1hz to 20hz active bandpass filter?

snip

Well, it would. You'd just have to use a lot of them...

Mook Johnson wrote: (top posting fixed)

A dsPIC would be way overkill for something that needs a bandwidth of

_Assuming_ that you're sampling no faster than 200Hz you could use just about any 8-bit micro, but you'd need a darn good anti-aliasing and reconstruction filters. You'll have a definite trade-off, but if you plan on doing a sum-and-decimate for your first filter you can sample fairly quickly with the minimal load on the processor, then you can do "fancy" filtering at a lower rate (this doesn't help you with reconstruction, though).

I'd look into one of the 24-bit sigma-delta converters that Analog Devices and other folks sell for instrumentation -- they have inherent anti-alias filters that operate in the digital domain, their accuracy is higher than any but the best front-end circuitry, and they're 8-pin.

Since we're on the subject of sampling rate vs. bandwidth, pay close attention to the depth of your data paths -- that 1Hz lower corner frequency combined with a high sampling rate will demand quite a few bits in the filters.

Greetings Gents,

I'm looking to fabricate a 1 HZ to 20 HZ active bandpass filter for an oddball active vibration dampening project I'm playing with. Signal source is a MEMS accelerometer -> bandpass filter -> amplifier -> mass driver, with the goal being to reduce the dampening time of a 2 meter long, 400mm diameter metal pipe.

The primary requirement for the filter is that the input and output must be in phase with each other, is this going to be a problem?

Secondarily, it would be nice for it to have a rather steep cutoff (24db /oct or better).

Any thoughts?

Take Care, James Lerch

Hello Tim,

Agree. Caps of that size will also have a lot of tolerance or be freaking expensive.

And get used to the thought that none of this neat little ceramic stuff is going to work here ;-)

Regards, Joerg

I agree with Tim

Give the dsPIC at microchip a look. It has 12 bit A2D and PWM outputs that can be used as slow DACs. They are in-circuit programmable and they have a C compler with a 1 month demo (forever if you don't mind your computer date being off).

The only thing you might have to buy is the ICE2 emulator to program the part $99.

Very good basic building block for an embedded system.

First, thanks gents I appreciate the ideas!

Secondly, I have the book " Digital Signal Processing: a Practical Approach" on it's way as we speak, as I was pretty certain I'd need a Pic of some kind to do what I want. Is this book going to be of assistance? (on the assumption I can climb the learning curve! )

In the mean time, while I await the book's arrival, for crude and simple prototyping using a PC as a control system, would something like this be a good starting point:

I have yet to climb the Pic programming learning curve, but I can hold my own in Visual Basic... (yes, it is sad, but if I can get it to work in that environment, I'm sure I can figure out how to get similar code stuffed into a pic or such.)

Also, depending on my final version of a mass driver, I might not need to filter out the 1hz to DC components (or depending on resonance of the final structure the 1hz mark may move upwards to 5hz or so...)

Thanks again for you help

Take Care, James Lerch

Hello James,

For playing around with it this might work. But you won't get a high resolution ADC and you may also need Labview ($$$).

Regards, Joerg

Agreed, especially when I consider this to be "Just playing around"

Indeed, and the reason I was interested in it in the first place (that plus the price point is attractive)

here's the spec page:

Analog input in continuous scan mode is 1.2kS/s, which for my experiment (1-20hz) should suffice (I think..)

Analog output is 100S/s, which will be a problem, but there are a lot of Digital outputs, maybe I can implement my own DA converter after the device....

Take Care, James Lerch

Forget Labviw, DasyLab and Matlab, they are all far too expensive. However, they include an universal library suitable for Visual studio compilers. But since the don't specify an ADC speed, I'd consider them of the DC type.

Rene