FS7755

Rev. 1.1

16/22

F1

Freq

2

I

V

×

0

CF

F2

DSP

Ins tantaneous Real Pow er Signal

( Fr eq Domain)

Time

Time

CF

F1

v( t)

i( t)

ω

ω

2

Fig12Real Power to Impulse Converter

As can be seen in Fig12, the frequency output CF is seen to vary over time, even under steady load

conditions. This frequency variation is primarily due to the cos(2ωt) component in the instantaneous real

power signal. The output frequency on CF can be up to 2048 times higher than the frequency on F1 and

F2. This higher output frequency is generated by accumulating the instantaneous real power signal over a

much shorter time while converting it to a frequency. This shorter accumulation period means less

averaging of the cos(2ωt) component. As a consequence, some of this instantaneous power signal passes

through the digital to frequency converter. This will not be a problem in the application. When CF is used

for calibration purposes, the frequency should be averaged by the impulse counter. This will remove any

ripple. If CF is measuring energy, e.g., in a microprocessor based application, the CF output should also

be averaged to calculate power. Because the outputs F1 and F2 operate at a much lower frequency, more

averaging of the instantaneous real power signal is carried out. The result is a greatly attenuated

sinusoidal content and a virtually ripple-free frequency output.

7

Interfacing the FS7755 to a Micro- Controller for Energy Measurement

The easiest way to interface the FS7755 to a micro-Controller is to use the CF high frequency output with

the output frequency scaling set to 2048×F1, F2. This is done by setting SCF = 0 and S0 = S1 = 1 (see

Table Ⅳ). With full-scale AC signals on the analog inputs, the output frequency on CF will be

approximately 5.57 kHz. Fig13 illustrates one scheme that could be used to digitize the output frequency

and carry out the necessary averaging mentioned in the previous section.