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ultra low frequency low pass filters.

M

Mook Johnson

Jan 1, 1970
0
Hey all,

When designing sub 1Hz filters for a very slow moving signal that can have
noise on it from 1Hz and upward, what is the safest way to achieve low DC
offset and only mild shifting of the corner frequency? Let say the
temperature range is from 0 to 125C.

The signal is a 0 - 5V analog signal analog signal being read by a 16 bit
a2d.

looking at two way of doing this.

1) good old S-K analog filter using X7R or NPO caps.
problem- Resistor in the network are 500K with 1uF (largest practical
size in X7R). The drift if X7R with temperature is tolerable but the offset
voltages, noise pickup and laternate path through the Polymimide PCBs at
160+C temperature are concerns.

2) DSP filter,
the antialias filters will have similar but reduced affects of the
straight analog option with teh only advantage being the lack of crossover
point drift. The increased performance of the "brickwall" filters will not
have a big benefit.

Any other circuit topologies or tricks to making this thing stable?
 
T

Tim Wescott

Jan 1, 1970
0
Mook said:
Hey all,

When designing sub 1Hz filters for a very slow moving signal that can have
noise on it from 1Hz and upward, what is the safest way to achieve low DC
offset and only mild shifting of the corner frequency? Let say the
temperature range is from 0 to 125C.

The signal is a 0 - 5V analog signal analog signal being read by a 16 bit
a2d.

looking at two way of doing this.

1) good old S-K analog filter using X7R or NPO caps.
problem- Resistor in the network are 500K with 1uF (largest practical
size in X7R). The drift if X7R with temperature is tolerable but the offset
voltages, noise pickup and laternate path through the Polymimide PCBs at
160+C temperature are concerns.

2) DSP filter,
the antialias filters will have similar but reduced affects of the
straight analog option with teh only advantage being the lack of crossover
point drift. The increased performance of the "brickwall" filters will not
have a big benefit.

Any other circuit topologies or tricks to making this thing stable?
If you use the DSP filter approach you will be able to optimize your
antialiasing filter for low offset. If you're filtering down to the sub
Hz you can _way_ oversample, use a cheap slow processor, and still have
an anti-aliasing filter with decent-sized caps. At this point you don't
have to give a damn about the capacitor drift, you only have to worry
about the DC offset.

As an added benefit you'll be filtering any ADC noise after it's been
sampled, thus increasing the effective ADC resolution (but not accuracy).
 
J

John Larkin

Jan 1, 1970
0
Hey all,

When designing sub 1Hz filters for a very slow moving signal that can have
noise on it from 1Hz and upward, what is the safest way to achieve low DC
offset and only mild shifting of the corner frequency? Let say the
temperature range is from 0 to 125C.

The signal is a 0 - 5V analog signal analog signal being read by a 16 bit
a2d.

looking at two way of doing this.

1) good old S-K analog filter using X7R or NPO caps.
problem- Resistor in the network are 500K with 1uF (largest practical
size in X7R). The drift if X7R with temperature is tolerable but the offset
voltages, noise pickup and laternate path through the Polymimide PCBs at
160+C temperature are concerns.

2) DSP filter,
the antialias filters will have similar but reduced affects of the
straight analog option with teh only advantage being the lack of crossover
point drift. The increased performance of the "brickwall" filters will not
have a big benefit.

Any other circuit topologies or tricks to making this thing stable?

2, for sure. The filter can be simple exponential smoothing, not
official "DSP", if you're not picky about rolloff shape.

Out = Out + (In-Out) / K

If that settles too sloppily for your taste, just do a 2nd or 4th
order version of same.

John
 
T

Tim Wescott

Jan 1, 1970
0
John said:
2, for sure. The filter can be simple exponential smoothing, not
official "DSP", if you're not picky about rolloff shape.

Out = Out + (In-Out) / K

If that settles too sloppily for your taste, just do a 2nd or 4th
order version of same.

John
On an ultra-cheap processor that's

Out = Out + ((In - Out) >> S);

Just make sure that your variables have at least 16+S bits.
 
T

Tim Shoppa

Jan 1, 1970
0
good old S-K analog filter using X7R or NPO caps.
500K with 1uF (largest practical
size in X7R).

Why limit yourself to ceramic caps? [Metalized] polyester and
polypropylenes are available up to 10uF and there's no problem at all
with using multi-megohm resistors if you want. FET op-amps (like you'd
use with high impedances like this) have a bigger offset voltage but
you seem vague as to what your offset requirements are. (You're also
vague about the temperature requirements, starting with 125C but then
complaining about Polymimide PCB's at 160C).

But your option 2:
2) DSP filter

seems like a no-brainer winner, because your acquisition rate is so
glacially slow and you're going to A-D the result anyway.

Tim.
 
J

John Larkin

Jan 1, 1970
0
On an ultra-cheap processor that's

Out = Out + ((In - Out) >> S);

Just make sure that your variables have at least 16+S bits.

