FS7755

Rev. 1.1

11/22

13.

Detail Description

1

Theroy of Operation

The two ADCs digitize the voltage signals from the current and voltage transducers. These ADCs are 16-bit

second order sigma-delta with an oversampling rate of 895 kHz. This analog input structure greatly simplifies

transducer interfacing by providing a wide dynamic range for direct connection to the transducer and also by

simplifying the anti-aliasing filter design. A programmable gain stage in the current channel further facilitates

easy transducer interfacing.

The real power calculation is derived from the instantaneous multiplication of the current and voltage signals. In

order to extract the real power component (i.e., the DC component), the instantaneous real power signal is a

low pass filter output. Fig5 illustrates the instantaneous real power signal. This scheme correctly calculates

instantaneous real power for non-sinusoidal current and voltage waveforms at all power factors. All signal

processing is carried out in the digital domain for superior stability over temperature and time.

2

I

V

×

ADC

ADC

V

1P

V

1N

V

2P

V

2N

PGA

x1, x8, x16, x32

F1

CF

F2

Instantaneous

i(t) & v(t) Signal

DSP

Instantaneous Real

Power Signal

Time

Time

i(t), v(t)

P(t)

Fig5

Signal Flow Diagram

The low frequency output of the FS7755 is generated by accumulating this real power information. This

low frequency inherently means a long accumulation time between output pulses. The output frequency is

therefore proportional to the average real power. This average real power information can, in turn, be

accumulated (e.g., by a counter) to generate real energy information. Because of its high output frequency

and shorter integration time, the CF output is proportional to the instantaneous real power. This is useful

for system calibration purposes that would take place under steady load conditions.

2

Power Factor Considerations

The method used to extract the real power information from the instantaneous power signal (i.e., by low

pass filtering) is still valid even when the voltage and current signals are not in phase. Fig6 displays the

unity power factor and a power factor = 0.5 conditions, i.e., current signal lagging the voltage by 60

assume the voltage and current waveforms are sinusoidal, the real power component of the instantaneous

power signal (i.e., the DC term) is given by:

60

cos

2

×

⎟

⎠

⎞

⎜

⎝

⎛ × I

V

This is the correct real power calculation.