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

[Ayush]

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.

[Coq_Rouge]

You have done something wrong. You should read the manual for both the SCB-68 and your DAQ card once more. In such cases I often use MAX, and a battery as source. Then a DAQ input is not connected (floating) you will see measurements like you are doing now. Some high level voltage often modulated with mains frequency.

[Ayush]

 I am using the receiver in ac coupled differential mode and the transmitter in referenced single ended mode. Is that a problem? What I don't understand is that the circuit is working for lower frequencies but not for higher ones.

[Henrik_Volkers]

What is your sampling rate?

Your amplitude modulation might indicate that you passed the nyquist freq. 🙂  

Even at 100kS/s you will see that kind of result. 

Have you provided a path for the bias current?

Message Edited by Henrik Volkers on 06-01-2010 04:00 PM

[johnsold]

The gain bandwidth product for a TL081 is 3 MHz.  At 40 kHz and a gain of 12.5 you have 0.5 MHz.  This leaves only a factor of six for the feedback network.  The open loop phase shift is about 90 degrees.  I suspect that you have created a fine oscillator.

If you need a filter at 40 kHz, use a faster amplifier or less gain.  If you need to use the TL081, build the filter with unity gain, then amplify the filtered results, possibly using two stages of amplification, each with a gain of about 3.5, rather than trying to do all the gain in one stage.

Lynn 

[Coq_Rouge]

I know this is not a electronic forum. But I think you should post a schematic of the electronics. The same also goes for your code.

[Ayush]

I made the changes that you had suggested. The gain is down to 0.6 but the problem still remains the same - it works at 20 kHz but not at 40. Also, at higher frequencies, I noticed that the output has only a frequency of 26 kHz and its harmonics. There is no sign of the input signal. I experimented a bit and found that the circuit works well below 26 kHz but not above it. Any ideas as to what may be causing this?

[Henrik_Volkers]

It's a well known fact that amplifiers oscillate, while oscillators don't 😉

It's OT but post the shematic and a pic of your board and the analog addicted here will take a look.

Have you buffered the OP power supply? 

Message Edited by Henrik Volkers on 06-02-2010 12:45 PM

[Ayush]

http://picasaweb.google.com/lh/sredir?uname=ayushlko&target=PHOTO&id=5478151867854637586&aid=5478151...

Sorry about this round about way of posting the schematic but this was really the only way I had.

[johnsold]

You really did not mean that R3 and R6 were 6 ohms, did you?

According to my filter book (Williams, Electronic Filter Design Handbook), the open loop gain should be well in excess of 2*Q^2 at the resonant frequency.  Since you specified a bandwidth of 2 kHz and a resonant frequency of 40 kHz, Q = 20.  The open loop bandwidth of the TL081 at 40 kHz is about 100.  This is probably marginal excess gain.

Ar is gain at resonant frequency.  Ar' is desired gain at resonant frequency. 

R2 = Q/(pi*f*C) = 58 kilohms.

Ar = 2*Q^2 = 800  !!! This is greater than the open loop gain of the op amp!!!

R1 = R2/(2*Ar) =  36 ohms

R3 = (R2/2)/(2*Q^2 - Ar') =  28 kilohms (for Ar' =1)

The specifications you gave are not compatible with this circuit at 40 kHz using the TL081.

Lynn