【正文】 t an idea of how low the source impedance has to be, consider a simple calculation of RC time constant. The input capacitor charges according to vcap = vin (1et/CR), where C is the sampling capacitor, R is the source resistance, and Vin is the voltage applied to the RC circuit. The difference in voltage between Vin and Vcap is: vin vcap = vin et/CR Therefore, given the value of the sample and hold capacitor (normally 1030pF), the charging time of the capacitor, and the error voltage that can be tolerated (1/2 LSB), one can calculate the maximum allowable signalsource resistance that will charge the capacitor in a given time. The input buffer can also serve as a filter for removing unwanted signals. Because the ADC is a sampling system, its output has spectral symmetry centered about half the sampling frequency. Thus, signals greater than half the sampling frequency cannot be distinguished from signals less than 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 frequency response should therefore be higher than 100kHz, but a slower response is also acceptable because the switching waveforms are reasonably repetitive. The system39。s 14bit resolution was not of prime importance in the design stage, because measurements to within % (10 bits) are perfectly adequate for most powermeter applications. Once digitized, the results are uploaded via an RS232 interface and displayed on a PC screen, with an update rate of once per second (like most multimeters). Once data arrives at the RS232 port, the power of the PC can be unleashed upon it. The full circuit diagram (Figure 2) will be discussed in sections: Figure 2. Circuit diagram. Analog Front End A precision resistive divider allows accurate voltage measurements up to 30V. The ORNA101 from Vishay Thin Film is a trimmed resistor work prising two 1k and two 10k values. The ratiometric accuracy of these four resistors is %, and so provides a precision fraction (1/11) of the applied voltage. The MAX12539。s theorem) are described in many papers and books. An antialiasing filter removes these unwanted frequencies. The effect of aliasing can be quite subtle if you consider the frequency spectrum of any signal that is not purely sinusoidal. A nonsinewave by definition has harmonics, and these higher harmonics translate downward to cause errors at the lower frequencies. Again, unless the input signal frequency range is definitely known, a bination buffer and antialiasing filter is always good design practice. Minimal filtering is required because the system in question takes only spot readings. For higher sampling frequencies, a simple lowpass filter using ponents C1, C2, C3, and C4 provides signal rolloff ahead of the ADC input. These ponents also serve to average out the readings taken by the ADC, and they effectively remove ripple from the input and output power measurements. Digitization The MAX125 is a selfcontained, simultaneoussampling, successiveapproximation dataacquisition system with onchip RAM. It samples two sets of four signals using 2x4 sample/hold amplifiers, which are then digitized sequentially by a single ADC block. The results are stored in RAM and read out sequentially on the parallel data bus under processor control. The MAX125 also includes a voltage reference whose spuriousfree dynamic range (SFDR) is suitable for many highresolution DSP applications. (The internal reference can be overridden by an external system reference.) The MAX125 also exhibits good INL (the maximum deviation of real from theoretical ADC output), good DNL (the deviation of real from theoretical ADC step magnitude) and monotonicity to 13 bits (an output code that increases or remains unchanged in response to an increasing analog input). To enable efficient use of the microprocessor, the MAX125 provides a pushpull interrupt output indicating when the conversion has finished. An internal buffer allows use of either the internal or an external reference. As for the analog input, the reference input should be driven from a lowimpedance source. This applies especially for ADCs that employ a successiveapproximation register (SAR), because the reference voltage feeds directly into the DAC39。C) needed for any digitisation system is straightforward. First, define the system39。C, divide by the reference voltage and multiply by 106. The result is normally about 2ppm/176。s mode of operation to receive inputs from all four channels of MUX A. Once this is established, the microprocessor ports are reconfigured as inputs to receive data from the ADC. When CONVST is pulsed low, the processor waits for an interrupt signal from the MAX125. It then reads out the conversion of channel 1 and sends the result (via the MAX3100 UART and the MAX202E) to the PC39。s serial clock line (