From: Bob Blick <bobblick@outlook.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Fri, 7 Feb 2020 19:08:29 -0800
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Hi Justin,=0A=
If you are having larger amplitude after a lowpass filter then it seems log=
ical that there is a new problem being introduced. Perhaps you have RF bein=
g rectified. Perhaps there is some power supply or reference voltage proble=
m. Perhaps your opamp's common-mode range is being exceeded. Or something e=
lse unrelated to the principle of adding a lowpass filter to an input. A pe=
rfect(or good-enough) lowpass filter will not act like a peak detector!=0A=
Friendly regards, Bob=0A=
=0A=
________________________________________=0A=
From: piclist-bounces@mit.edu <piclist-bounces@mit.edu> on behalf of Justin=
 Richards =0A=
Sent: Friday, February 7, 2020 6:42 PM=0A=
To: Microcontroller discussion list - Public.=0A=
Subject: Re: [EE]: Stability AND Gain of =ECg scales - best of both worlds=
=0A=
=0A=
Implementing the filter as suggested has produced similar results to my=0A=
intuitive application of filtering capacitors.=0A=
=0A=
That is, it trades off high frequency small amplitude oscillations for low=
=0A=
frequency high amplitude oscillations.=0A=
=0A=
This filter produced approx 0.125Hz full scale deflections from 20Hz=0A=
barely detectable deflections.=0A=
=0A=
Starting to think oscillations is not the correct term here as the output=
=0A=
looks more like a single diode rectifier with a small mark space ratio.=0A=
=0A=
The solution to improve stability always seems to be reducing the gain=0A=
which reduces the usable range.=0A=
=0A=
Perhaps a total redesign where the meter movement is servo driven in both=
=0A=
directions.=0A=
=0A=
Justin=0A=
=0A=
=0A=
On Sat, Feb 8, 2020 at 8:36 AM Jason White  wrote:=0A=
=0A=
> Possibly of interest,=0A=
>=0A=
> Sallen-Key Low-pass Filter - you get a two pole low-pass filter with a=0A=
> single op amp.=0A=
>=0A=
> I have attached a schematic of the said filter arrangement. OKAWA has=0A=
> a excelent calculator [1] for that topology. For fun I calculated what=0A=
> your component values should be using their tool.=0A=
>=0A=
> The calculator yields the following component values for a lowpass=0A=
> filter with a 0.1Hz corner frequency.=0A=
>=0A=
> R1 =3D 1.6M=D9=0A=
> R2 =3D 1.6M=D9=0A=
> C1 =3D 1uF=0A=
> C2 =3D 1uF=0A=
>=0A=
> At 10Hz this filter will reduce the incoming signal magnitude by 80dB=0A=
> - that is 1/10000 or 0.0001. If filtering will solve your problem -=0A=
> something like this will probably do a good job.=0A=
>=0A=
>=0A=
> [1] Sallen-Key Low-pass Filter Design Tool:=0A=
> http://sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm=0A=
>=0A=
> --=0A=
> Jason White=0A=
=0A=
=0A=
-- =0A=
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive=0A=
View/change your membership options at=0A=
http://mailman.mit.edu/mailman/listinfo/piclist=0A=
.