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Shunt resistor selection for Energy Probe

With 20x amplification, Energy Probe requires the correct selection of a shunt resistor to provide the best possible dynamic range in power measurement, while avoiding saturation of the input of Energy Probe.

A shunt resistor with a value that is too low reduces measurement dynamic range, resulting in less resolution in the power data. A shunt resistor with a value that is too high can cause the input stage of the Energy Probe to saturate, which causes a flat line in the charts that are related to Energy Probe in the Timeline view.

To avoid input saturation, the drop across the shunt resistor must never be more than 165mV. You can also use the following equation to determine whether your shunt resistor is appropriate:

RShunt(max) = 165 x Vsupply / ( 1000 x Power ) 

Vsupply is the input/core voltage. Power(max) is the maximum power that the Energy Probe measures. Rshunt(max) is the maximum value of the shunt resistor. A shunt resistor value that is greater than Rshunt(max) might cause input saturation.

This equation provides the absolute maximum value for the shunt resistor. Use a value that is more than five percent lower than this to allow for component tolerances.

Note

When connecting the Energy Probe, consider the following:
  • The black and white probe closest to the green wire is Channel 0.
  • For best results, attach Channel 0 to the power source which best represents the CPU load. Streamline aligns the power data with the software activity by maximizing the correlation of Channel 0 with the CPU load.
  • The probe white wire is V+. The black wire is V-.

Examples

  • 5V power supply, 8W(max), Rshunt(max) = 100 milliohms.

  • 1V core voltage, 2.5W(max), Rshunt(max) = 50 milliohms.

  • 1.5V core voltage, 0.4W(max), Rshunt(max) = 500 milliohms.

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