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> Bandpass Filter With Single Supply, Bandpass filter with single supply
Umesh
Posted: December 07, 2009 02:09 pm
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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
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Sch3mat1c
Posted: December 07, 2009 05:20 pm
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Is it distorting?


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Umesh
Posted: December 08, 2009 04:28 am
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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.
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Sch3mat1c
Posted: December 08, 2009 05:48 am
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Give it the same total supply voltage and change all ground connections to a virtual ground at half the supply voltage.

Tim


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Umesh
Posted: December 08, 2009 06:38 am
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I alreatdy biased all the ground points to VCC/2, but it was not working for me.
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Dr Wattson
Posted: December 08, 2009 03:32 pm
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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?
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Umesh
Posted: December 10, 2009 05:44 am
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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]
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Sch3mat1c
Posted: December 10, 2009 06:08 am
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I don't suppose input is also referenced to 2.5V?


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GPG
Posted: December 10, 2009 11:14 am
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^^Vee is positive in your diagram
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Umesh
Posted: December 10, 2009 12:30 pm
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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.
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Dr Wattson
Posted: December 10, 2009 02:15 pm
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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.
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CWB
Posted: December 10, 2009 02:56 pm
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yeah ... there were quite a few posts that went the way of the ghost ...
but still better than loosing the whole thing .


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Umesh
Posted: December 29, 2009 12:27 pm
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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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