Author Topic: Sallen key filter DC biasing problem  (Read 358 times)

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Offline edgaras006

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Sallen key filter DC biasing problem
« on: February 21, 2019, 01:48:14 am »
Hello everybody,

I designed 8th order low pass filter based on sallen key topology. Tested its transfer function in multi sim and it looked great, but when I assembled it turned out that filter shift DC level of bipolar +-10V signal and halved it. So when I feed in +-10V signal I get -100mv..-10,1V signal. Despite shifted DC level, filter has a correct transfer function, so I am a little lost. What have done wrong?

Thanks in advance.
 

Offline Benta

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Re: Sallen key filter DC biasing problem
« Reply #1 on: February 21, 2019, 04:03:03 am »
First, a comment (please take it in a positive sense): that's one of the messiest schematics I've seen for some time.

Now for your problem: what does "Input +/-10 V from low impedance source" mean? If the output of your source is AC coupled, you have no DC reference and the circuit will fail. You should never allow external sources to set the DC bias, this must be internal to your circuit.
« Last Edit: February 21, 2019, 04:05:11 am by Benta »
 

Offline edgaras006

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Re: Sallen key filter DC biasing problem
« Reply #2 on: February 21, 2019, 07:38:04 pm »
Thanks for the compliment about schematic. It would have been way nicer if I would have not displayed all real components.

A source is DC coupled differential amplifier. It seems I have soldered something wrong (possibly shorted something) because another identical PCB works just fine.
 

Offline bson

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Re: Sallen key filter DC biasing problem
« Reply #3 on: February 22, 2019, 08:36:14 am »
Circuit seems fine, and I have no difficulty understanding the schematic, so fine by me.  My #1 guess would be you did something wrong with the supplies, like grounded a -12V supply pin.  Second guess would be you accidentally swapped a cap and a resistor.
 

Offline Audioguru

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Re: Sallen key filter DC biasing problem
« Reply #4 on: February 22, 2019, 02:24:45 pm »
I tidied up part of the schematic:
 

Offline edgaras006

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Re: Sallen key filter DC biasing problem
« Reply #5 on: February 25, 2019, 04:38:03 am »
It turned out to be damaged opamp. I replaced it and the circuit started up just fine. It looked like as if input transistor gate was damaged by ESD (even if I had ESD wrist strap and was working on ESD mat).

Thank you, everybody, for suggestions and advises. Audioguru did a really nice job, thanks.
 


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