Texas Instruments | Data Converters for Industrial Power Measurements (Rev. B) | Application notes | Texas Instruments Data Converters for Industrial Power Measurements (Rev. B) Application notes

Texas Instruments Data Converters for Industrial Power Measurements (Rev. B) Application notes
Application Report
SBAA117B – July 2004 – Revised December 2005
Data Converters for Industrial Power Measurements
Miroslav Oljaca, Tom Hendrick ......................................................................... Data Acquisition Products
ABSTRACT
Sources of noise generation are pervasive in contemporary industrial applications.
Unwanted noise presents one of the major problems in measuring power or protecting
circuits in such an environment. Knowing the characteristics and understanding the
behavior of noise sources are valuable to designers and users of electronic circuits.
This application report discusses both of these topics. The first part of this report
defines some fundamental concepts of electronic noise; the latter portion addresses the
selection and design of filters used in conjunction with analog-to-digital converters
(ADCs). A design example for measuring voltage and current with dedicated
transformers, using the ADS8364, is provided along with equations and experimental
results.
1
2
3
4
5
Contents
Types of Noise ......................................................................................
Noise Filter Design for Differential Signals ......................................................
Noise Filter Design for Single-Ended Signals...................................................
Measurement Results ..............................................................................
Conclusion ...........................................................................................
1
2
3
4
7
List of Figures
1
2
3
4
5
6
7
8
9
10
1
Typical Noise Sources .............................................................................
Typical Power Measurement Application Circuit (Differential Signal) of the ADS8364....
Frequency Response of the Equivalent Differential and Common-Mode Filters ...........
Typical Power Measurement Application Circuit (Single-Ended Signal) of the ADS8364
AC Performance Curves as Function of Filter Capacitor C1 from ...........................
THD and Harmonics as Function of Filter Capacitor C1 from ................................
4096 Point FFT of ADS8364 with Optimized Differential Input Filter Circuit from .........
AC Performance Curves as Function of Filter Capacitor C1 from ...........................
THD and Harmonics as Function of Filter Capacitor C1 from ................................
4096 Point FFT of ADS8364 with Optimized Single-Ended Input Filter Circuit from .....
2
2
3
4
5
5
5
6
6
7
Types of Noise
Noise in electronic circuits is most frequently caused by electromagnetic interference (EMI), radio
frequency interference (RFI), and ground loops.
In terms of ac power, common-mode noise is the noise signal between the neutral and the ground
conductor. This type of signal interference differs from normal-mode (or differential) noise, which is
referenced from the line (hot) and the neutral conductor.
Common-mode noise impulses tend to be higher in frequency than the associated normal-mode noise
signal. This characteristic is to be expected, since the majority of common-mode signals originate from
capacitive-coupled normal-mode signals. More coupling among the line, conductors, neutral and ground
occur as operating frequency increases. Electronic equipment is 10 to 100 times more sensitive to
common-mode noise than it is to normal-mode noise.
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SBAA117B – July 2004 – Revised December 2005
Data Converters for Industrial Power Measurements
1
www.ti.com
Noise Filter Design for Differential Signals
The amount of noise present on the power line can be surprising at any given time. The source of this
noise typically arises from the electrical distribution system external to the building as well as the
distribution system within the building. The noise itself is the result of the power line’s dynamic nature
because of the ever-changing loads carried on the line. Figure 1 shows the typical noise found on a power
line.
R1
+VOUT
VDIFF
C1
-VOUT
VCM
Figure 1. Typical Noise Sources
Conventional power transformers and isolation transformers will not block normal-mode noise impulses.
However, if the secondaries of these transformers steer the neutral bonded to ground, they will convert
normal-mode noise to common-mode. From the standpoint of microelectronic circuits, common-mode
noise is potentially more harmful than normal-mode.
In a data acquisition system (such as power measurement), noise effects can be reduced by taking
advantage of an ADC with differential inputs. Balancing the impedances allows one to convert noise
sources into common-mode signals that can be rejected by an ADC with differential inputs.
Differential signals are more suitable for use in most industrial applications. Common-mode noise is
dramatically reduced, if not completely eliminated, when measuring differential signals. For industrial or
high-noise environment applications, several Burr-Brown products from Texas Instruments are intended to
give designers maximum advantage by using fully-differential signal paths.
