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I'm trying to design a 40dB active bandpass filter with a centre frequency of 130KHz and a passband of 80KHz. I don't really know much about filter design so I used the TI filter design tool to design one. I wanted to power the design using a single 3S lipo battery so I added a rail splitter to the design. And since the output of the filter was going to a single Ended ADC, I added a summing amplifier at the end to bias the signal by 1.65V. Here's what my final design looks like.

enter image description here

I designed this on a stripboard and the circuit was working so I decided to make a PCB for it. My isue currently is my PCB design is extremely noisy. I grounded the input to record the self-noise of the pre-amp and I get the following waveform (amplitude is about 600mV). enter image description here I took the FFT of the signal and it seems to be at around 235KHz or so. (centre is 130KHz, 50KHz division).

enter image description here

If I add a sine wave to the input of the board, I get the following output: (Input is a 130kHz 0.01Vpp signal) enter image description here And the FFT of the signal looks like this: enter image description here

The goal is to eventually design a low-noise preamp and I'll be connecting a hydrophone to it. However, right now, I'm a bit stumped as to what the problem could be. Simulation works fine, stripboard design is fine as well.

I've used 0603 package RC components but used Aluminium SMDs for the electrolytic caps. My plan is to eventually switch to tantalums in the second revision. Could anyone possibly see an issue with this?

For completeness, this is what the PCB looks like: enter image description here

I'm still waiting on the voltage regulator but I'm currently probing from the output of the filter.

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  • \$\begingroup\$ Your 'noise' look more like an oscillation. Isolate the source by connecting R1 to ground instead of U1 output. Where are the power supply decoupling capacitors for the op amps? The 3.3V regulator is connected to the virtual ground, which can only source 20mA. So why are you using a 3A regulator? \$\endgroup\$ – Bruce Abbott Dec 12 '19 at 8:06
  • \$\begingroup\$ @Bruce Abbott, I assumed with the 220uf and 1uF present at the virtual ground, I wouldn't need decoupling capacitors. Regarding the regulator, the circuit it's powering draws about 17mA so I thought it'd be fine . I probably need to rethink the design regarding that \$\endgroup\$ – MaskedAfrican Dec 12 '19 at 21:48
  • \$\begingroup\$ Just a comment, but you should really work on component placement. Everything should be grouped functionally. Keep associated components as close to their ICs as possible. Add decoupling capacitors to the supply rails of op amps, place them physically close to the ICs they decouple. \$\endgroup\$ – Captainj2001 Jan 7 '20 at 15:42
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Issue was fixed by changing power supplies. I had the Pre-Amp connected to a bench power-supply which seemed to be the issue. I swapped those out for batteries and the issue seemed to be fixed

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