Design of tunable cutoff frequency filter

[Junus2012]

Dear friends,

I am trying to design programmable or tunable fc for my LPF. For this puspose I need to have tunable resistor and I will leave the capacitors constans.

Toward this, a MOS transistor is an option, but I believe it will kill the concept of using Active RC filter rather than the gm-c filter because it will result in a comparable non-linearity, especially talking about large signals. While I have seen plenty of papers by using the MOS as a variable resistor but most of those people are processing week bio-signals.

I have in the used technology a very high sheet resistance (HRIS) with size even can be made smaller than equivalent MOS resistor. Here is my question, if such resistor is available why people not using it frequently? most of them they use poly2 resistor which occupy large die area.

would it be better to have tunable HRES bank than using MOS resistor?

Thank you very much

 

[sutapanaki]

People usually use bank of resistors and/or capacitors to tune the frequency. Of course, this is coupled with appropriate tuner, for example in the form of a VCO, just one of the possible approaches.

 

[BigBoss]

Using a MOS Transistor at the place of resistor is not a practical application in IC circuit except some particular systems.
Adjustable Filters are realized by either either Cap bank or Res Bank or both. Because Drain-Source Resistance of a MOS Transistor is not linear and unwanted Filter Response will eventually be appeared. ( Process Variations and Mismatch are more dominant on MOS Transistors rather than Capacitors and Resistors ) and the Temperature on MOS Transistors' Specifications can be dragged down.

 

[crutschow]

What is the filter operating frequency?

 

[dick_freebird]

MOS "resistors" have lousy linearity except when
hard-switched either "on" or "off".

People have been doing digital step attenuators
and filters using FDSOI switches and segmented
resistors / capacitors in RF applications. A question
is, can you tolerate discrete steps or must the
filter be continuously tunable? Often a step-size
smaller than the production tolerance of the LSB
could be called "good enough" provided that the
control scheme is compatible (like, if you were of
a mind to make a voltage controlled center
frequency, but the bigger picture includes a DAC
to make the tune-voltage from some digital brain,
then skip the middle-man and just feed digital
tune word to a digital capacitor, digital resistor,
whatever, straight up.

 

[Junus2012]

Dear friends,

Thank you for your contribution, for sure no linearity is comparable to the real resistor like poly resistor,

However, authors are justifying the use of non-linear MOS resistor in fully differential circuit where second order distortion can be canceled.

In the same time a lot of work have been presented to linearize the MOS resistor, as an example the one below

**broken link removed**

or this one

A 1-V 450-nW fully integrated biomedical sensor interface system

This paper presents a 1 V 450 nW fully integrated bio-signal acquisition IC in 0.35 μm CMOS technology which includes a tunable band-pass filter, a variable gain amplifier, and a 12-bit ADC. The ultra-low power is achieved by using an energy-efficient system architecture and a novel tunable...

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