2
Noise Filter Design for Differential Signals
One such ADC specifically designed for applications in a noisy environment is the ADS8364. The input
signal is ±2.5V around 2.5V. In industrial power metering, for example, the output of the voltage
measurement transformer is typically ±10V. Before applying the meter outputs to the ADC, the signal
needs to be scaled and offset to fit the differential input range of the converter.
The resistors R2A and R2B from Figure 2 will offset the output signal from the transformer Tr1 around the
reference voltage (VREF) of the ADC. The resistor network of R1A and R2A, along with R1B and R2B, will
scale the differential input signal to the full-scale range (FSR) of the ADC. The scaling factor or gain of this
resistor network is presented in Equation 1. To have this circuit work properly, resistors R1A and R1B must
be equal in value; R2A and R2B, as well, must be of equal value. The attenuation of the differential and the
common-mode signal is the same.
R1A
Tr1
+
VL
230V
50Hz
ADS8364
C1
R1B
− REFIN REFOUT
±10VPP
C2BB
R2B
R2A
C2A
CR
Figure 2. Typical Power Measurement Application Circuit (Differential Signal) of the ADS8364
2
Data Converters for Industrial Power Measurements
SBAA117B – July 2004 – Revised December 2005
www.ti.com
Noise Filter Design for Single-Ended Signals
G DIFF G CM R2A
R2B
R 1AR 2A
R 1BR 2B
(1)
The resistors R1A and R1B need to have a high enough impedance to avoid loading the signal source.
They also provide additional input-overload protection, isolating the ADC input from extreme external
signal sources.
The next step is to implement a low-pass filter, to filter the noise from the differential signal applied to the
ADC. Adding capacitor C1 between the (+) and (–) inputs of the ADC will create the differential filter.
Finally, the common-mode signal that occurs is first attenuated by the resistor divider, then further
attenuated by the implemented RC filter. The cutoff frequency of the common-mode signal can be
calculated by Equation 2.
BW CM 1
2
1
R R
R 1AR2A
1A
2A
C2A
2
R R
R 1BR2B
1B
2B
C 2B
(2)
The –3dB differential bandwidth of this filter can be expressed by Equation 3.
1
BW DIFF 2
2R R2A
R 1A
1AR 2A
C1
C2A
2
(3)
Any mismatch between the time constants of corresponding common-mode filters will unbalance the input
bridge and reduce high-frequency common-mode rejection. Capacitor C1 connected across the bridge
output effectively reduces any ac common-mode rejection errors from component mismatch.
For example, making C1 ten times larger than C2A or C2B provides a 20x reduction in the common-mode
rejection error arising from a C2A/C2B mismatch. Figure 3 shows the frequency response of the differential
filter and mismatched common-mode filters. From the previous example, the differential filter had an
attenuation of –26dB for the cutoff frequency of the first common-mode filter. The difference in the –3dB
bandwidth between these two common-mode filters, which is presented as a differential signal, will be
attenuated by –26dB.
5.0
0.0
CMB
Amplitude (dB)
-5.0
-10.0
DIFF
-15.0
CMA
-20.0
-25.0
-26dB
-30.0
-35.0
-40.0
0.1
1k
10k
100k
Frequency (kHz)
Figure 3. Frequency Response of the Equivalent Differential and Common-Mode Filters
3
Noise Filter Design for Single-Ended Signals
In cases where the common-mode noise is not a concern, the differential signal is replaced with a
single-ended signal. In this configuration, one side of the transformer is tied to ground. Another side acts
as a signal source. As in the previous example, the output signal from the transformer is ±10V and needs
to be scaled to match the differential input of the ADS8364. The resistor network from Figure 4 will scale
the input signal and offset so that the full-scale range (FSR) will be from 0V to 5V.
SBAA117B – July 2004 – Revised December 2005
Data Converters for Industrial Power Measurements
3
www.ti.com
Measurement Results
+5V
Tr1
R4
±10VPP
R1
R2
+
VL
230V
50Hz
ADS8364
R3
C1
− REF REFOUT
IN
CR
Figure 4. Typical Power Measurement Application Circuit (Single-Ended Signal) of the ADS8364
The resistor network of R1, R2, R3 and R4 will scale the single-ended input signal to the FSR of the ADC.
