AN2512 datasheet(9/38 Pages) STMICROELECTRONICS

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AN2512

Application description

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1.4.1

Current sensing circuit

The STPM01 has two external current sensing circuits, primary and secondary current

channels.

Normally, the second current circuit is used in single-phase meter implementation when the

anti-tamper feature is required. In this way it is possible to read also the current flowing into

the neutral wire to have a comparison with the current flowing into the line wire and detect

possible tampers.

In this application only the primary channel has been used. As a consequence, the

configuration of STPM01 is:

●

PST= 2 if a current transformer is used (this is the case of this meter);

●

PST= 0 (or 1) if a Rogowski coil is used

in the latter case ADDG bit can be used to have a further gain of x8.

The current channel uses a current transformer to sense mains current. The burden resistor

is used to produce a voltage between VIN1 and VIP1 proportional to the current measured.

1.4.2

Anti-aliasing filter

The anti-aliasing filter is a low-pass filter. It has a negligible influence on the voltage drop

between IIN1 and IIP1. Its aim is to reduce the distortion caused by the sampling, also

called aliasing, by removing the out-of-band frequencies of the input signal before sampling

it with the analog-to-digital converter.

Filtering is easily implemented with a resistor-capacitor (RC) single-pole circuit which

obtains an attenuation of -20dB/dec.

1.4.3

Voltage sensing circuit

A resistor divider is used as voltage sensor.

The 740 k

Ω resistor is separated into three, 2x270 kΩ and 1x200 kΩ, in-series resistors,

which ensure that a high voltage transient does not bypass the resistor. This also reduces

the potential across the resistors, thereby decreasing the possibility of arcing. The following

resistors are used to implement resistor divider:

●

R=R14+R15+R16=740 K

Ω,

●

R5=470

Ω.

Capacitor C11 and resistance (R19+ R15) create a filter which prevents Electromagnetic

Interference (EMI).

1.4.4

Crosstalk cancellation network

The voltage front-end handles voltages of considerable amplitude, which makes it a

potential source of noise. Disturbances are readily emitted into current measurement

circuitry where they interfere with the actual signal to be measured. Typically, this produces

a non-linear error at small signal amplitudes and non-unity power factors. At unity power

factor, voltage and current signals are in phase and crosstalk between voltage and current

channels merely appears as a gain error, which can be calibrated. When voltage and

current are not in phase, crosstalk has a non-linear effect on the measurements, which

cannot be calibrated.

AN2512 Datasheet(PDF) 9 Page - STMicroelectronics

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