turbo wrote on Jul 8th, 2010, 4:51am:Hi,
I have the following real time situation:
Input signal: A signal with amplitude between 0 and 1V. The signal frequency is 1 (one) Hz.
2. To use a Gm-C filter: but here to have such a large time constant and keeping Capacitor size minimum (let say 100pF) the required transconductance is of the order of few pA/V which seems to be very challenging. Moreover, for operation to be ultra low power this would necessitate a bias current of few pico Amperes, almost in noise domain!
A) Cannot agree with you about gmC not being suitable: I have seen gmC filters with reasonable cap sizes and cutoffs in this range. Consider that you can use circuit tricks to reduce the effective gm of your circuit, e.g. if you make a differential pair for a gm stage, then you can cross-couple the outputs to achieve a cancellation and reduction in gm. I think that a factor of 10 reduction on gm ought to be possible.
You wouldn't need pA of bias current, only some 10-100 nA or so which you can achieve with suitable current source types.
There are surely other tricks to lower gm as well. Keep in mind that you use a very small W/L. At lower current levels, you need to fight against slipping into weak inversion as gm/Id of a transistor is maximized when entering into weak inversion.
B) Consider using a series of MOS switches as R for your RC. This also works, provided that the MOS switches are biased weakly. This may however be a bit nonlinear.
Vivek