Data Sheet AS8118

Revision 1.8, 15-Feb-05

Page 15 of 30

Defined Current and Calibration Time Method

The AS8118 generates a pulse whenever the internal energy accumulator contains a value, which is greater than a

programmed threshold. This threshold is the parameter Pulse_lev. This parameter depends on the basic meter properties,

Firstly, an ideal value for Pulse_lev is calculated. This is the value, which would have to be programmed into the AS8118

device if the meter system was perfect.

The following formula calculates this ideal value of Pulse_lev:

435924

20

230

*

I

A

*

V

V

)

ideal

(

lev

_

Pulse

max

mains

=

A calibration is performed to compensate for system non-idealities like resistor tolerances etc. The effect of these non-

idealities is that with the ideal Pulse_lev value the pulse rates will not be correct. In order to calibrate the meter, a new

Pulse_lev

value has to be found. Figure 6 shows the basic calibration setup. CAL pulses from a meter built with the AS8118

are counted during a defined time period tc, while an accurately defined calibration current Ical, is being measured.

Calboard

AS8118

Meter

I

cal

time

base

t

c

Figure 6

Basic calibration setup for defined current and calibration time method

Again, if the system was perfect we would expect a certain number of pulses to be counted, the ideal number of pulses, Ni:

1000

3600 *

Ical

*

V

*

tc

*

PR

Ni

mains

=

where tc is in seconds.

PR is the pulse rate on pin CAL, which can be calculated from the PROM parameters F_mon_sel and F_cal_sel:

Fcal

*

Fmon

PR

=

where Fmon [imp/kWh] is the pulse rate selected by F_mon_sel and Fcal is the multiplier selected by F_cal_sel.

If pin LED is used for calibration then PR is the pulse rate on pin LED (selected by F_led_sel).

The corrected value for Pulse_lev can now be calculated using the following formula:

Ni

Nr

*

)

ideal

(

lev

_

Pulse

)

corrected

(

lev

_

Pulse

=

,

where Nr is the real number of pulses, i.e. the number of pulses counted during tc.