AN2299

RC calibration

Rev2

21/26

5

RC calibration

The purpose of RC oscillator calibration is to bring the working frequency into the band of

interest (4.0 < F < 4.2MHz) in order to properly set some absolute definitions of time in the

device. The calibration can be obtained either by changing the value of current settling

resistor which should be connected between pin CLKIN and Vss or by programming of one

or two OTP bits (CRC). The first possibility allows a precise calibration to a certain value

while the second one allows only coarse calibration in few steps. In general, it is not so

important to have a precise frequency of a certain value as long it stays stable under

temperature variation or aging. On the other hand, the value of this frequency directly

influences the power constant of the meter. For example, if Rogowski coil is used, the

constant C changes twice as much as frequency F (dC/C = 2dF/F) but, if CT or shunt is

used, the constant C changes as frequency F (dC/C = dF/F).