On Jul 28, 4:24=A0am, "steveu" <ste...@coppice.org> wrote:
> >The following document says on page 5 an anti aliasing filter is used
> >on the A/D input channels of an energy meter. =A0Warning the document is
> >31 pages long, only click on the link if you have a broadband internet
> >connection. =A0http://focus.ti.com/lit/an/slaa409a/slaa409a.pdf. =A0My
> >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. =A0 Furthermore on page 6 the
> >document says the sampling frequency is 4096 samples per second which
> >gives a Nyquist frequency of 2048 cps. =A0 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 thos=
e
> 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 simila=
r
> 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, a=
nd
> do so without upsetting performance in the band of interest. The band of
> interest varies somewhat between power utilities, but they generally expe=
ct
> 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 fo=
r
> accuracies of 0.1% or better, which is about 0.009dB. So, really flat gai=
n
> response is required. In audio a few degrees of phase is seldom a big dea=
l.
> In comms a degree of phase is seldom a big deal. In energy measurement, a=
t
> 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 industria=
l
> area. Industrial systems using big arcs, or huge amounts of RF for heatin=
g,
> 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 differenti=
al
> 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 c=
an
> be quite wide tolerance) will not inject unbalanced amounts of ground pla=
ne
> 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 ou=
t.
> Such balance is naturally achieved in the current sensor circuit, as long
> as the component tolerances aren't too wide. In the voltage sensor circui=
t
> we need to take care, because the impedance into one leg is zero (a groun=
d
> 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 inherentl=
y
> out of band, which it is appropriate to filter away. Applying an anti-ali=
as
> 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 typica=
l
> 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 pur=
e
> voltage signal, and affect the energy measurement result. The interesting
> case is when the voltage signal has harmonic content, as this may correla=
te
> 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 int=
o
> 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 an=
d
> 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 consum=
er
> 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