AN-1304: ADE7912/ADE7913 DC Measurement Performance (Rev. 0) PDF

AN-1304
APPLICATION NOTE
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ADE7912/ADE7913 DC Measurement Performance
by Nathan Benamati and Petre Minciunescu
INTRODUCTION
RECOMMENDED DC MEASUREMENT PROCEDURE
The ADE7912/ADE7913 isolated 3-channel, Σ-Δ ADCs target
the polyphase energy metering applications using shunt current
sensors. However, because the devices can be used to sense dc
signals, this application note presents their dc measurement
performance. In energy metering applications, the current
channel is used to sense the voltage across shunt current sensors
and the voltage channels are used to measure voltages across
resistor dividers. From a dc measurement perspective, this
separation is not meaningful because every channel can be
used to sense dc signals.
The ADE7912/ADE7913 data sheet declares an IP, IM channel
ADC offset error typical value of −2 mV, and a V1P, VM or
V2P, VM channel ADC offset error typical value of −35 mV.
This offset affects the accuracy of the dc measurement and must
be eliminated. To estimate the dc offset of every channel, follow
these steps:
1. Apply 0 V at the ADE7912/ADE7913 between the IP and
IM pins, between the V1P and VM pins, and between the
V2P and VM pins.
2. Read the ADC output registers IWV, V1WV, and V2WV
at least 50 times over 1 second. The period between the
readings is not important, although as a good
programming practice, the readings should be periodical.
3. Average the readings to obtain the dc offset.
The ADE7912/ADE7913 ADCs have a ±31.25 mV input range
for signals between the IP and IM pins, and ±500 mV between
the V1P and VM pins and between the V2P and VM pins.
This application note describes the performance of the
ADE7912/ADE7913 when dc signals are applied at the inputs
of the three Σ-Δ ADCs.
To ensure the stability of the dc measurements, follow these
steps:
1.
2.
3.
Rev. 0 | Page 1 of 8
Read the ADC output registers IWV, V1WV, and V2WV at
least 50 times over 1 second.
Average the readings.
Subtract the dc offset to obtain the dc measurement.
AN-1304
Application Note
TABLE OF CONTENTS
Introduction ...................................................................................... 1
Assessing the Performance of DC Measurements ........................3
Recommended DC Measurement Procedure ............................... 1
Conclusions ........................................................................................5
Revision History ............................................................................... 2
REVISION HISTORY
4/14—Revision 0: Initial Version
Rev. 0 | Page 2 of 8
Application Note
AN-1304
ASSESSING THE PERFORMANCE OF DC
MEASUREMENTS
2.5
For simplicity, the performance was assessed on the ADE7978/
ADE7933 evaluation kit. The rms measurements in the
ADE7978 were used as a proxy for the ADE7912/ADE7913 dc
measurements.
1.5
Figure 1, Figure 2, and Figure 3 present the standard deviation
values obtained on every A, B, C, and N data path of the ADE7978.
STANDARD DEVIATION (%)
0.5
0
–0.5
–1.0
–2.0
–2.5
0.1
1
10
100
PERCENTAGE OF FULL SCALE (%)
12258-001
DC signals with variable amplitude were provided from a
National Instruments NI_PXI-4461 card between the IP and IM
pins, between the V1P and VM pins, and between the V2P and
VM pins. The IM and VM pins were connected to ground as
the ADE7912/ADE7913 data sheet specifies maximum IM and
VM voltage to ground of ±25 mV. One thousand measurements
were executed for every signal level and then the standard
deviation of the measurements was calculated. Note that one
measurement means the average of 50 readings over 1 second.
1.0
–1.5
Figure 1. Channel IP, Channel IM DC Measurement Repeatability
2.5
AVRMS, +σ
CVRMS, +σ
AVRMS, –σ
CVRMS, –σ
2.0
1.5
STANDARD DEVIATION (%)
Because the kit contains four ADE7933 devices, four rms
measurements were used to assess the performance of each
current and voltage channel. The high-pass filters in the current
and voltage data paths of the ADE7978 were disabled.
BIRMS, +σ
NIRMS, +σ
BIRMS, –σ
NIRMS, –σ
AIRMS, +σ
CIRMS, +σ
AIRMS, –σ
CIRMS, –σ
2.0
BVRMS, +σ
NVRMS, +σ
BVRMS, –σ
NVRMS, –σ
1.0
0.5
0
–0.5
–1.0
–1.5
–2.5
0.1
1
10
100
PERCENTAGE OF FULL SCALE (%)
12258-002
–2.0
Figure 2. Channel V1P, Channel VM DC Measurement Repeatability
2.5
AVRMS, +σ
CVRMS, +σ
AVRMS, –σ
CVRMS, –σ
2.0
STANDARD DEVIATION (%)
1.5
BVRMS, +σ
NVRMS, +σ
BVRMS, –σ
NVRMS, –σ
1.0
0.5
0
–0.5
–1.0
–1.5
–2.5
0.1
1
10
PERCENTAGE OF FULL SCALE (%)
100
12258-003
–2.0
Figure 3. Channel V2P, Channel VM DC Measurement Repeatability
Rev. 0 | Page 3 of 8
AN-1304
Application Note
Table 1. Standard Deviation of DC Measurements at
Ambient Temperature TA = 25°C
88.7
80
60
42.2
17.4
20
8.9
4.9
2.3
1.4
1.0
0.7
0.7
–1.7
–0.9
–0.5
–0.3
–0.2
0
–4.3
–9.5
–20
–19.5
–40
Standard Deviation (%)
I Channel
V1 Channel
V2 Channel
0.008
0.003
0.003
0.02
0.02
0.02
0.22
0.21
0.15
0.40
0.42
0.37
0.94
1.06
0.83
1.83
2.2
1.7
–48.3
–60
40
33.3
30
20
Figure 4, Figure 5, and Figure 6 present the worst errors
obtained over these measurements.
