DocID028405 Rev 1

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UM1955

Current sensing configuration

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2.2

How to set up the board

With this expansion board, users can measure the current of a USB-powered device or any

5 V application. It is important not to go beyond 5 V in order to avoid operational amplifier

damage. To help protect the circuit, an ESDAULC6-1U2 unidirectional ESD protection

device has been implemented.

Monitoring USB-powered device current: connect the charger (wall adapter or PC) to the

Micro-USB port. Then connect the bottom USB port to the required device.

Monitoring other application current: it is also possible to measure the current of

applications that are not USB powered. The power supply voltage should be connected to

2.3

Theoretical output

The output voltage of the operational amplifier is proportional to the measured current, as

the following formula illustrates:

Equation 4

Therefore,

Equation 5

Similar to the instrumentation amplifier configuration, the operational amplifier for this

configuration is used with a 5 V power supply voltage. Thus, in order to obtain a range

limited to 3.3 V avoid damage on the microcontroller side, a divider bridge has been added.

This additional structure is recognizable by the last term in the above formula (composed by

R30 and R39). Thanks to this structure, a DC 5 V output voltage is reduced to a 3.2 V.

Note that the maximum frequency of this circuit is limited to 330 Hz based on operational

amplifier GBP and the circuit gain.

Equation 6