Hi harpoon, I was absent for 8 days, therefore my late response.

I think, you shouldn`t rule out RC opamp filters from the beginning.
Based on a filter design program I arrived at a maximum capacitance of 3.5 µF and Rmax=10 kOhms.
The filter topology is based on integrators (two opamps for 2nd order).
Single-opamp topologies are possible but require larger capacitances (150 µF).

Hi buddypoor,

I should really have said that I am looking for an on-chip solution.

3.5uF cap on a chip is really too big ! For a 2fF/um^2 MIM cap, this equates to a cap area of ~42mm x 42mm ... much bigger than a standard wafer reticle size !

The filter is there to filter low f noise (drifts) and DC offsets, I was actually thinking of putting an amplifier in there as well to reduce the noise effect of the ADC downstream ... so pretty much have to be before the ADC.

So you can see where I am going with this ...

So far from literature searches ...

gm-c : Problematic generating a low enough gm (consistently over PVT) or a big enough C

switched-c : have to contend with very big C ratios

interesting real world challenge ! ...  ;)

here are my first order back of envelope calc ...

at 0.2 Hz, the time constant needed is ~0.8s

tau = Cf/Cin * 1/fclk

fclk needs to be high-ish, at least an order higher than the 100Hz bandpass upper corner, say it is 2kHz

this means Cf/Cin is roughly 1600 !

if Cin is ~100fF, Cf needs to be ~160pF ... which is big on chip (but maybe not that unreasonable).

Maybe I should re-visit my cap-multiplier circuit again for this ...

hi buddypoor,

thanks for your comments ...

Lex wrote on Aug 21^st, 2013, 11:44pm:


I am afraid not ... I am looking at an implementation suggested in this paper ...
http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=34666

looks promising ...

harpoon wrote on Aug 22^nd, 2013, 5:45am:


It is a bandpass with a mid frequency of app. 4.5 Hz.
Corner frequencies (3 dB) at 0.2 Hz and 100 Hz.
Topology: Tow-Thomas (2 integrators) - that means: 2 capacitors.
The given values (10k, 3.5µF) are correct.

Please note that sometimes there is a confusion regarding the order of a bandpass.
Lowest possible order is n=2 (Denominator polynominal of 2nd order).
Some authors/programs claim that this would be first  order (n=1).
However, this is not correct: The order is determined by the denominator.

For a higher order (n=4 with 4 capacitors) I get: Rmax=10k and Cmax=79 µF