STPM11ATR datasheet(38/45 Pages) STMICROELECTRONICS

38 / 45 page

STPM1x calibration

STPM11/12/13/14

38/45

Each calibrator value can be changed from a binary form to a decimal correction form, using

the following formula:

Kv=(Cv/128)*0.125 + 0.75 and the same for Ki.

Let us choose as initial value Ai=32

From the target power constant CP of the meter and the actual values of VRMS and IRMS,

which are applied to the meter under calibration, the error of power measurement can be

calculated:

err = 100(fx/f -1) [%], where fx is the real frequency read at LED output.

Now, a final unit less power reduction factor can be calculated:

pF = (pD - err)/100

This final power reduction factor can be considered as a product of voltage and current

reduction factors which are produced from corresponding calibration constants. So, an

obvious solution to obtain the voltage and current reduction factors is to calculate a common

reduction factor as a square root of pF. This result must fall within the indicated range,

otherwise the device cannot be calibrated:

768

≤R = 1024 pF + 0.125 < 1024

In order to obtain the corresponding calibration constants, the reduction factor must be

transformed:

CV = CC = R - 768

By using separately the integer and the fractional part of the common reduction a better fit of

calibration constants can be produced. Simply, let's set one of the two calibration registers

(e.g. CV) to the lowest integer value of R, while the other (CC) should be set to the nearest

integer value of R. Examples:

R-768=128.124; in this case set CV=128; set CC=128

R-768=127.755; while in this other one set CV=127; set CC=128.

Note:

STPM13/14: each current channel must be calibrated separately. In order to do this, follow

these steps:

Apply the nominal test voltage to the voltage sensor, and the nominal test current to the

primary current channel sensor. Do not apply such current on the secondary current

channel sensor.

Adjust the voltage and primary current calibrators (see above).

Disconnect the nominal test current from the primary current channel sensor, and apply it to

the secondary current channel sensor.

Adjust only the secondary current calibrators, so that the same power is computed.

Table 18.

Calibration results

Description

Value

Value of Calibrator

Kp = Kv*Ki = 0.765625

Frequency at LED

f = P*In*Vn/3600000 = 40.8889 Hz

Voltage divider

Sv = (F*DL*Vbg2)/(fM*Vn*In*Gv*Gi*Kp*Ai*Av*Si)= 0,6324mV/V

Voltage divider resistor

R1=R2*(1000/Sv-1)

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