14.8
10
V1RMS MAX
V1RMS MIN
6.4
3.4
1.6
0.7
0.4
0.2
0.1
0.1
–1.5
–0.6
–0.4
–0.2
–0.2
–0.2
0
–3.0
–10
–6.2
–16.6
–20
–30
–31.5
–40
0.1
1
10
100
PERCENTAGE OF FULL SCALE (%)
The ADC offset varies over temperature and power supply; this
affects the accuracy of the dc measurements. The ADE7933
temperature measurement was offset compensated at 25°C
and a VDD supply of 3.3 V, the nominal value. TheADE7933
temperature range is between −40°C and +85°C and the voltage
supply range at the VDD pin is between 2.97 V and 3.63 V.
Figure 5. Channel V1P, VM Drift Over Temperature
60
53.1
50
40
25.6
30
V2RMS MAX
V2RMS MIN
20
ERROR (%)
The ADE7933 devices were set at +85°C and for VDD equal to
2.97 V, 3.3 V, and 3.63 V; dc signals of various amplitudes were
measured. Then, the ADE7933 devices were set at −40°C and
the measurements over VDD were repeated.
100
12258-005
Measurement Error (%)
I Channel
V1 Channel
V2 Channel
0.02
0.02
0.02
0.25
0.25
0.2
0.5
0.5
0.5
1.2
1.2
1.0
2.0
2.5
2.0
10
Figure 4. Channel IP, IM Drift Over Temperature
ERROR (%)
Table 2. ADE7912/ADE7913 DC Measurement Error
Specification
1
PERCENTAGE OF FULL SCALE (%)
12258-004
–80
–100 –97.1
0.1
Based on these results, the specification of the dc measurements
at ambient temperature TA = 25°C is then stated in Table 2.
Dynamic Range
10 to 1
100 to 1
200 to 1
500 to 1
1000 to 1
IRMS MAX
IRMS MIN
40
10.4
10
5.2
2.6
1.1
0.5
0.3
0.1
0.1
–2.8
–1.2
–0.5
–0.3
–0.2
–0.1
0
–10
–5.6
–11.7
–20
–30
–27.8
–40
–50
–55.2
–60
0.1
1
10
PERCENTAGE OF FULL SCALE (%)
Figure 6. Channel V2P, VM Drift Over Temperature
Rev. 0 | Page 4 of 8
100
12258-006
Dynamic Range
1 to 1
10 to 1
100 to 1
200 to 1
500 to 1
1000 to 1
100
ERROR (%)
Table 1 presents the standard deviation numbers when the dc
signals have certain levels relative to full scale. The performance
remains acceptable down to approximately 1% of full scale,
when the standard deviation of the dc measurements reaches
values around 0.21% for the current and V1 channels, with the
V2 channel being a little better at 0.15%.
Application Note
AN-1304
Table 3. Channel V1 DC Measurement Specifications Over
Temperature
3.
4.
Dynamic
Range
10 to 1
100 to 1
200 to 1
500 to 1
1000 to 1
5.
V1 Channel
Measurement Error (%)
0.4
3.5
6.5
17
34
Temperature
Coefficient (ppm/°C)
62
540
1000
2615
5230
Channel V1 has the best performance of all three channels,
with a derived specification and temperature coefficient
presented in
Table 3. The temperature coefficient was computed as follows
for the dynamic range of 100:1:
TempCoeff V 1 =
3.5 × 10 −2
= 538.5 ≅ 540 ppm /  C
25 − ( − 40 )
Determine the dc offset at these two temperatures.
Calculate the temperature coefficients of the offset between
t1 and tr and between tr and t2. Consider the first coefficient
to be a constant below the room temperature and the
second to be a constant above room temperature.
Use the temperature sensor readings to adjust accordingly
the dc measurement.
Offset = Offsetr + (t − tr) × Temperature Coeff
Result = Register value – Offset
Note that small nonlinearities can appear when using a Σ-Δ
ADC for dc measurements. These nonlinearites typically appear
when the effective input dc signal (input + inherent offset) is
near zero. This effect should have little to no impact on the
quality of the measurement given the previously stated
accuracy.
CONCLUSIONS
The errors over temperature appear mainly because of the dc
offset variation. The temperature sensor embedded into the
ADE7912/ADE7913 may help in mitigating this temperature
variation. The following method estimates the temperature
coefficient variation over the temperature range of the meter
and uses it to compensate the dc offset:
For dc measurements, it is recommended to use the V1
channel of the ADE7912/ADE7913. Provide dc signals between
±500 mV. Make an offset calibration at room temperature. The
errors of the measurements at room temperature are under
0.25% for a 100:1 dynamic range. Over temperature, the errors
are below 3.5% for a 100:1 dynamic range.
1.
To improve the accuracy over temperature, bring the instrument containing the ADE7912/ADE7913 to two temperatures,
compute the temperature coefficients, and use the result to
adjust the dc measurements.
2.
Determine the dc offset, Offsetr, at room temperature, tr,
following the procedure indicated in the Recommended
DC Measurement Procedure section.
Bring the ADE7912/ADE7913 to two temperatures, t1 and
t2, placed on either side of the room temperature as far
away as possible: t1 < tr and t2 > tr.
Rev. 0 | Page 5 of 8
AN-1304
Application Note
Rev. 0 | Page 6 of 8
Application Note
AN-1304
NOTES
Rev. 0 | Page 7 of 8
AN-1304
Application Note
NOTES
I2C refers to a communications protocol originally developed by Philips Semiconductors (now NXP Semiconductors).
©2014 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
AN12258-0-4/14(0)
Rev. 0 | Page 8 of 8
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