Product Tutorials

Power Meter Tutorials

Background

Common Mode Voltage Rejection

What is common mode

The input circuitry of the power meter is floating from ground (earth potential). There is a so called stray capacitance between this input circuitry and the case.
When a high voltage containing high frequencies is applied to the input terminals, it produces high frequency current through the input circuitry and the stray capacitance to earth.
It causes error that the current produce the potential difference.
Both paths not being identical causes a potential difference at the input of the DA-converter, that did originally not exist at the terminals.

Influence of common mode voltage

The measured value of a current influenced by a common mode voltage is shown below:

NOTE: Few percentage error caused by common mode voltage can be bypassed.

Common mode rejection ratio specification

An applied Common mode voltage will generate noise on the input circuitry which will result in a detected Urms and Irms on the display after A/D conversion, which is most critical for current measurements due to low shunt voltage.
YOKOGAWA specifies the effect of Common Mode Voltage as follows:
When 600Vrms at 50/60Hz Common Mode Voltage is applied to the Voltage or Current input the displayed noise value will be less then 0.01% of the selected range for all ranges. When we express the value in dB against selected range we get:

Common mode rejection ratio specification

Practically this means that in the 10V range the displayed value will be lower then:

This also means that in the 1A range the displayed value will be lower then:

The Yokogawa method gives immediately the error influence on the final result in % of the selected range. Most other manufacturers specify the Common Mode Voltage effect by using strictly the CMRR formula:

Common mode rejection ratio specification

Common mode rejection ratio specification

For example:

Voltage input:
120dB Common Mode Rejection Ratio at 50Hz
Current input:
140dB Common Mode Rejection Ratio at 50Hz

When 600Vrms is applied to the inputs the maximum measured voltage can be calculated as follows:

For the Voltage input with 120 dB CMRR at 50Hz, the applied 600 Vrms will give a maximum measured voltage of:

In the 10V range this means a maximum error of 0.006 % of range.
For the Current input with 140 dB CMRR at 50Hz, the applied 600 Vrms will give a maximum measured voltage of:<

In the current range the voltage should be transferred to a current value by using the formula:

When a 0.1 ohm shunt in 1 A range has been used it means that 60 mV noise is equivalent to 600 mA error which is an error of 0.06% of range.

Conclusion: Yokogawa uses a different way of specifying with the advantage that the error can easily be determined for current and voltage. Other manufacturers follow the strict CMRR formula with the disadvantage that it is rather complicated to calculate the error in the current range caused by the Common Mode Voltage.