Filtering ideas

Depending on "what" you need to measure on your signal, it might be a good application for a lock-in amplifier. This technique is often used to measure very small signals buried in noise.

A more modern term is "synchronous demodulation".

-- Many thanks,

Don Lancaster voice phone: (928)428-4073 Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552 rss:

email: snipped-for-privacy@tinaja.com

Please visit my GURU's LAIR web site at

Anyone have any thoughts on this Elliptic filter ->

It's the highest order Elliptic active filter I could find. It's not bad, but would need at least 4 stages.

Paul

You can make as high of an order filter as you like out of one single op-amp; the problem is that the sensitivity of the filter's response to component variations and op-amp imperfections increases super-linearly with respect to the order of the filter you're trying to build. This is why you tend to see most "high performance" active filters built out of bi-quad casecades (with typically two op-amps per section) and topologies with only one op-amp tend to either need trimming or else somewhat more relaxed specs. (And I've never seen someone try to pull off, e.g., a 4th order filter with one op-amp in anything other than SPICE, but who knows... somebody probably has!)

As such, I'd suggest you take your circuit there and run it through the a Monte-Carlo simulation in SPICE and see if you like the results!

---Joel

Opamp elliptics are a bit of a waste of time unless you have a particular frequency to notch out. Certainly useless if you want that 1MHz break frequency. I'd guess a 30kHz filter might be appropriate. Would suggest just stacking some simple 2nd or 3rd order Butterworth designs. john

"w2aew" a écrit dans le message de news: snipped-for-privacy@j52g2000cwj.googlegroups.com...

If you have a good DSO then you have the lock-in of choice. Lots of averaging with maybe a small preamplifier front end if the signal is too low. I have no pb looking at sub-uV signals directly feeding them to my scope (no preamp because I'm a lazybones). A small RC to BW limit the noise and 4K averaging nicely do the rest.

--
Thanks,
Fred.

Thanks for input. I see that you're 100% correct. Have you tried a program called "Filter Solutions 2006?" It's a dream program. I'm very disappointed in the Elliptic filters. The best bandpass filter I found using a Pos SAB active implementation with lower & higher corner freq. = 1KHz & 10KHz is a Chebyshev I. It requires only 4 op-amps for a 7th order bandpass. Even after entering LM741 amp parameters I still get

-208db at 60Hz and -89db at 25KHz. Nearly 1db at 1KHz and 10KHz. It's unbelievable! I'm curious what the disadvantages to a Pos SAB active implementation since it uses less than half the parts-- 4 op-amps compared to 10. Here's the Spice Net List:

MULTIPLE ANALYSES

• V1 1 0 AC 1

*V1 1 0 PULSE 0 1 0 5.033E-08 0 R111 1 2 4E+04 C112 2 7 1.069E-08 R113 2 0 2500 R109 2 3 1E+04 C101 3 4 1.504E-08 C102 4 5 1.828E-09 R103 5 0 5.737E+05 R104 4 7 2500 R106 6 0 1E+04 R107 6 7 1795 C108 4 0 2.183E-09 X101 5 6 7 OPAMP PARAMS: A=3E+11 B=4E+06 R211 7 8 2E+04 C212 8 13 6.06E-09 R213 8 0 5000 R209 8 9 1E+04 C201 9 10 1.001E-08 C202 10 11 7.2E-09 R203 11 0 1.051E+05 R204 10 13 5000 R206 12 0 1E+04 R207 12 13 2821 C208 10 0 1.469E-09 X201 11 12 13 OPAMP PARAMS: A=3E+11 B=4E+06 R311 13 14 2E+04 C312 14 19 6.288E-09 R313 14 0 5000 R309 14 15 1E+04 C301 15 16 7.512E-09 C302 16 17 1.434E-08 R303 17 0 7.002E+04 R304 16 19 5000 R306 18 0 1E+04 R307 18 19 2269 C308 16 0 9.47E-10 X301 17 18 19 OPAMP PARAMS: A=3E+11 B=4E+06 R401 19 20 1E+04 C402 20 21 1.451E-09 R403 21 0 1.557E+04 R404 20 23 3.513E+04 R406 22 0 1E+04 R407 22 23 7.787E+04 C408 20 0 1.451E-09 X401 21 22 23 OPAMP PARAMS: A=3E+11 B=4E+06 .AC DEC 200 0.1 2.5E+04 .PLOT AC VDB(23) -200 0 .PLOT AC VP(23) -200 200 .PLOT AC VG(23) 0 0.005 .TRAN 0.05 10 0 .PLOT TRAN V(23) -0.5 0.3 .END

