【正文】 easured. If any interval exceeded seconds, the PC counter was reset in anticipation of the first character in the subsequent data packet. Results The device under test (DUT) was the MAX1705, a boost DCDC converter configured in PWM mode. Loads were applied to the evaluation kit, and the results taken by freezing the MAX125 PC program and reading values from the screen. The results were then confirmed using the (now obsolete) digital multimeter, and pared with the expected efficiency as given by graphs in the datasheet: Table 1. Load/ Vin/V Iin/A Vout/V Iout/A Efficiency/% (exp. Eff/%) 95 50 92 100 83 390 56 Calibration, Errors and Tweaks Though digital purists may argue differently, the analog board layout must be impeccable to obtain the best circuit performance. All power rails to the analog ICs should be thick, and decoupled to ground with capacitors physically close to the devices: at least 1uF (tantalum) in parallel with 100nF (ceramic). A good ground plane should be included, especially around the MAX471, because high frequency, high amplitude currents occupy that territory. The resistors on the MAX471 should be precision types with low inductance, because any tolerance on these ponents directly affects the results. All analog PCB tracks should be as short as possible and far away from digital lines. The MAX125 reference amplifier should be decoupled at input and output, close to the IC. Ideally, the board should incorporate separate analog and digital ground planes, connected at a single point. It was observed that the 10MHz clock appears in unexpected places, so if this signal is to be routed across the pc board it should be physically remote from analog inputs, especially the resistor works and with a ground track in between. To further minimize noise radiation, this signal can be filtered to remove clock harmonics and reconstituted using a simple digital gate placed close to the processor and ADC. A 39。C es and goes so fast (in 1181。C for a 12 bit system, and 30ppm/176。C. Next, decide on the maximum drift (in volts) that can be tolerated (normally 1/2 to 1 LSB) over the temperature range, and hence the drift in V/176。s input code changes, the input impedance of the reference input also changes, thereby modulating the reference voltage. Fortunately, the MAX12539。5V, which allows a degree of safety under fault conditions and plenty of headroom for input and output voltage measurements. Input and output currents are measured using the precision current amplifier MAX471. This device measures the voltage across an internal 30m resistor, and delivers an output current whose amplitude equals 500uA per ampere of highside current. Thus, the value of an external resistor scales the output voltage, and suitable resolution is achieved in this case by selecting a value of . The MAX12539。APPLICATION NOTE 1138 Practical Data Acquisition using a Windows1based Power Meter Abstract: This application note describes the design of a PCbased, 14bit data acquisition system. It takes a system approach, includes all the necessary building blocks: analog, digital, hardware, and software. It discusses each step, testing systems separately before integrating them, and detailing pitfalls learned along the way. Many articles have been written about the building blocks in a typical data acquisition system, but few address the entire system, from analog input to PC display. To cover all the problems encountered in designing a plete data acquisition system, the engineer might have to amass ten articles. The following application note describes the design of a PCbased, 14bit data acquisition system. It takes a system approach, includes all the necessary building blocks: analog, digital, hardware, and software. It discusses each step, testing systems separately before integrating them, and detailing pitfalls learned along the way. The Design Specification The task: Design a power meter based on a 14bit simultaneoussampling ADC with onchip RAM (MAX125). The need for a power meter is apparent to anyone who has tried measuring the input and output characteristics of a DCDC converter using conventional instruments. The design allows users to perform load measurements on the device under test without connecting an endless spaghettimass of test leads. Figure 1 shows the pleted power meter display on a personal puter (PC) monitor. Figure 1. Windows PC output with example readout. To cater for boost, buck, and linear implementations, the measurement range was chosen as 30V for both input and output. Most quality DCDC converters operate at 100kHz or higher. The system39。s faultprotected input has a range of 177。s R2R ladder. As the DAC39。s operating temperature range. For this application, assume a normal lab temperature range of 30176。C for a 14 bit system, 10ppm/176。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 ca