I have designed a fourth order bandpass filter using the opamps TL081C. It is designed to operate at a center frequency of 40 kHz with a bandwidth of 2 kHz and a gain of -12.5 . I am using an SCB-68 board for data transmission and reception, one channel each for transmitter and receiver. I have grounded the adjacent channels to both the transmitter and the receiver in order to avoid crosstalk. However, at the output, I get a signal which looks like an amplitude modulated signal and is, most probably, noise. The amplitude of this signal is always around 9V, no matter what is the amplitude of the input (even if the input is 0V, the result is the same). If I scale down the whole BPF to operate at a center frequency of around 20 kHz, everything works fine. Therefore, I think the problem lies not in the design of the BPF but the use of the board at higher frequencies. Please reply as soon as possible.

LabVIEW

bandpass filter amplifies only noise at high frequencies and saturates the opamp output

Free tool to play with:

http://www.national.com/analog/amplifiers/webench_filters

will end in an circuit including 'reasonable' values.

And don't get frustrated ... what you are currently learning by (*DOING!*) practice is rare nowadays.

Greetings from Germany
Henrik

LV since v3.1

“ground” is a convenient fantasy

'˙˙˙˙uıɐƃɐ lɐıp puɐ °06 ǝuoɥd ɹnoʎ uɹnʇ ǝsɐǝld 'ʎɹɐuıƃɐɯı sı pǝlɐıp ǝʌɐɥ noʎ ɹǝqɯnu ǝɥʇ'

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LabVIEW

bandpass filter amplifies only noise at high frequencies and saturates the opamp output

Re: bandpass filter amplifies only noise at high frequencies and saturates the opamp output