*OpAmp Complex Model 1=+in 2=-in 10=Vo A=G*Bw Product(Hz) B=Bw(Hz) .SUBCKT OPAMP 1 2 10

+PARAMS: A=1.E+12 B=1.E+06 Rin 1 2 2.e+06 Cin 1 2 5.e-12 E1 3 0 1 2 1.0 Rp 3 4 1.0 Cp 4 0 {1/(2*PI*B)} E2 5 0 4 0 {A/B} Rout 5 10 500. .ENDS OPAMP

[snip]

Designing elliptic filters CAN be a bit of a headache if you're winging it.

If you really want to do it right may I suggest...

"Synthesis of Filters" Herrero and Willoner Prentice Hall EE Series © 1966 Library of Congress 66-27547

Once upon a time I had the whole General Parameter Filters chapter programmed into an ancient tape-card-reading hp calculator (or was it TI... it was early '70's).

Probably ought to redo it in Larkin-suggested PowerBasic.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

It sure is! Unless you're cooling the with, e.g., liquid helium, thermal noise alone is well above -208dB in any reasonable bandwidth! :-)

Most likely sensitivity to component value variations -- run a Monte Carlo simulation on each and see.

a

used

particular

Yes, the prog is excellent. I recognised the circuit drawing. Don't believe those incredible dB figures. In normal practice you're lucky to get anything down to -80. To spot those filter differences, initially pick a SAB type circuit design, select a component on it, then on the drop down box go for a multiple run Monte-Carlo analysis using random values for all the R's and C's. You'll see some -really- ugly spreads on that original pristine response. Then do the same using leapfrog and GIC implementations. john

Do you just want to detect the signal? Why bother if you already know when to expect it? Or is this an experiment to see whether detection is possible?

Anyway, I have three suggestions:

1) Filter out-of-band noise. 2) Average, if possible 3) Use correlation to detect

If you just want to observe the signal, you can probably just use an oscilloscope with averaging, since you have a trigger.

If you really do want to detect the signal, well, I'm not a signal processing expert either, but correlation is probably the best way to detect it. I guess you would digitize, then correlate.

Also, any properties of the signal which are known will help you detect it. For example, is it repeated regularly? If so, and if the period is known, your detection can be improved dramatically.

You might want to try some other newsgroups. comp.arch.dsp?

--Mac

Assuming that the stimulus and reponse are repetitive, the best way to handle this is with synchronous averaging. The idea is that you sample a series of response time points that are triggered by the stimulus. If there were no noise, then each set of points would look identical. The noise causes each actual response to be wildly different because of the added trash, but if you average enough of these together the true response appears. You get a 3 dB improvement for each doubling of the number of responses averaged.

Note that this kind of averaging is done on a point-by-point basis: Point 1 of response 1 is averaged only with point 1 of response 2, 3, 4, etc, independent of other time points.

The beauty of synchronous averaging is that it does not affect the waveform in any way... there is no rounding, no phase shift, no step response, no ringing. But it may take a lot of averages to get the desired smoothness.

This is the way researchers record evoked neural responses from scalp electrodes. The desired response is totally buried in huge amounts of noise, but because it is repeatable, it can be extracted after 100s of passes are averaged.

I have some on-line tutorials about this at

The synchronous averaging titles are about halfway down the page. Here is a direct link to the first one: At the bottom of each article are links to next and prior, etc.

Hope this helps!

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

Home of DaqGen, the FREEWARE signal generator

In article , wrote: [....]

Since everyone else is talking about the signal, I'll ask a question about the "noise". Is it really random or is it mostly harmonics of the local mains frequency?

If it is harmonics of the mains, you can help things a lot by timing the experiment such that it slides over one cycle of the mains in every N samples and then averaging N samples together. This tends to reject the low harmonics of the mains.

