THE DIGITAL WATTMETER

This design project is meant to cover several aspects of digital design. First, it is a very high speed system - one that would be too fast for most micocontrollers and hence the CPLD approach is realistic (as contrasted with our previous projects which would be better implemented in a micro controller). It also illustrates some realistic applications involving analog functions as well as digital. It also illustrates some digital arithmetic processes.

The wattmeter will take the form shown below.

The AVERAGE POWER delivered to a load may be computed as the INSTANTANEOUS POWER averaged over time (at least one complete cycle, often longer). The instantaneous power is simply v(t)*I(t). We shall assume analog circuit which will provide two voltages Vv and Vi which are proportional to the voltage and current in the circuit to be measured. We shall assume that the frequency of the signal may be as high as 50KHz. This means that, to obtain reasonable accuracy at 20 samples per cycle, we will need to take 1 sample every microsecond. During this 1 microsecond, we will have to multiply two 8-bit numbers and compute the average power. The National ADC06081 ADC and the DAC0800 will be assumed for this design The ADB08061 is a dual channel ADC and so both signal may be converted with the one device.

The challenge in this design will be (1) reading, studying, and interpreting the specifications for the ADC and DAC and applying them to the design, and (2) Obtaining the product of the two digital inputs.

Some consideration must be given to the maximum input frequency and the affect of sampling the current at a slightly different time than the voltage. An analysis of these two consideration is provided

Before we proceded with the design, some actual results will be seen (to prove that it can, in fact, be done)..

The method of multiplication to be used is Booths' Algorithm, which you should check out.

(To be continued...

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