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**JC1** · 08-02-2005, 12:48 AM

Analog Devices

Hi Guys,

Not sure if Carl can promote Analog Devices parts too much, but these have some potential, and can be bought from Digikey.

Really like the 12-bit 1 Meg sample per second on some of these. That's 1 us samples along the decay curve. And the speed to begin processing.

http://www.analog.com/en/prod/0,,762...7021%2C00.html

JC

**JC1** · 08-05-2005, 12:51 AM

Guest

Is anybody registered on Eric Foster's PI classroom?

They are still struggling with the filter problem.

Give them a link to the first post, please.

Driving me crazy. Tried to register never got my password.

Thanks.

**Reg** · 08-05-2005, 01:42 AM

Hi JC,

I copied your post as well as mention were it was on the Geotech forum and sent the info to Eric. So, he should have it by now.

Thanks for the info. It was quite informative. The overshoot was obvious when simulating a simple filter using EWB (Electronic Work Bench) and plugging in the the values Eric mentioned. I simply mentioned that when the input was a short pulse there would be a noticeable gain. Eric has this program so I am sure he has looked at it by now.

Reg

**Ferric Toes** · 08-05-2005, 07:57 AM

Hi JC and Reg,

Many thanks for the help and info., but I haven't had time yet to look at it further. The EWB simulation uses the Bode plotter and sig. gen., which I presume sweeps through the frequency band you set up. Using the values I have, shows no peak in the response. If I increase the feedback capacitor, then I start to get peaking. I haven't tried a pulse input yet. The input to the filter is the differential integrator output, which is a dc level (near 0V), with superimposed residual random noise, only a few tens of mV at most.

With the values I have on the PI Forum, the gain is just over 9db and flat to 5Hz. -3db point is 21Hz and the response is -60db at approx 600Hz.

If I take out the feedback capacitor, the gain doesn't change much, but the flat bandwidth reduces to 1Hz, -3db at 15Hz, and -60db at 10kHz.

Maybe that is the problem. Out to the -3db point the bandwidth of the 2nd order filter is wider, so letting through more lf noise. Perhaps the next move is to increase the capacitor values on the 2nd order, to give a -3db point of 15Hz.

Eric.

**Reg** · 08-05-2005, 02:50 PM

Hi Eric,

I should have been more clear in my post about seeing the overshoot. With the EWB program I used the signal generator set for a sq wave of 1 hz and 50% duty cycle for the input and a scope instead of the bode plotter to look at the outptut of the circuit built using the parameters you posted. This setup showed the overshoot JC mentions.

Then I reduced the duty cycle of the sq wave to 7%. This sort of simulated a brief noise pulse. Now, the scope signal displayed an increase in voltage when the filter was used. Disconnect the feedback capacitor and the voltage increase disappears.

About the only way to use this type of filter for noise reduction appears to use unity gain where the output is tied directly to the -input. Then it appears to work fine. (This is what JC1 mentions).

With this type of analysis (Sq wave in from generator and observing with a scope instead of a bode plotter), the increase in voltage level becomes obvious when the circuit has gain. So, it appears there would be an increase in noise level using this type of filter if there is any gain. Interesting. I had never given this a thought until now.

I am not sure why the bode plotter didn't show anything, but it didn't on mine either.

PS. I am too old and been out of college too long to try to figure out mathimatically just what is going on in the filter circuit, but obviously there is an overshoot, which would indicate a slightly underdamped filter. Maybe, JC1 can jump in here and help me out and explain it in simple terms. I guess that is why I have programs like EWB and spell checkers. PS I kant spel ether.

Reg

**Carl-NC** · 08-05-2005, 06:14 PM

The difference in 3dB bandwidths is only enough to make the total noise equal. A single pole response has a noise bandwidth that is PI/2 times its 3dB bandwidth (i.e., 57% higher). A two-pole response has a typical noise BW that is 11% higher than the 3dB BW. This makes the noise BW of the two almost identical. As already mentioned, noise peaking could be the culprit.

Not sure if Carl can promote Analog Devices parts too much...

I could, but I recently left Analog Devices, and now work for Maxim. So I really should promote Maxim.

- Carl

OK, call me slow, but I just got the anagram!

