Goal: I am designing a low-noise, 8-pole, low pass filter to use in the output stage of a lock in amplifier. The filter needs to attenuate wide-band noise and a doubled carrier frequency in the range of 40 kHz to ~800 kHz. The higher the attenuation of the stopband is, the better.

What I have tried: I used the TI online filter design tool (https://webench.ti.com/filter-design-tool/design/7) to create a 30kHz 8-pole Sallen-Key LPF. The schematic is included below in figure 1. I designed the filter in Eagle using ultra-low noise opamps (LMH6626). The design schematic is included in figure 2, and the board layout in figure 3. I have assembled and tested the filter and am not getting the results I expected.

enter image description here Figure 1

enter image description here Figure 2

enter image description here Figure 3

The result I have achieved: I tested the circuit by feeding in a 1MHz BW noise signal and taking an FFT of the output. I found that the passband was severely attenuated in addition to the stop band. Further, there was a resonance peak in the stop band at around 250kHz.

My questions:

  1. Am I doing something wrong in the design or implementation of this filter?
  2. Why is the passband attenuated more than what is simulated? It should be a unity gain filter. Am I accidentally creating an attenuator by using the SMA connectors and cables? I thought that the opamps acted as a buffer.
  3. Is there a different topology that you would recommend for this application?
  • \$\begingroup\$ Bell 103 FSK modem work, mark/space of 1270 Hz/1070 Hz and 2225 Hz/2025 Hz, required 10th order receive and 6th order transmit filters. The receive filter wasn't fun -- as in hard -- to get right. I don't know if you imagine just ask TI and plug-and-play. Maybe so. Maybe not. But I would be feeling especially diligent given past experience. \$\endgroup\$ Jul 28 at 11:06
  • \$\begingroup\$ If you really want low noise design then you will need to change your resistor values. Some of your resistors have almost 10x (thermal) noise then your opamp... \$\endgroup\$
    – Rokta
    Jul 28 at 12:58

1 Answer 1


The LMH6626 is not unity gain stable!

enter image description here

  • \$\begingroup\$ I can't believe I missed this thank you so much! I have been using it on a different project as a voltage follower 🤦‍♂️. \$\endgroup\$
    – Joshua S.
    Jul 28 at 1:53

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