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Need suggestion on demanding design on bandpass filter/ Notch filter with gain

Other Parts Discussed in Thread: OPA2131, OPA131, OPA4131

We are not very experienced on designing analog circuits. And we are trying to design a bandpass filter or a notch filter. Could I ask for some comments on the filter choices?

Description of our project

There are signals of DC, 1kHz, 2kHz, 3kHz, 4kHz, 5kHz, 6kHz and 7kHz. DC has similar value comparing with the amplitude of 1kHz. However, 2kHz, 3kHz, 4kHz, 5kHz, 6kHz and 7kHz are 40dB smaller than 1k.

Our target is to measure 2kHz signal with a lock-in Amplifier based on a 16-bit ADC. The lock-in amplifier is an imperfect one, which measures the amplitude of the sum of 2kHz, 4kHz and 6kHz instead of pure 2kHz. Therefore, the 2kHz is to be

1) extracted from harmonics of 2kHz(avoid influence on Lock-in Amplifier) and 1kHz,DC(avoid saturation)

2) amplified by 40dB(measured by a 16-bit ADC).

Where do the signals come from:

We are trying to measure the linearity of a system by injecting 1kHz signal into the system and observing the output. The output composes, as mentioned above, 1kHz signal, and harmonics of 1kHz signal which are 2kHz signal, 3kHz signal, 4kHz signal and etc. which are caused by distortion of the system.

There is another source of the 3kHz signal. Since 1kHz wave is produced by square wave going through a 4th order low-pass filter, there is 3kHz harmonics produced as well. The 3kHz harmonic then goes into the system and appears at the output of the system.

Proposed solutions:

Here are our two choices:

  • Use a notch filter(-40dB to -50dB) to get rid of DC and 1kHz and then amplify the signal left by 40dB. Then a fourth order lowpass filter will be used to reject harmonics of 2kHz. (with passband of 2kHz, 4th order passive lowpass filter is enough for us)

  • Use a bandpass filter with center frequency at 2kHz, which has -40dB suppression at 1kHz. The bandpass filter will reject all unwanted signal and an amplifier of 40dB will be added afterwards.

Need comments

Our most important concern is noises, and there is no specification on how low the noises should be, as we are researching on the possibility to reduce noises as much as possible...

  • Could I ask for some comments on the choices of previous designs?

  • And is it possible to design Notch filter with gain of 45dB? Or a bandpass filter with gain of 45dB? We are thinking to put gain in the filter to reduce noises as much as possible.

  • Will gain on Notch filter/ Bandpass filer cause any stability issue?

Any comments on any point will be really appreciated. Thanks for reading.

Best regards, Richie