The scaling factor (or gain) of this resistor network must be the same as in the previous example. In order
to have this circuit work properly, resistors R1, R3 and R4 must be equal and each have twice the value of
resistor R2.
The next step is to implement a low-pass filter, in order to filter the noise from the single-ended signal
applied to the ADC. Adding capacitor C1 between the (+) input and the ground of the ADC creates the
filter.
4
Measurement Results
Measurements were made with the ADS8364 operating with a clock of 3.8MHz. The sampling rate was
38kHz. These conditions resulted in a conversion time of 4.47µs and an acquisition time of 21.84µs. In the
case of the differential signal, the choice of resistor values for R1A and R1B was 4kΩ, and 1.3kΩ for R2A
and R2B. These resistor values provide a gain of 0.245. For the transformer output voltage of ±10V, the
full-scale ADC input is 4.9V. The value of capacitor C1 was changed from 0 to 3.3nF.
Keeping the operating conditions the same for the ADC as those of the single-ended signal, the resistor
values for R1, R3 and R4 were 6.48kΩ, and 3.24kΩ for R2. These resistor values provide a gain of 0.25.
For the transformer output voltage of ±10V, the full-scale ADC input will be 5V. The value of capacitor C1
was changed from 0 to 1.8nF.
It is essential to realize that the input filter time constant, from equivalent R and C1, must not exceed
one-fifth of the ADC sampling time. For greater time constants, performance degradation can be expected.
Measurements were taken and the results were analyzed. Table 1 presents the measurement results for
the differential signal. Figure 5 and Figure 6 present graphical representations of the measurement data
from Table 1.
Table 1. Experimental Measurement Data for Differential Signal in Figure 2 2
Capacitor (nF)
4
AC Performance
Harmonics
C1
C2A
C2B
SNR
SINAD
SFDR
THD
3rd
5th
7th
9th
0.00
0.00
0.00
84.8
83.7
90.6
–90.0
–90.6
–102.3
–102.4
–114.1
0.56
0.00
0.00
85.3
84.8
98.2
–94.3
–98.2
–98.5
–101.0
–122.0
1.00
0.00
0.00
85.3
84.9
98.1
–94.8
–98.1
–99.8
–101.6
–122.8
1.20
0.00
0.00
85.5
85.0
98.4
–94.7
–98.7
–99.2
–101.1
–115.5
1.50
0.00
0.00
85.3
84.9
98.2
–95.4
–99.6
–99.7
–101.8
–124.3
1.80
0.00
0.00
85.3
85.0
98.1
–96.3
–101.4
–100.4
–102.4
–110.1
2.20
0.00
0.00
85.3
84.9
97.9
–95.6
–103.1
–100.1
–100.5
–105.1
3.30
0.00
0.00
84.2
81.8
88.4
–85.6
–88.4
–92.3
–93.8
–96.7
1.80
0.18
0.18
86.0
85.6
100.4
–96.2
–101.6
–100.4
–101.6
–113.8
Data Converters for Industrial Power Measurements
SBAA117B – July 2004 – Revised December 2005
www.ti.com
Measurement Results
100
98
Amplitude (dB)
96
SFDR
94
92
90
tAQ = 21.84ms
R1A = R1B = 4kW
R2A = R2B = 1.3kW
SNR
88
SINAD
86
84
82
80
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
C1 (nF)
Figure 5. AC Performance Curves as Function of Filter Capacitor C1 from Table 1
-80
Amplitude (dB)
-85
THD
-90
Harmonic 05
tAQ = 21.84ms
R1A = R1B = 4kW
R2A = R2B = 1.3kW
-95
-100
-105
Harmonic 07
Harmonic 03
-110
0
0.5
1.0
1.5
2.0
C1 (nF)
2.5
3.0
3.5
Figure 6. THD and Harmonics as Function of Filter Capacitor C1 from Table 1
After preliminary measurement, capacitors C2A and C2B were added. The lowest measured THD was for a
value of C1 around 2nF. Choosing 1.8nF for C1 and 0.18nF for C2A and C2B will give an equivalent
capacitance of 1.9nF.
Adding common-mode filter capacitors C2A and C2B will improve SNR. The FFT results of the final circuit
are presented in Figure 7.
