[Moved]How to design a Low Pass Filter for low frequency?

[quickanswers]

I want to design a low pass filter working in low frequency with cut-off frequency of 15Hz, with C: 2pF so resistance needed is R: 5307.85562633 MΩ. So it's not possible with 1st order circuit. So what can be done for 2nd order circuit? I've seen gm-C filters but not getting how to design it and how to calculate the overall Gm for the whole circuit in Cadence Virtuoso. Give valuable inputs.


Thanks and Regards.

 

[KlausST]

Hi,

Give valuable inputs.

We expect this from you in first place.

No information about technology, no precision, no filter targets.

one number with 2 digit, the other with 12 digits precision ...

It´s neither complete nor consistent informations.

Before we can give valueable answers we need an almost complete information about your requirements and specifications.

Klaus

 

[barry]

Why 2pf? Why 2nd order, why not 56th order? The order of your filter determines the rolloff characteristics, not the cutoff frequency. As Klaus points out, you provide zero information.
Maybe you should study up on filters.

 

[BradtheRad]

By working with ideal components (simulated) it's easy to achieve a first-order type which theoretically does what you want. However garden-variety real-life components have lower internal resistance and that dictates your capacitor needs a much greater Farad value.

Example, Sallen-key type built around 1 or more op amps: internal resistance about 1MΩ. Useful resistors to put in your circuit might be 100k. Therefore to achieve a few Hz, multiply your cap value by several thousand.

 

[quickanswers]

Hi,


We expect this from you in first place.

No information about technology, no precision, no filter targets.

one number with 2 digit, the other with 12 digits precision ...

It´s neither complete nor consistent informations.

Before we can give valueable answers we need an almost complete information about your requirements and specifications.

Klaus

Well the thing is that, I'm making this for a fabrication project purpose. Whole circuit is CMOS based and using C, not using R. For capacitor, 2pf would be big in size when fabricated. So it's special instruction not to take size bigger than that. That's why I specified 2pf capacitor and for that I'll need a much higher resistance which is not achievable for CMOS based resistance.

Technology node is 180nm SCL.
I'll need a high roll off factor for that too, because it will be used for accepting frequency in the range from 0.5Hz to 15Hz only.

That's why my question was how to achieve that high value of R using only CMOS? I've gone through gm-C filters and that is used for low frequency purpose. So that's why I've a challenge to calculate Gm(Transconductance) for whole circuit. And Gm = 1/Rout, so I'll get my high resistance value from there. The lower the Gm, the more the resistance and lower Gm can be used for lower frequency. I hope now I'm able to give some information about what I need.


Thanks.

 

[BradtheRad]

From an electrical viewpoint your setup is near the realm of static charge. The resistance (5 Gohm) can come from high voltage yet low Amperes. To take readings requires methods on a different principle than our normal instruments. Calibrating may require rarely used methods and specialized knowledge. Even after calibration your instruments may be inaccurate and lose calibration easily.

100M and 200M resistors exist. You might be able to string together enough of them to make 5 Gohm. Or derive 5 Gohm from semiconductors. In any case your voltmeter(?) must not load the output meaning its own resistance should be many times 5 Gohms. Otherwise your 2pF capacitance won't yield a rolloff curve within your specified band of .5 to 15 Hz.

 

[barry]

Well the thing is that, I'm making this for a fabrication project purpose. Whole circuit is CMOS based and using C, not using R. For capacitor, 2pf would be big in size when fabricated. So it's special instruction not to take size bigger than that. That's why I specified 2pf capacitor and for that I'll need a much higher resistance which is not achievable for CMOS based resistance.

Technology node is 180nm SCL.
I'll need a high roll off factor for that too, because it will be used for accepting frequency in the range from 0.5Hz to 15Hz only.

That's why my question was how to achieve that high value of R using only CMOS? I've gone through gm-C filters and that is used for low frequency purpose. So that's why I've a challenge to calculate Gm(Transconductance) for whole circuit. And Gm = 1/Rout, so I'll get my high resistance value from there. The lower the Gm, the more the resistance and lower Gm can be used for lower frequency. I hope now I'm able to give some information about what I need.


Thanks.

you still don’t give enough information. and a low pass filter doesn’t have a lower frequency spec (“0.5 to 15 Hz”); that would be a band pass filter. i think maybe you need to learn more about filters.

 

[KlausST]

Technology node is 180nm SCL.

So this is IC design (silicon level design) --> then this should be moved to the IC design section.
(currently you posted on the PCB level design section)

and using C, not using R

You will need both.

***

A filter is defined by
* pass frequency (and accuracy)
* passband ripple
* stop frequency (and accuracy)
* attenuation at stop frequency
* order of filter (may be determined by the values above)
* filter characteristic (determines phase shift and transition from pass to stop band)
..maybe more like input impedance, output impedance, power consumption if active, noise, ...

For that low frequency ... mayve
* some type of switched capacitor filter, or
* an ADC followed by a digital filter
is more suitable.

Without true filter requirements ...
including informations about the signal source
and the goal of the signal processing
... we can not give good assistance.

Klaus