Peculiar filter bandpass filter requirements

[Fish]

I have a very strange problem. I wish to design a bandpass filter with a lower cutoff of .001 Hz and an upper cutoff of 5 kHz.

To satisfy this large bandwidth requirement and extremely small low frequency cutoff, the circuit needs to be built for a very low center frequency. As a result, the quality factor, and hence gain at the center frequency is vanishingly small. To restore the signal passed by such a filter would require something absurd like ~1e24 subsequency amplification.

Any recommendations on filter topologies, filter order, what else I could try would be appreciated.

 

[JimB]

My first thoughts are to use two separate filters, a 5khz low pass and a 0.001 hz high pass.

But what on earth do you want a cut off frequency of 0.001 hz for?

JimB

 

[Nigel Goodwin]

I agree with JimB, a high-pass filter at 0.001Hz sounds rather stupid, why on earth would you possibly want such a device?. It looks more likely that a 5KHz low-pass filter would do all you required?.

Although you 'could' design a filter for such a low frequency, the capacitors involved would be huge, and the cost would probably be prohibitive?.

 

[Fish]

Yeah, I know its very strange, and I'm not sure it will work. My friend and I are working on a project and he suggested that we build a bandpass with such a low cutoff in order to reduce low frequency drift in a sensor circuit. I'm not an electronics expert, but it seems to me that it should in theory work, and why I made this post. Is it really that unrealistic?

 

[Nigel Goodwin]

Yeah, I know its very strange, and I'm not sure it will work. My friend and I are working on a project and he suggested that we build a bandpass with such a low cutoff in order to reduce low frequency drift in a sensor circuit. I'm not an electronics expert, but it seems to me that it should in theory work, and why I made this post. Is it really that unrealistic?

I would suspect the filter would probably drift more than the sensor?, and you will require massive coupling capacitors - as by definition it can't be DC coupled!.

What kind of sensor is it anyway?.

If you want to explore the filter possiblity, you might have a look at the free software from MicroChip at **broken link removed**, although it says it only goes down to 0.1Hz - but making the capacitors 100 times larger will give you an idea of the capacitor sizes required.

 

[bmcculla]

You can buy chopper stabilized op amps that have very low offset voltage - might cut down on your drift.

You also might try using a temperature sensor to compensate for any drift due to temeprature.

You could implement your filter using DSP techniques but you will need a really long FIR filter - long time window to see the low frequencies. If you sampled slowly you might be able to do it without a ton of processing and memory.

 

[Russlk]

This circuit will maintain zero volts out within the range of the op amp, but signals above .001 hz will pass. You can follow it with a low pass to limit the bandwidth. The gain is (1+Rgain/10K)/2.