FS7755

Rev. 1.1

17/22

FS6611

MCU

Counte r

Time r

CF

REV P*

UP/ DOWN

* REV P must be used i f the meter is bi di recti onal

or di re c ti on of e ne rgy fl ow i s ne e de d

< 10%

CF

Tim e

Average

Frequency

Frequency

Ripple

±

Fig13 Interfacing the FS7755 to MCU

As shown, the frequency output CF is connected to an MCU counter or port. This will count the number of

pulses in a given integration time that is determined by an MCU internal timer. The average power

proportional to the average frequency is given by:

Average Frequency = Average Real Power =

Timer

Counter

The energy consumed during an integration period is given by:

Energy=Average Power×Time=

Time

Counter

×Time=Counter

For the purpose of calibration, this integration time can be 10 to 20 seconds to accumulate enough pulses

to ensure correct averaging of the frequency. In normal operation, the integration time can be reduced to

one or two seconds depending, for example, on the required update rate of a display. With shorter

integration times on the MCU, the amount of energy in each update may still have some small amount of

ripple, even under steady load conditions. However, over a minute or more, the measured energy will have

no ripple.

7.1 Power Measurement Considerations

Calculating and displaying power information will always have some associated ripple that will depend on

the integration period used in the MCU to determine average power and also the load. For example, at

light loads, the output frequency may be 10Hz. With an integration period of two seconds, only about 20

pulses will be counted. The possibility of missing one pulse always exists, since the FS7755 output

frequency is running asynchronously to the MCU timer. This would result in a one-in-twenty (or 5%) error

in the power measurement.

8

Transfer Function

8.1

Frequency Outputs F1 and F2

The FS7755 calculates the product of two voltage signals (on Channel 1 and Channel 2) and then low

pass filters this product to extract real power information. This real power information is then converted to a

frequency. The frequency information is output on F1 and F2 in the form of active low pulses. The pulse

rate at these outputs is relatively low, e.g., 0.34Hz maximum for AC signals with S0 = S1 = 0 (see Table III).

This means that the frequency at these outputs is generated from real power information accumulated

over a relatively long period of time. The result is an output frequency that is proportional to the average

real power. The averaging of the real power signal is implicit to the power to impulse converter. The output

frequency or pulse rate is related to the input voltage signals by the following equation.