Very low noise high pass filter design

filternoisenoisefloor

We are trying to design a high pass filter, which mainly goals are:

  • very low noise. Low noise is the first concern.
  • suppresses 1kHz by around 50dB and passes 2kHz.
  • Gets rid of DC
  • Dont care on other frequencies.

Description of our project:

We are to measure a very weak 2kHz signal in accompany with very large 1kHz signal(70dB larger than 2kHz signal) with a 16-bit ADC based Lock-in Amplifier.

Since the 2kHz signal is too weak and we need to achieve a resolution coming from a 16-bit ADC, 2kHz signal has to be amplified first. However, since 1kHz is too large, we have to filter it out first, in order to prevent saturation, before amplification.

About specifications of "how low the noises should be". The requirement is as low as possible. We are willing to try ANY method/combination/components to reduce the noises.

Here are points that I know:

passive filters:

  • Need inductors, and at low frequency it may be very hard to find a
    large and accurate one? (I do not want to make inductor by hand)
  • low noises comparing with active filters.

active filters:

  • easy to implement
  • relative larger noises…

Need your help!

Since the noises are our most important concern, could I ask for some suggestions on tradeoffs of choosing:

  • active filter based on OpAmp,
  • active filter based on BJT/MOSFET or
  • passive filter?
  • implement active filters based on IC
  • implement active filters based on switched capacitors

Could I have some general comments?

Any suggestions are appreciated.

Regards,
Richie

Best Answer

Yes, we only want to eliminate 1kHz

A notch filter can be used for this: -

enter image description here

This is a 10 kHz design and note figure 6 - how it responds as you get between 9 kHz and 11 kHz with the input signal.

This article describes the circuit in more detail and provides examples of other notch filters across the audio range. The article is by TI and is entitled: -

An audio circuit collection, Part 2

Regarding the op-amps, because you are using a lock-in amplifier it's not critical BUT just in case go for op-amps that are below 10 nV/\$\sqrt{Hz}\$ specified on noise. Devices that spring to mind are OP1177, AD8605, ADA4528 but there are plenty with lower noise.

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