Lowpass filter, cutoff 10kHz, fast response time, very low noise addition

[Mathieu Fregeau]

Hi,

I am looking to build a lowpass filter (cutoff about 10kHz) with very fast
time response (I am sampling the signal at 125kHz). However, this filter is
to process a signal from a thermocouple and thus needs VERY low noise
addition from the filter as the signal of such sensors is in the order of
millivolts. I believe active filtering will add considerable noise, and a
passive filter would be preferable.

Is that possible to obtain a very fast response lowpass filter in a passive
way? (and how do I compute the values of each components?) How to I choose
the order of the filter?

I think this is a very simple project but I'm not an electrical engineer...
Is there is any online tools or software that can help designing a passive
lowpass filter of high order and tells you the time response curve of the
circuit (upon a step function let say)?

Regards,

---------
Mathieu Fregeau
University of Washington
Department of Aeronautics and Astronautics
Aerospace Research Building - Box 352250
Seattle, WA 98195-2250
Phone (office): 206-543-1070
Phone (lab): 206-616-5557
Fax: 206-543-4719
[email protected]

 

[Tony]

Hi,

The answers are all generally yes. But to start with, a lowpass filter
by definition doesn't have a "very fast time response". You must
accept the time response that goes with the filter you select.

It can certainly be passive and low noise - just add a series resistor
R that doesn't upset the DC accuracy or ADC noise too much, then shunt
the output to ground with a cap so R.C = T (the filter's time constant
- about 16us for 10kHz).

But you probably want more than 20dB/decade rolloff, and the best
simple circuit that does that is a variation of the above single pole
circuit. It uses a low noise bipolar transistor wired as a simple
common emitter amplifier, with resistors from supply to collector, and
from collector to base. Then instead of the single cap to deck that
was on the single pole filter, you wire in two caps - one to the
transistor's base, the other to its collector. Eg, try making the
series resistor 10k, the other two resistors 33k, and the two caps
1n0. It's only 2 poles, but produces no DC error and very low noise,
so maybe it's enough? To be sure you need to know what it is you're
trying to reject (what frequencies, how much rejection).

Failing that, you could make an L-C filter; at 10kHz, you could get
away with regular cheap axial chokes, but you'd need to allow for the
very poor Q (not easy, and not the sort of thing you can tweak to get
it right).

Or, if you only want one, use "proper inductors" and hang the expense.

And I have to ask - who makes thermocouples that respond up to 10kHz?

Hi,

I am looking to build a lowpass filter (cutoff about 10kHz) with very fast
time response (I am sampling the signal at 125kHz). However, this filter is
to process a signal from a thermocouple and thus needs VERY low noise
addition from the filter as the signal of such sensors is in the order of
millivolts. I believe active filtering will add considerable noise, and a
passive filter would be preferable.

Is that possible to obtain a very fast response lowpass filter in a passive
way? (and how do I compute the values of each components?) How to I choose
the order of the filter?

I think this is a very simple project but I'm not an electrical engineer...
Is there is any online tools or software that can help designing a passive
lowpass filter of high order and tells you the time response curve of the
circuit (upon a step function let say)?

Regards,

---------
Mathieu Fregeau
University of Washington
Department of Aeronautics and Astronautics
Aerospace Research Building - Box 352250
Seattle, WA 98195-2250
Phone (office): 206-543-1070
Phone (lab): 206-616-5557
Fax: 206-543-4719
[email protected]

Tony (remove the "_" to reply by email)

 

[John Popelish]

Hi,

I am looking to build a lowpass filter (cutoff about 10kHz) with very fast
time response (I am sampling the signal at 125kHz). However, this filter is
to process a signal from a thermocouple and thus needs VERY low noise
addition from the filter as the signal of such sensors is in the order of
millivolts. I believe active filtering will add considerable noise, and a
passive filter would be preferable.

Is that possible to obtain a very fast response lowpass filter in a passive
way? (and how do I compute the values of each components?) How to I choose
the order of the filter?

I think this is a very simple project but I'm not an electrical engineer...
Is there is any online tools or software that can help designing a passive
lowpass filter of high order and tells you the time response curve of the
circuit (upon a step function let say)?

Regards,

Probably the lowest noise kind of low pass filter you can make is an
LC (series L, shunt capacitor). But thermocouples are often prone to
common mode noise (voltage or current that is common to both leads of
the couple) because the couple is often grounded, and the voltage at
that point may be very noisy, compared to signal ground at the
receiving end. I have often had very good results suppressing this
kind of noise bu adding a common mode filter inductor in series with
both leads of the couple, at the receiving end. This sort of thing is
made to be used as a power line filter in switch mode supplies and is
very available. It adds only an ohm or two to the couple, so open
couple detection methods work right through it. Here is an example of
what I am talking about:

http://rocky.digikey.com/WebLib/Panasonic/Web data/ELF-17N.PDF

The beauty of this type of filtering is that it can have a very low
frequency attenuation for common mode noise, but pass differential
mode voltages (the couple voltage) with essentially no filtering.

 

[hrh1818]

I don't kow what type of thermocouple you are using but most
thermocouples are low pass filters with a 3 db frequency less than 100
cycles per second. They won't measure rapid changes in temperature.
This means with your 125 kHz sampling frequcny you could use a simple
one pole low pass RC filter with a 100 cps 3 db point. Then if you
have any unusual noise problems like 60 cycle power line noise or high
frequency noise from an induction heater you could get rid of this
noise with a digital filter. As a side note it seems unusual you are
not using an A/D comverter better suited for thermocouple signals then
this 125 kHz device you are using.