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UAF42 60Hz Notch Filter Creating 2kHz Noise

Helmut Forren
Mastermind 6805 points
Other Parts Discussed in Thread: UAF42

I'm using the UAF42AU in a configuration matching sbfa012.pdf to make a 60Hz notch filter.  I've put it between three amplifiers in series.  My original signal is very small, so I need these three amplifiers to get to where I need to be.  However, 60Hz noise is creeping in, that doesn't respond to shielding because my gain is so high at the first stage (200dB).

The first notch filter, after the first amplifier, successfully removes the 60Hz.  However, it seems to INTRODUCE new noise at around 2kHz with magnitude around 4Vp-p.  While this noise is subsequently squelched by an RC filter, it's large presence at a middle stage reduces the maximum gain I can apply at that stage.  And I can't let the noise get clipped, or else the statistics of the desired signal get adversely effected.

So why might this 2kHz noise (and that is an approximate frequency) being created?  How can I prevent it from being created?

Note that my subsequent stage at signal "V60PAout" goes into an RC filter, with R = 1K and C = 1nF and 1uF in parallel to ground.

  • 0
    TI__Guru* 93105 points

    Hello Helmet,

    Is the 2 kHz noise still present when the V60PAout input is disconnected from the 200 dB gain amplifier, and then grounded? The UAF42 is intended to be driven by a source impedance which is ideally zero Ohms. I am wondering if that if the source impedance represented by the filter and preceding amplifier stage isn't involved in some way.

    Can you share the circuit that precedes the UAF42 notch filter? It would seem that you would need a balanced input amplifier circuit that has good CMRR to reject the 60 Hz noise. A gain of 200 dB is enormous and running it single-ended may make it very difficult to keep 60 Hz out.

    Regards, Thomas

    PA - Linear Applications Engineering