0
Frequency Spectrum (4096 Point FFT)
FS = 38.0000kHz, FIN = 1.271kHz
-20
Amplitude (dB)
-40
-60
-80
-100
-120
-140
-160
SNR = 86.029 SINAD = 85.627 SFDR = 100.418 THD(9) = -96.159
Figure 7. 4096 Point FFT of ADS8364 with Optimized Differential Input Filter Circuit from Figure 2 2
SBAA117B – July 2004 – Revised December 2005
Data Converters for Industrial Power Measurements
5
www.ti.com
Measurement Results
The same measurements are done using the schematic from Figure 4. Table 2 presents the measurement
results for the single-ended signal. Figure 8 and Figure 9 present graphical representations of the
measurement data from Table 2.
Table 2. Experimental Measurement Data for Single-Ended Signal in Figure 4 4
Capacitor (nF)
AC Performance
Harmonics
C1
SNR
SINAD
SFDR
THD
2nd
3rd
5th
7th
0.00
83.37
82.26
92.55
–88.74
–92.55
–108.11
–92.91
–97.49
0.15
85.67
84.67
95.93
–91.57
–95.93
–100.81
–96.42
–100.82
0.33
85.96
84.86
95.65
–91.35
–95.65
–100.10
–96.36
–100.65
0.51
85.77
84.66
95.57
–91.12
–95.57
–99.42
–96.50
–100.27
0.68
86.09
85.00
95.64
–91.51
–95.64
–100.33
–96.83
–100.60
0.82
86.36
85.27
95.78
–91.83
–95.78
–102.92
–97.05
–99.91
1.00
86.35
85.05
95.00
–90.92
–95.00
–98.98
–96.65
–99.95
1.20
85.69
82.97
90.56
–86.30
–92.31
–90.56
–93.09
–96.16
1.50
83.50
77.94
81.92
–79.35
–88.49
–81.92
–86.76
–89.76
1.80
81.04
74.34
77.72
–75.39
–85.42
–77.72
–83.00
–85.76
100
98
96
Amplitude (dB)
94
SFDR
92
tAQ = 21.84ms
R1 = R2 = R4 = 6.48kW
R3 = 3.24kW
SNR
90
88
86
84
SINAD
82
80
0.0
0.5
1.0
1.5
2.0
C1 (nF)
Figure 8. AC Performance Curves as Function of Filter Capacitor C1 from Table 2
-80
Amplitude (dB)
-85
THD
-90
Harmonic 02
tAQ = 21.84ms
R1 = R2 = R4 = 6.48kW
R3 = 3.24kW
-95
Harmonic 05
-100
-105
Harmonic 03
-110
0.0
0.5
1.0
1.5
2.0
C1 (nF)
Figure 9. THD and Harmonics as Function of Filter Capacitor C1 from Table 2
6
Data Converters for Industrial Power Measurements
SBAA117B – July 2004 – Revised December 2005
www.ti.com
Conclusion
The FFT results of the single-ended circuit are presented in Figure 10.
0
Frequency Spectrum (4096 Point FFT)
FS = 38.0000kHz, FIN = 1.271kHz
-20
Amplitude (dB)
-40
-60
-80
-100
-120
-140
-160
SNR = 86.356 SINAD = 85.721 SFDR = 95.779
THD(9) = -91.828
Figure 10. 4096 Point FFT of ADS8364 with Optimized Single-Ended Input Filter Circuit from Figure 2 2
5
Conclusion
The preceding analysis shows a simple and effective way to implement a high-performance ADC for
industrial power measurement. Understanding noise sources and propagation through the measurement
circuit is essential in determining the final design. Proper filtering will not only reduce harmful differential
and common-mode noise; it will also permit the ADC to operate at its maximum performance. After
choosing an input resistor divider that will properly scale and offset the input signal (as well as protect the
input of the ADC), it is necessary to make several measurements with different capacitors. These
capacitors will reduce input bandwidth of the ADC, increasing the SNR performance and the effective
number of bits. The value of these capacitors is directly proportional to the value of the input resistors as
well as to the sampling time of the ADC. This application report shows that an ADC operating with
relatively high source impedance and a properly designed differential filter can provide measurements with
an SFDR greater than 98dB as well as THD better than –96dB with SNR over 85dB.
SBAA117B – July 2004 – Revised December 2005
Data Converters for Industrial Power Measurements
7
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