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Sallen-Key Low-pass Filter VS Buffered Voltage Bias

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Hello.
can I ask you what is the differences between these two harware configurations ?

I have an Arduino Due in my hand and I want to implement the project in my home.

First (with CT sensor only, not the transfomer) I’d Tried the low pass filter with a cut off freq of 500 Hz and I notice the differences compared the “simple- standard” configuration which have a lot of noise on the AnalogRead input.
Noise is definitelly reduced.also with low Wattage

Never tried the Buffered Voltage Bias.

References here:

Sallen Low pass solution

Buffered voltage bias solution

Thanks for explainetion

You are talking about two completely different things, the only link between them is the bias voltage generates an artificial d.c. offset to the input to the ADC, which the filter is intended to remove so as to leave what the ADC would have measured if it could accept negative-going inputs.

The buffered bias is intended to supply the bias voltage to several inputs from one source, thus potentially saving components whilst providing a more stable voltage.

The problem with a filter - any filter - is striking the balance between the time it takes to settle after switch-on, and the unwanted removal of the lowest frequency in the desired band - in our case in Europe, the 50 Hz mains frequency. If you go past first order filters, then you need to take processing time into account too.-

There are at present two techniques in use, a third was used initially.

The first to be used was a high pass filter. This removed the d.c. component, but required a comparatively long settling time because it could not be initialised to somewhere near the expected value.

That was replaced in emonLib by the present technique, where the d.c. component is extracted by a low-pass filter, then subtracted from the signal to leave only the desired a.c. with no offset.

In emonLibCM, the nominal offset is removed first, then the average signal over the whole sampling interval computed and the remaining d.c. component is extracted after calculation of the average (for the power) or the rms average (for the voltage and current).

But, if you use the buffered bias and you have a spare ADC input, then you can measure the bias voltage directly with the spare input and simply subtract that from the signal - what you have then is the emonLib method without needing to use a software filter.

I tought that both is implemented because they decrease unwanted noise of outhern possibles sources.
I’m sorry, I’m learning step by step because this kind of thing still hard to understand for me

Why? there’s nothing to be sorry about. If…

then I - and most of us here - shall do all we can to help you. There is much to learn, you cannot expect to understand it all immediately. So do take it step by step, and always ask if you are unsure.

No, as far as we are able to ascertain, the noise mainly comes from the power supply (to the Arduino or emonTx). The emonTx when powered from the a.c. adapter has a lower noise reading than when powered via the programmer and USB from my laptop computer.

So…in your opinion,about the ardware point of view, an active low pass filter as in the link abow, in between the arduino input and the probe, is good enought to maximize the reading system (reduce noise at min and have consistent reading between you and the energy company bill)?

thanks for explaination

The emonTx does not have an “anti-alias filter”. It would be a good idea to include one of those, and that would also help to reduce any high frequency noise that is picked up via the transformer (a.c. adapter or current).

But you must remember that some current waveforms can have high frequencies in them, and so filtering those out will in general give you an incorrect measurement. Whether that is something that matters is for you to judge.

My opinion is that there are much bigger sources of error than that - noise from the USB or computer power supply is one and errors in the current transformer can become significant at low currents.

If you want to add an active filter in the current input, noise from the filter itself could be a concern, because - if you use our ‘standard’ 100 A c.t. - that represents a maximum power of 24 kW, and many people want to measure currents and powers very much smaller than that. If you think that the burden resistor generates about 1 V at maximum, when you try to measure even a 100 W tungsten lamp, the voltage is only about 4 mV.

Furthermore, The c.t’s errors increase as the current gets smaller - this is inherent in the operation of every current transformer. The YHDC SCT-013 series c.t. is only specified over the range 10% - 120%, i.e. 2.4 kW is the lowest power at which YHDC guarantee the performance.