**Ferric Toes** · 08-05-2005, 06:45 PM

Thanks Carl. Obviously needs looking into further. Talking of anagrams, how about Random Recall, Corralled Man, or Cornmeal Lard. You can have fun with names at http://www.wordsmith.org/anagram/advanced.html

Eric.

**Carl-NC** · 08-06-2005, 01:07 AM

Cornmeal Lard, I like that... though Random Recall seems more appropriate.

I wonder if anagrams are indicators of the future? My youngest daughter is "Endemic Moral Hell" and my teenage daughter is "A Normal Mans Death". The latter one is looking more & more likely...

**JC1** · 08-06-2005, 04:24 AM

Excellent

Thanks Guys,

This was bugging the heck out of me.

Wow! Have to go through the posts.

All has to do with Group Delay. Different delays for different frequencies. This is long subject but google Group Delay Active Filters and you should get some answers.

Reg has got it right, putting in square wave, which we have all learned from Minelab contains "multiple frequencies".

Look for overshoot, multiple order filters will need to be Bessel, Thompson, linear phase, etc. to avoid this problem.

Really busy, consulting.

Carl is now with Maxim, another good company.

Congrats.

Used to design electronics for various NASA payloads, used alot (and I mean alot) of Analog Devices high rel parts.
Pretty white ceramic with gold leads.

Really good results with Analog Devices parts. Burr Brown too.

Sorry for the delay, group that is.

JC

**JC1** · 08-06-2005, 04:31 AM

Filter Pro

Oh and took a look at Filter Pro.

Group delay is shown as the black line.

It needs to be flat and roll off. Try the different filter types and see the peak in group delay.

I use Circuit Maker Pro for fun and speed. And Pspice for the serious stuff. Both of these show group delay if you tell it too.

Not so sure with EW. Played with a few versions of EW friends had, but not so impressed.

JC

**JC1** · 08-06-2005, 04:37 AM

More

Thanks Reg,

Hi Eric, hope this helps. More to active filters then meats the eye (frequency response).

Have Fun, need sleep.

JC

**JC1** · 08-06-2005, 05:10 PM

Group Delay

Hi Eric and All,

Here is a link to a Maxim app. note for fun. Eric I have not studied your specific circuit, but I promise you peaks in the group delay will cause you problems. Been there done that years ago. Time response,,, for all of this discussion there is NO peak in the frequency response.

For those who have EW Pro version I believe it should show this, not sure about basic version.

here is the link, look at the first graphic, it also discusses delay equalization. The graph shows the types of filters and the "peaks" in the "group delay" {time response, flat in frequency response.

http://www.maxim-ic.com/appnotes.cfm...te_number/2159

**Wirechief** · 08-06-2005, 05:38 PM

EWB

Hello Eric, Reg and Carl with others,
Is Electronics Workbench obtainable for individuals and is it expensive?Thanks, Wirechief.

**JC1** · 08-06-2005, 05:55 PM

active filters

Cut and Paste {I did not write the following, but it is true}
More Maxim applications.

It's a jungle out there.

A small tribe, in the dense wilderness, is much sought after by head hunters from the surrounding plains. Known throughout the land for their esoteric expertise, this is the tribe of the Analog Engineers, who live in the farthest regions of the left half Plains, past the jungles of Laplace.

The guru of analog engineers is the Analog Filter Designer, who sits on the throne of his kingdom and imparts wisdom. You never get to see him even, with an appointment, and you call him "Sir." {This is correct}

The countless pages of equations found in most books on filter design can frighten small dogs, and digital designers. This article clears a path through the brush for the practical engineer and unravels the mystery of filter design, enabling you to design continuous-time analog filters quickly and with a minimum of mathematics.

The Bessel filter gives a constant propagation delay across the input frequency spectrum. Therefore, applying a square wave (consisting of a fundamental and many harmonics) to the input of a Bessel filter yields an output square wave with no overshoot (all the frequencies are delayed by the same amount). Other filters delay the harmonics by different amounts, resulting in an overshoot on the output waveform.

http://www.maxim-ic.com/appnotes.cfm...te_number/1795

another good primer. read section two on filter characteristics, not far in. Pulse response.

http://focus.ti.com/lit/an/sloa049b/sloa049b.pdf

Too much fun. Now where is that anagram thing.

JC

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