For the mains frequency its self, you could also add a notch filter to remove it before the ADC.

--
--
kensmith@rahul.net   forging knowledge

Hi,

I'm looking at an op-amp graph of total noise vs source resistance. The noise increases with an increase of source resistance. Does this also apply to reactance?

The way I understand it is resistance is thermal noise, but reactance from inductors and caps don't caused any noise. Real L's and C's have some R, but in that sense they have noise.

---------------------------------

Some details:

I have an input coil antenna that's 22 ohms R and about 10 mH. Nothing fancy, just a round loop coil with a lot of turns. Typical frequencies: from 1KHz to 1MHz. At say 1MHz the reactance would be just over 6 M ohms. The chart, "Total Noise vs Matched Source Resistance" at

According to the chart a 6 M ohm source resistance would have outrageous noise. Is it safe to say the 6 M ohm reactance (for 1MHz signals) will cause no noise? Rather the noise will come from the 22 ohms R? According to the chart, 22 R at 1KHz is ~ 1 nV/SqrtHz. Not sure what it would be at 1 MHz, but it seems lower than 1KHz.

The charts is for "Matched Source Resistance." The coil is one continuous resister, so if I place the 2 coil leads directly to the op-amp then is that considered matched? I'm using a typical differential op-amp has two input R's. So if the coil's total R is 22 ohms then would that equate to two 11 ohm input R's? Very confusing, lol.

Many thanks, Paul

Noise is due to thermal noise *and* shot curent noise. The shot noise current of an amplifier is droped accross the source impedance (and Rin). If the source is an inducter, the voltage noise caused by the amp input current shot noise will increase with frequency despite the fact that the inductor itself does not generate any noise.

Kevin Aylward B.Sc. snipped-for-privacy@anasoft.co.uk

SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design.

"There are none more ignorant and useless,than they that seek answers on their knees, with their eyes closed"

W R O N G You are off by several orders of magnitude. it's 63k. But even this is impossible, because the capacity between the windings will not allow you to reach much more than 600 ohms. That would be a Q of 30. This coil would be useless above a few kHz. 1MHz will be attenuated by 40dB. But apart from that did you ever see a 10mH coil?

The chart, "Total Noise vs Matched Source

You do not understand that table either. Better to read up in Win's book on noise. too long to explain here.

You just have too little knowledge to understand. Read AoE again.

--
ciao Ban
Apricale, Italy

LT1028 is a good and expensive bipolar OP-amp. But at high Z it is a bad choise. Better to use a JFET- or MOSFET-device at high Z.

Nature is normally very hard to fool! I suppose we are talking about a LC-circuit near resonance. That means the LC-circuit will act as a transformer for R, and it will show a noise source of the same magnitude as its equivalent parallell resistance, which can be 6Mohm. If it is far from resonance, it will still be a bad match to a bipolar device. And if it is an antenna, there may also be noise in the rf-field that it picks up.

--
Sven Wilhelmsson
http://home.swipnet.se/swi

Let me see if I understand this. A pure inductor cause shot noise, but no thermal noise? How can I calculate a differential op-amp's output noise if the source is mostly inductive? ->

Note that in my case R1 & R2 are also reactive-- 22 ohm resistance and

6 M ohm inductance.

The capacitance is extremely small. So there's no resonance.

I presume that the noise would be a lot greater if it was 6 M ohm resisters rather than inductors.

Thanks for any help, Paul

--- snip ---

A pure inductor does not cause shot noise, but the OP-amp does. The source impedance would shunt this shot noise, so when Z-source is low the shot noise is not dominant. When Z-source is high, it is!

If X_inductive = 6 Mohm at 1 MHz then L would be ~ one Henry. ( Or do you mean milli-ohm? ) The same value, 6 Mohm, for X_capacitive would imply C ~ 0.03 pF! That is small indeed, but I do not understand how it could be.

My guess is that it would be about the same at the output of the OP-amp! The input capacitance of the amplifier can, if properly matched, balance the reactance. But then we have a resonant circuit.

--
Sven Wilhelmsson
http://home.swipnet.se/swi