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.
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.
- Am I doing something wrong in the design or implementation of this filter?
- 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.
- Is there a different topology that you would recommend for this application?