Cirrus Logic AN365 User Manual
A
v(t)
i(t)
R
Figure 1. Rogowski Coil Current Sensor
AN365
Application Note
Using the CS5480/84/90 Energy Measurement IC
with Rogowski Coil Current Sensors
1. Introduction
Rogowski coil, or di/dt, current sensors have been increasingly implemented in AC power and energy measurement
applications because of their exceptional linearity, high current capacity and dynamic range, light weight, and
electrical isolation. The Cirrus Logic energy measurement ICs CS5480, CS5484 and CS5490 support various types
of Rogowski coils.
This application note presents accuracy results from testing the CS5480 with three different types of Rogowski coil
sensor. The CS5484 and CS5490 use the same core technology. Testing results of the CS5484 and CS5490 show
nearly identical results. Comparable power meters were constructed using the CS5480 and three Rogowski coils:
Pulse Electronics PA3202NL, TAEHWATRANS TR9L, and Sentec Mobius. Accuracy test results are presented that
demonstrate that the CS5480 can achieve an energy measurement accuracy of 0.1% over a 4000:1 dynamic range
when interfaced to a Rogowski coil.
2. Rogowski Coil Overview
A Rogowski coil current sensor is a helical coil of wire wrapped around an AC line conductor and used to measure
the flow of electric charge through the conductor. Since a Rogowski coil has an air core instead of an iron core, the
measurements show excellent linearity with practically no saturation problems. In addition, the Rogowski coil rates
highly for electrical isolation from the buss bar, and is light weight with low material cost. The Rogowski coil is
increasingly being implemented when measuring high-current AC power and energy.
Cirrus Logic, Inc.
http://www.cirrus.com
Copyright Cirrus Logic, Inc. 2012
(All Rights Reserved)
MAR’12
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vt
AN
0
–
l
------------------
di t
dt
-----------
=
[Eq. 1]
V kI=
[Eq. 2]
The voltage v(t) that is induced in the Rogowski coil is proportional to the rate of the charge of current (di(t)/dt) and
is based on Faraday's law. The voltage v(t) produced by a Rogowski coil is calculated using Equation 1:
where,
A = Area of each small loop
N = Number of turns
l=2R and is the length of the winding
µ
= Permeability of free space
0
When the AC line current, I, is a 50Hz or 60 Hz sinusoidal, Equation 1 can be simplified to Equation 2.
where
k = Sensitivity constant that represents the output voltage per ampere at 50Hz or 60Hz.
Most Rogowski coil manufacturers provide a sensitivity constant for the coil. See Table 1:
Model Manufacturer Output Voltage/Ampere at 50Hz
PA3201NL Pulse 416µV/A
TR9L Taehwatrans 1.7mV / A
Mobius Sentec 80µ V / A
Table 1. Rogowski Coil Parameters
3. Rogowski Coil Support
Since the output of a Rogowski coil is proportional to the derivative of the instantaneous primary current, an
integrator is required to retrieve the original current signal. The CS5480 incorporates selectable digital integrators
for both current channels. The integrators have the frequency responses illustrated in Figure 2 to compensate for
the 90 degree phase shift and 20dB /decade gain generated by the Rogowski coil.
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10
0
10
1
10
2
10
3
-90
-80
-70
-60
-50
-40
-30
Frequency (Hz)
Phase (degrees)
Phase of Rogowski Integrator
10
0
10
1
10
2
10
3
0
10
20
30
40
50
60
Frequency (Hz)
Magnitude (dB)
Magnitude of Rogowski Integrator
Figure 2. Frequency Response of the Digital Integrator
CS5480
IIN 1-
IIN 1+
27nF
27nF
27nF
27nF
V+
V-
GND
GND
IIN1-
IIN1+
J1
Rogowski Coil
PA 3202 NL
Current
1kΩ100Ω
1kΩ100Ω
Figure 3. Typical Connections Between a Rogowski Coil and the CS5480
3.1 Connection Between a Rogowski Coil and the CS5480
Since the Rogowski coil has an inherent 20dB/decade gain, a single-pole low-pass RC anti-aliasing filter is not
capable of attenuating the high-frequency noise. Two cascaded low-pass RC filters are required to produce a
40dB/decade attenuation at high frequency. Figure 3 illustrates the typical connections between a Rogowski
coil sensor and the CS5480. If the Rogowski coil has a shielding lead, it should be connected to ground.
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3.2 PGA Selection on the Current Channel
The CS5480 current channel incorporates a programmable gain amplifier (PGA) with two selectable input gains.
The Config0 register bits I1PGA[1:0] select either 10x or 50x gain for the current channel. The two PGA settings
dictate the maximum input voltage that can be applied to the IIN± inputs.
CS5480 I-channel PGA Maximum Input Voltage
10x
50x
Table 2. PGA Settings
167mV
35mV
RMS
RMS
The Rogowski coil output voltage produced with the maximum AC line current (I
) should not exceed the max-
max
imum input voltages listed in Table 2. Some margin should be considered according design requirements.
3.3 Filter Selection
The CS5480 high-pass filter (HPF) and integrator are set using the Config2 register. To support a Rogowski coil,
HPF must be enabled on the voltage channel and the integrator must be enabled on the current channel.
3.4 Calibration and Compensation
To compensate for the tolerances and variations in components and to remove the residual offset and noise in
the system, a gain calibration, phase compensation, AC offset calibration, and power offset correction should
be performed. For more information, see the CS5480 data sheet, entitled Three Channel Energy Measurement
IC, for details regarding the calibration and compensations process.
4. Measurement Accuracy Results with Rogowski Coils
The CS5480 load performance (measurement accuracy under different load conditions) has been tested with three
different types of Rogowski coil. The active and reactive energy load performance is tested with a single energy
pulse. The meter constant is set at 2000 impulses per kWh, or 2000 impulses per kVarh. The I
is calculated based on CS5480 I
U
= 240V and line frequency of 50Hz.
n
register values. All of the tests were conducted at room temperature and with
RMS
load performance
RMS
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4.1 Load Performance with Rogowski Coil PA3202NL
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.01 0.1 1 10 100
Percentage Error (%)
Load Current (A)
PF = 1 Lagging PF = 0.5 Leading PF = 0.5
Figure 4. Active Energy Load Performance
4.1.1 Active Energy Load Performance
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Power Factor Load Current (A) Current Dynamic Range (x:1) Error
80 1 0.04%
8 10 0.04%
0.8 100 0.04%
PF = 1
0.16 500 0.04%
0.08 1000 0.07%
0.04 2000 0.05%
0.03 3333 0.06%
0.02 4000 0.02%
80 1 0.05%
Lagging PF = 0.5
8 10 0.05%
0.8 100 0.04%
0.16 500 0.06%
0.08 1000 0.05%
0.04 2000 0.06%
80 1 0.04%
Leading PF = 0.5
8 10 0.04%
0.8 100 0.05%
0.16 500 0.04%
0.08 1000 0.05%
0.04 2000 0.07%
Table 3. Active Energy Load Performance
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