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Maxim Integrated - Integration Nation
Steve Taranovich

Filter Techniques in Circuit Integration

Steve Taranovich
steve.taranovich
steve.taranovich
7/31/2013 12:38:32 AM
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Re: Re : Filter Techniques in Circuit Integration
Another nice technique for integrated electronics---thanks SunitaT---the other advantage of active-RC filters is to decrease the value of the filter's capacitors

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SunitaT0
SunitaT0
7/30/2013 11:56:24 PM
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Re : Filter Techniques in Circuit Integration
In the design of frequency-selective integrated circuits, the lack of inductors is a serious drawback. In many apps, this drawback can be overcome by using conventional active-RC filter methods. In designing active-RC filters, one uses a combination of resistors, capacitors, and gain blocks in linear feedback loops to get the desired frequency selectivity without the need for inductors.

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B_Albing
B_Albing
7/27/2013 12:20:09 AM
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Re: Some minor issues
@KCP - thanks - a good article to read for reference.

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kendallcp
kendallcp
7/26/2013 2:03:34 PM
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Newbie
Re: Some minor issues
Hi Jeff

Floating inductors (and other higher order elements) have for decades been the bane of filter designers trying to migrate from tried and tested LC network design into something more active, whether for integration or just to avoid baseball-sized limps of ferromagnetic material.

When asked (which is, for such an esoteric subject, quite infrequently), I usually counsel to avoid any of the active floating element techniques.  They present a lot of matching and common mode problems.  There are two main ways to achieve this, facets of the same bigger network synthesis picture:

1  Use only grounded inductors - they are easy to synthesise.  Of course, chip designers like grounded capacitors too.  Something has to give way, especially if you've got series-resonant circuits...

2  Transform your design so that a floating inductor becomes a floating resistor (or even a floating capacitor, that's rare though).  That's the basis of Bruton-transformed FDNR filters, one of the most elegant conceits in the entire active filter canon.  I wrote about this a while back:

http://www.analog-eetimes.com/en/bruton-charisma-make-those-inductors-vanish-using-savvy-scaling.html?cmp_id=71&news_id=222901145

 

 

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Netcrawl
Netcrawl
7/24/2013 8:14:57 PM
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Re: MOSFET-C
Yes I read it wonderful stuff, its contained detailed information of VLSI works, the filters are being discussed effectively, good sources of reference. 

 

 

 

 

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steve.taranovich
steve.taranovich
7/24/2013 3:23:05 PM
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Re: Some minor issues
Hi Jeff---excellent questions:

1. In a parallel circuit, the smaller quantity (reactance) has the greater effect on the total current. Think of the example of resistances in parallel, the smaller reactance draws more current and has a greater effect on the total Z. An option is to keep the "inductive" reactance high enough in the series RLC combinations, so that its effect would be negligible.

However, in cascading these types of parallel filter architectures, it might be better to change the filter topology---you can still simulate an inductor in any other configuration. Check out the references at the end of this blog for more details.

2. Actual implementaions show 600 MHz to 3.8 GHz tuning range with a noise figure of 5---see reference [1] at the end of the article.

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B_Albing
B_Albing
7/24/2013 3:04:40 PM
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Re: MOSFET-C
@Scott - I assume this is much like the way we used to use JFETs as voltage variable resistors. Of course, they were depletion mode devices, so we needed a negative gate voltage. But otherwise, it seems similar - right down to the pesky problem of the need to characterize channel resitance vs. gate voltage so you could get predictable performance.

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JeffL_2
JeffL_2
7/24/2013 2:59:35 PM
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Newbie
Some minor issues
1. Your figure 2(a) seems to suggest that it emulates a grounded inductor. You mentioned that this could be used (among other things) to make an analog BPF. In many BPF implementations a substantial fraction of the inductors would NOT be in a grounded configuration (series LC section for ezample). Does a "differential" connection (effectively 2 Figure 2(a)s in tandem) work OK or would capacitive imbalances create problems? If there are problems is it necessary to change the filter topology or is there another configuration that will properly synthesize an ungrounded inductor?

2. To roughly what RF frequency are these techniques considered practical and can you give instances where they have been put into production at those frequencies?

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steve.taranovich
steve.taranovich
7/23/2013 4:45:27 PM
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Re: MOSFET-C
Good reference Scott. I've seen Tsividis work in VLSI on the IEEE XPlore site for MOSFET-C filters---good stuff

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Scott Elder
Scott Elder
7/23/2013 4:41:07 PM
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MOSFET-C
Yannis Tsividis has published alot of work on MOSFET-C filters.  This is where the MOSFETs are tuned as a resistor by driving the gate voltage.  I built one many years ago for a multi-megahertz bandpass filter inside the read channel of a hard disk drive.

Nowadays it seems that the objective is to move the signal into the digital domain as soon as possible (i.e. ADC) and solve the rest of the problem using DSP.  A simple anti-alias filter in front of the ADC is all that is required.  

 

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