Right; I usually do it that way. And I usually throw away a couple
more bits so I don't have to worry about 2's comp rollovers and stuff,
which always hurts my head.

What's interesting is trying to do a higher-order filter using only
shifts to make the coefficients. You get some strange unorthodox
transfer functions.

John
 
J

Jim Thompson

Jan 1, 1970
0
good old S-K analog filter using X7R or NPO caps.
500K with 1uF (largest practical
size in X7R).

Why limit yourself to ceramic caps? [Metalized] polyester and
polypropylenes are available up to 10uF and there's no problem at all
with using multi-megohm resistors if you want. FET op-amps (like you'd
use with high impedances like this) have a bigger offset voltage but
you seem vague as to what your offset requirements are. (You're also
vague about the temperature requirements, starting with 125C but then
complaining about Polymimide PCB's at 160C).

But your option 2:
2) DSP filter

seems like a no-brainer winner, because your acquisition rate is so
glacially slow and you're going to A-D the result anyway.

Tim.

As I've pointed out in previous posts...

Using integrators of the form 1/(p+1) eliminates problems with
capacitor dissipation factor at low frequencies. (p = s/(2*pi*Fo),
normalized Laplace variable)

However this means using a state-variable-like circuit structure.

See "StateVariableFilter(P+1).pdf" on the S.E.D/Schematics page of my
website for details.


...Jim Thompson
 
R

Rich Grise

Jan 1, 1970
0
Hey all,

When designing sub 1Hz filters for a very slow moving signal that can have
noise on it from 1Hz and upward, what is the safest way to achieve low DC
offset and only mild shifting of the corner frequency? Let say the
temperature range is from 0 to 125C.

The signal is a 0 - 5V analog signal analog signal being read by a 16 bit
a2d.

looking at two way of doing this.

1) good old S-K analog filter using X7R or NPO caps.
problem- Resistor in the network are 500K with 1uF (largest practical
size in X7R). The drift if X7R with temperature is tolerable but the offset
voltages, noise pickup and laternate path through the Polymimide PCBs at
160+C temperature are concerns.

2) DSP filter,
the antialias filters will have similar but reduced affects of the
straight analog option with teh only advantage being the lack of crossover
point drift. The increased performance of the "brickwall" filters will not
have a big benefit.

Any other circuit topologies or tricks to making this thing stable?

Sure. Just use a straight, plain old ordinary D/A, and filter it in
software. Heaven sakes, you only have 2,000,000,000 nanoseconds to deal
with a cycle of input! ;-P

Cheers!
Rich
 
K

Ken Smith

Jan 1, 1970
0
Hey all,

When designing sub 1Hz filters for a very slow moving signal that can have
noise on it from 1Hz and upward, what is the safest way to achieve low DC
offset and only mild shifting of the corner frequency? Let say the
temperature range is from 0 to 125C.

Since you didn't say you wanted your result in digital form, I will assume
that you really need an analog output.

One of the problems with the ADC-DSP-DAC method is that the DC offset in
the converters can add up to a fair amount. A trick for dealing with this
is to make an integrator with a very low gain and feed it the difference
between the input and the output. So long as the integrator's gain is way
down before the phase shift of the filter gets big, the exact gain of the
integrator doesn't matter, you can use it to tune out the offset. This
lets you get away with inexact parts and yet have an exact cut off.


A DSP or even a micro controller making an IIR filter can have a very
sharp cut off without too much cost in hardware. You may want to follow
the DAC with an analog filter and dither it to increase the effective
number of bits. Since the frequencies you want to do are very low, 24
bits worth of DAC can be bought so you should be able to make 26 or 28
bits worth of results.
 
R

Robert Baer

Jan 1, 1970
0
Mook said:
Hey all,

When designing sub 1Hz filters for a very slow moving signal that can have
noise on it from 1Hz and upward, what is the safest way to achieve low DC
offset and only mild shifting of the corner frequency? Let say the
temperature range is from 0 to 125C.

The signal is a 0 - 5V analog signal analog signal being read by a 16 bit
a2d.

looking at two way of doing this.

1) good old S-K analog filter using X7R or NPO caps.
problem- Resistor in the network are 500K with 1uF (largest practical
size in X7R). The drift if X7R with temperature is tolerable but the offset
voltages, noise pickup and laternate path through the Polymimide PCBs at
160+C temperature are concerns.

2) DSP filter,
the antialias filters will have similar but reduced affects of the
straight analog option with teh only advantage being the lack of crossover
point drift. The increased performance of the "brickwall" filters will not
have a big benefit.

Any other circuit topologies or tricks to making this thing stable?

You alerady jave excellent responses concerning filtering.
The PCB material you mentioned can be replaced with much less
expensive PCB material made by Matsushita; see the PCB reference on the
http://oil4lessllc.com/ site.
 
M

Mook Johnson

Jan 1, 1970
0
Thanks guys,

As I learned more about filters I found a IIR filter that would do the
trick, was stable and simple to impelemnt. I could use a PIC if I wanted to
since the signal is so SLOW.

Thanks for all the imput.
 
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