GoldExI : FAQ : How to filter 50 Hz noise

Question

How can I filter out 50 Hz noise?

Answer

TurboAveraging feature provides unique possibility to effectively filter out unwanted frequencies in the spectrum of the input signal. If you want to remove e.g. 50 Hz noise, you have to choose such a number of averaging samples and such a sample frequency that averaging will take place exactly during integer number of periods of 50 Hz i.e. during N*20 ms. For example, the following settings in Options | Averaging will do the job very well:

Number of samples    = 400
Start averaging from = 0
Turbo Sampling Rate  = 20000 (Samples/s)

For sampling input signal 400 times with a rate of 20 kSamples/s it will take exactly 20 ms -- a period of 50 Hz noise. Choosing

Number of samples = 800

will average during 2 periods, and so on. The first experiments showed (see the plot below) that such an averaging technique drastically (about factor of 7 in our case) reduces 50 Hz noise.

In this plot you can see the width of noise track (double noise amplitude) as a function of parameter GetVoltageAveragingTimes at fixed value of Turbo Sampling Rate = 20 kSamples/s. The background noise level of about 200 nV is always present and is related to non 50 Hz sources like zero-drift of preamplifier. It is obvious from this plot, that as Averaging period approaches 20 ms (GetVoltageAveragingTimes approaches 400) the noise amplitude drastically decreases. The same happens for 40 ms, 60 ms etc., but the dependence becomes less pronounced since the averaging over several periods takes place.
This technique can be also used to suppress 2 parasitic frequencies in the same time e.g. 50 Hz (20 ms) and 80 Hz (12.5 ms). Choosing

Number of samples    = 1000
Start avearging from = 0
Turbo Sampling Rate  = 10000 (Samples/s)

will result in 100 ms of averaging which corresponds to 5 periods of 50 Hz component and 8 periods of 80 Hz component. I hope the following table will help you in typical cases.

1st noise
frequency
(net)

2nd noise
frequency
(e.g.monitor)

optimal
averaging
period

Averaging
Times

Sampling
Rate

50

60

100 ms

1000

10 000

50

70

100ms

1000

10 000

50

75

40ms

0800

20 000

50

80

100ms

1000

10 000

50

85

200ms

1000

05 000

50

100

20ms

0400

20 000

50

120

100ms

1000

10 000

1st noise frequency (net)	2nd noise frequency (e.g.monitor)	optimal averaging period	Averaging Times	Sampling Rate
50	60	100 ms	1000	10 000
50	70	100ms	1000	10 000
50	75	40ms	0800	20 000
50	80	100ms	1000	10 000
50	85	200ms	1000	05 000
50	100	20ms	0400	20 000
50	120	100ms	1000	10 000