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Bandpass Filter With Single Supply, Bandpass filter with single supply

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Umesh	Posted: December 07, 2009 02:09 pm
Newbie Group: Members+ Posts: 9 Member No.: 27,543 Joined: December 07, 2009	Hi, I have created a band pass filter to filter 250kHz, 10kHz band width using multifeedback filter topology. I was trying to verify the design with simulation and was found the result to be satishfactory while using a dual power supply op-amps. After that I have choosen an op-amp with same GBP (150MHz) to operate in single power supply (with proer biasing)and and filter responce is chaged drastically. I was trying to verify through simulation only, and could not decided whether should I go to check in hardware. Every op-amp is working proper in dual supply but with sinlge suppy it is not working. Could any body help me to come up with issue? If you need some more info then I can send in mail the full circuit. Regards, Umesh

Sch3mat1c	Posted: December 07, 2009 05:20 pm
Forum Addict ++ Group: Moderators Posts: 18,194 Member No.: 73 Joined: July 24, 2002	Is it distorting? -------------------- Answering questions is a tricky subject to practice. Not due to the difficulty of formulating or locating answers, but due to the human inability of asking the right questions; a skill that, were one to possess, would put them in the "answering" category.

Umesh	Posted: December 08, 2009 04:28 am
Newbie Group: Members+ Posts: 9 Member No.: 27,543 Joined: December 07, 2009	Yes! it is distorting and my frequency responce is getting changed. Whem I am using dual supply I am getting nice frequecny responce with bandwidth of 10kHz and a gain of 20DB at 250kHz, when siumlating with single supply it started behaving like all pass filter between 10kHz - 100MHz with gain of zero DB.

Sch3mat1c	Posted: December 08, 2009 05:48 am
Forum Addict ++ Group: Moderators Posts: 18,194 Member No.: 73 Joined: July 24, 2002	Give it the same total supply voltage and change all ground connections to a virtual ground at half the supply voltage. Tim -------------------- Answering questions is a tricky subject to practice. Not due to the difficulty of formulating or locating answers, but due to the human inability of asking the right questions; a skill that, were one to possess, would put them in the "answering" category.

Umesh	Posted: December 08, 2009 06:38 am
Newbie Group: Members+ Posts: 9 Member No.: 27,543 Joined: December 07, 2009	I alreatdy biased all the ground points to VCC/2, but it was not working for me.

Dr Wattson	Posted: December 08, 2009 03:32 pm
Member Group: Trusted Members Posts: 198 Member No.: 26,161 Joined: September 12, 2009	Two questions: 1) Are you certain that you biased it correctly? 2) Did you add an input buffer to prevent the input biasing affecting the circuit?

Umesh	Posted: December 10, 2009 05:44 am
Newbie Group: Members+ Posts: 9 Member No.: 27,543 Joined: December 07, 2009	Here is the way I am biasing the filter in single supply. http://picturepush.com/public/2617934][img=http://www1.picturepush.com/photo/a/2617934/220/2617934.jpg][/url]

Sch3mat1c	Posted: December 10, 2009 06:08 am
Forum Addict ++ Group: Moderators Posts: 18,194 Member No.: 73 Joined: July 24, 2002	I don't suppose input is also referenced to 2.5V? -------------------- Answering questions is a tricky subject to practice. Not due to the difficulty of formulating or locating answers, but due to the human inability of asking the right questions; a skill that, were one to possess, would put them in the "answering" category.

GPG	Posted: December 10, 2009 11:14 am
Forum Addict ++ Group: Spamminator Taskforce Posts: 6,174 Member No.: 792 Joined: April 25, 2004	^^Vee is positive in your diagram

Umesh	Posted: December 10, 2009 12:30 pm
Newbie Group: Members+ Posts: 9 Member No.: 27,543 Joined: December 07, 2009	VCC is positive and VEE is negative. My input is 2.5V dc along with some harmonics (order 100mV) of 250Khz alond with some higher ordr haromonics. I am trying to filter out 250kHz harmonics.

Dr Wattson	Posted: December 10, 2009 02:15 pm
Member Group: Trusted Members Posts: 198 Member No.: 26,161 Joined: September 12, 2009	Why DC couple a bandpass filter? If infact the input is AC coupled it must also be buffered and biased to Vcc/2. Also, I question the connection of the resistor between the two opamp inputs, I've seen versions connected the same as you have it and versions where it connects the inverting input to ground instead. I've tried a simulation of both versions and it appears to make little difference. So the only thing left (I think) is the input and whether or not it is biased and coupled correctly. I had already posted something similar but it appears the forum database restoration nuked my post.

CWB	Posted: December 10, 2009 02:56 pm
Forum Addict ++ Group: Spamminator Taskforce Posts: 19,624 Member No.: 15,154 Joined: May 15, 2008	yeah ... there were quite a few posts that went the way of the ghost ... but still better than loosing the whole thing . -------------------- "Know how to solve every problem that has been solved" R. Feynman '88

Umesh	Posted: December 29, 2009 12:27 pm
Newbie Group: Members+ Posts: 9 Member No.: 27,543 Joined: December 07, 2009	Hi, I seems that spice model I was using for simulation was not correct. Finally I got spice model of op-amp opa300 and it was working fine with single supply. Thanks to every one for their quick responce. Regards, Umesh.

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