Re: Why use an anti aliasing filter with an energy meter.?
- From: hrh1818 <hrhan@xxxxxxx>
- Date: Tue, 28 Jul 2009 11:17:49 -0700 (PDT)
On Jul 28, 4:24 am, "steveu" <ste...@xxxxxxxxxxx> wrote:
The following document says on page 5 an anti aliasing filter is used
on the A/D input channels of an energy meter. Warning the document is
31 pages long, only click on the link if you have a broadband internet
connection. http://focus.ti.com/lit/an/slaa409a/slaa409a.pdf. My
understanding is you only need an anti aliasing filter if you want to
reconstruct the signal or find the frequency content of a signal.
Neither criteria apply to an energy meter. Furthermore on page 6 the
document says the sampling frequency is 4096 samples per second which
gives a Nyquist frequency of 2048 cps. But the anti aliasing filter
is a single pole filter with the *+3db point at approximately 5000
hz.
Hence I ask did the designers make a minor mistake or am I missing
something?
Howard
This is a good question, which deserves an answer that drags on a
bit.....
The term anti-aliasing is being incorrectly applied here. Its one of those
situations where one person started using an inappropriate term, and
everyone else copied. The filter is really there to suppress wideband
noise, which could badly mess up the operation of the sigma delta
converter.
If you look at competing solutions, they all use a similar circuit. For
example inhttp://www.analog.com/static/imported-files/data_sheets/ADE7758..pdffigure
34 you will see the same time constant used with a slightly different RC
configuration. Their circuit will be more fussy about board layout, which
is why the TI one is a little different. If you look at applications
information for other energy measurement devices you will see very similar
single pole RC filters, with time constants not a million miles away.
So, why do all these devices come with a recommendation to use this kind
of filter? Well, its a compromise that works well. Aliases in the current
and voltage waveforms matter far less than others have suggested. What
matters very much is wider band noise energy. We need to suppress that, and
do so without upsetting performance in the band of interest. The band of
interest varies somewhat between power utilities, but they generally expect
to capture at least the first 20 harmonics well.
The gain and phase of the voltage and current samplers need to be very
well controlled across the band of interest. In audio, 3dB is not a big
deal. In comms, 0.2dB is not a big deal. In energy measurement we look for
accuracies of 0.1% or better, which is about 0.009dB. So, really flat gain
response is required. In audio a few degrees of phase is seldom a big deal.
In comms a degree of phase is seldom a big deal. In energy measurement, at
0.5PF, 0.03 degrees of phase error gives a 0.1% measurement error. Really
well controlled phase is required.
You can get energy in the gigahertz range on a power line in an industrial
area. Industrial systems using big arcs, or huge amounts of RF for heating,
produce huge amounts of wideband energy on the power lines in the
surrounding area. That could really mess up the operation of energy
measurement devices, or perhaps even damage them. Really wideband noise
needs separately filtering from each of the differential inputs, because
the differential performance of the circuit starts to break down so far
from its operating band. This is the purpose of the two small capacitors.
The bigger capacitor filters medium frequency noise, where the differential
functionality of the input does perform well. On a perfect board only two
33nF capacitors to ground would be needed. However, the three capacitor
design is much more tolerant. Imbalance between capacitor values (which can
be quite wide tolerance) will not inject unbalanced amounts of ground plane
noise at the differential inputs. Capacitors not grounded at exactly the
same point on the ground plane will also not inject inject differential
amounts of ground plane noise.
The differential RC filter fits the need for accurate phase very nicely.
If the filter is well balanced the phase shifts in the two legs cancel out.
Such balance is naturally achieved in the current sensor circuit, as long
as the component tolerances aren't too wide. In the voltage sensor circuit
we need to take care, because the impedance into one leg is zero (a ground
connection) and the impedance into the other leg is about 1k ohms (the
voltage divider). If we don't tweaks the resistors to get close to a zero
phase shift we get two bad consequences. One is a phase shift that varies
with frequency, so higher harmonics are not handled correctly. The other is
a phase shift that varies with temperature. Capacitors generally have a
horrible temperature characteristic, and in a consumer grade meter we don't
want to use exotic devices. If there is a substantial phase shift, there
will also be a substantial variation with temperature and the meter's
accuracy will vary considerably with temperature for poor power factor
loads.
Others raised the issue that not suppressing frequencies >Fs/2 will lead
to measurement error. This is not really true. There is nothing inherently
out of band, which it is appropriate to filter away. Applying an anti-alias
filter has as much potential to corrupt the results as not applying one.
Let's look at the real measurement requirement....
In essence there should be a pure sine wave voltage signal, and a current
signal somewhere between pure sine wave and massively polluted with
harmonics. In practice, the utility's cables have some impedance, so the
voltage signal may have some harmonic content. In the real world a typical
residential area voltage signal is quite pure, and an industrial area
signal may have 10% harmonic content. 15% would be pretty bad for even a
heavy industrial area. The current waveform for an incandescent lamp load
will be a pure sine wave, and the waveform with a half wave load will be
horrible. >40% harmonic content is common for a current signal.
If the voltage signal is pure it won't matter how much harmonic content
there is in the current signal, or which harmonics they are. It also won't
matter if the harmonics are aliased. They just won't correlate with a pure
voltage signal, and affect the energy measurement result. The interesting
case is when the voltage signal has harmonic content, as this may correlate
with harmonic content in the current signal. If there are actual harmonic
components beyond Fs/2 (rather than interference or noise), and they are
filtered away by a true anti-alias filter, they won't contribute to the
energy measurement as they should. So, anti-alias filtering corrupts the
result. If these harmonics are not filtered away, they will fold back into
the band. They will fold back similarly in both the voltage and current
signal, will correlate, and will contribute to the energy measurement.
Unfortunately, they won't have the correct relative phases, so they will
contribute at the wrong effective power factor. Does it matter very much?
The lower harmonics are always far bigger in amplitude, so the potential
error is not that big, whatever we do about the aliases. The bottom line is
we are really no worse off processing the aliases than we would be if we
had an efficient anti-alias filter.
So, if we try to produce a real anti-alias filter we have some very tough
gain and phase constraints to meet if we are not to significantly affect
the most important harmonics - the fundamental, and 3rd to 11th. If we
ignore the aliases, which start at the 40th harmonic for a 50Hz supply and
4096 per second sampling, they aren't usually big enough to make a whole
lot of difference.
Rune mentioned 400Hz power. I should point out that the MSP430F47197 is
designed for 50Hz and 60Hz energy measurement in high volume light consumer
applications. 400Hz energy meters demand a high sampling rate, and are
quite specialised products. I don't know of any single chip solutions
targeted at that application.
I hope that sheds some light on the issues.
Steve
Steve, thank you for taking the time for providing a detailed response
to my question.
I conclude the filters should really be called power line noise
rejection filters instead of
of anti aliasing filters. Your response is highly appreciated.
Howard
.
- References:
- Why use an anti aliasing filter with an energy meter.?
- From: hrh1818
- Re: Why use an anti aliasing filter with an energy meter.?
- From: steveu
- Why use an anti aliasing filter with an energy meter.?
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