AS8218 datasheet(17/123 Pages) AMSCO

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Data Sheet AS8218 / AS8228

Revision 3.0, 31-May-06

Page 17 of 123

The current input signal levels may be programmed by means of on-chip programmable gain settings. The

required gain setting is selected as follows:

Current Input Gain Settings

Gain

Input Voltage

Comments

Current Inputs I1P, I1N

-30mV

≤VI1P≤30mV

Shunt mode; default setting

-38mV

≤VI1P≤38mV

CT mode or shunt mode

-150mV

≤VI1P≤150mV

CT mode

Current Inputs I2P, I2N

-30mV

≤VI2P≤30mV

Shunt mode

-38mV

≤VI2P≤38mV

CT mode or shunt mode

-150mV

≤VI2P≤150mV

CT mode; default setting

Notes:

1) VI1N and VI2N are connected to VSSA.

2) Refer to the Settings Register (SREG) in the DSP section for programming of the Gain Settings.

For optimum operating conditions, the input signal at the Maximum Current (Imax) condition should be set at

±30mVp, when the Gain = 20, or ±150mVp, when the Gain = 4.

The default Gain, the AS8218 / AS8228 ICs current input gain settings without any programming required, is

Gain = 20 for the I1 input and Gain = 4 for the I2 input.

The value of an ideal shunt resistor, may be calculated as follows:

Assuming an Imax rating of 60A (rms) → 84.85A (peak), then a shunt value of 350µΩ would be suitable.

354

shunt

thus a standard 300µ

Ω shunt resistor may be selected.

The mains currents are sampled at 3.4956kHz, assuming that the recommended crystal oscillator frequency of

3.5795MHz, is used.

The current transformer(s) must be terminated with a voltage setting resistor (RVS) to ensure the optimum

voltage input level to the current input(s) of the AS8218 / AS8228 ICs. The value of RVS is calculated as

follows:

)

(

VS =

where IL

= CT RMS secondary current at rated conditions (Vmains; Imax)

Vin(p) = The peak input voltage to the IC at rated conditions (Vmains; Imax). For example, if Gain = 4,

Vin(p) should be set at 150mVpeak.

Example: A current transformer is specified at 60A/24mA and the Gain = 4:

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