system can freeze the results if they need to be logged. The measured efficiency figures were far from consistent. This was accounted for by noticing ripple on the current and voltage waveforms. The resistors were bypassed with 10nF capacitors to reduce the currentripple readings, but the resistivedivider works measuring voltage were left unbypassed. Thus, the measured ripple (20mV on the input and 100mV on the output) accounted for the 177。74HC00 works well for this purpose. A number of factors should be considered to obtain absolute dc accuracy. The MAX873A reference offers excellent drift specifications
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