The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Analog Design >> low voltage switched capacitor filter
https://designers-guide.org/forum/YaBB.pl?num=1241092997

Message started by manodipan on Apr 30^th, 2009, 5:03am

Title: low voltage switched capacitor filter
Post by manodipan on Apr 30^th, 2009, 5:03am

Hi guys,
Actually i am designing some low voltage SC filter.....so the sampling stage is stray insensitive bottom plate sampling...my question is that i am simulating by connecting voltage source directly to the sampling stage......do i need to put some buffers also pin parasitics will also come into picture....so i want clarification s on that as otherwise i have to use low voltage opamp in unity gain configuration....thanks a lot...

Title: Re: low voltage switched capacitor filter
Post by thechopper on May 7^th, 2009, 7:20pm

Hi manodipan,

Whether you need a buffer before the sampling stage or not will depend on the actual output impedance of the circuit driving your sampling stage. IF such impendace is not too large then as long as you guarantee that during the sampling time the signal will fully charge your sampling cap then it should be ok.
Otherwise you might need to use some sort of buffering.

Hope this helps
Tosei

Title: Re: low voltage switched capacitor filter
Post by raja.cedt on May 8^th, 2009, 1:23am

hi manodipan,
i don't see any advantage of putting buffer,in fact it will add some noise and power dissipation at the same time the bandwidth should be grater than your sampling frequency and you may not sample with sampling frequency what you want to be.As you said it is low voltage then designing op amp also big challenge. As an experiment you can use an VCVS and op amp buffer ..you will see increased integrated noise in case of op amp buffer.

Thanks,
Rajasekhar.

Title: Re: low voltage switched capacitor filter
Post by Berti on May 8^th, 2009, 3:51am

Rajasekhar,

I don't understand your comment:

Quote:

i don't see any advantage of putting buffer,...

As Tosei wrote, I also think that it depends on the driving circuit whether you need an additional buffer or not.

If it is a prototype circuit (stand-alone test circuit), I would recommend to take a look at the bondwire inductance and parasitic capacitances to make sure that the ringing decays fast enough.

Cheers