UM0971 datasheet(18/45 Pages) STMICROELECTRONICS

18 / 45 page

Board description

UM0971

18/45

Doc ID 17701 Rev 1

2.6

Current sensing and conditioning network

2.6.1

Bipolar current reading configuration

The details of bipolar current sensing (also referred to as Alternating AC) reading

configuration is shown in Figure 11. In this configuration, the alternating current signal on

the shunt resistor, with positive and negative values, must be translated to be compatible

with the single positive input of the microcontroller ADC converter used to read the current

value. This means that the op amp must be biased in order to obtain a voltage on the output

which makes it possible to measure the symmetrical alternating input signal.

Basically, the output signal from the op amp is made up of two terms: a bias voltage Vbias

and an amplification of voltage drop across the shunt resistor (G). The formulas below show

the relationships between network components and signal values.

Figure 11.

AC current reading configuration

Equation 5

Where:

Equation 6

and:

Table 7.

Virtual neutral point reconstruction

Jumper

Position

Description

Between 1-2

Three resistors used

Between 2-3

DC bus voltage divider

Vout

Vbias

Rshunt I

⋅

()

⋅

Vbias

--------

⎝⎠

⎛⎞

⋅

-------------------------------------------------------

------------

⋅

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