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TIPD122: TIPD122 0.1Hz to 10Hz Noise Filter performance issue

Part Number: TIPD122

I have fabricated a 0.1Hz to 10Hz noise filter based on reference design TIPD122 for μV amplifier circuit testing. I am have problems with noise in the high-pass filter stage limiting the over device performance. When jumper JP7 is installed the high-pass filter input is grounded and I get at 45mVpp ripple at Vout. The complete filter has 100x gain for an input referred noise of 450μVpp! When I probe R9 at the HPF output I get a waveform exactly 1/10 the output so the source of the problem must be the HPF stage which is giving me a roughly 4.5mVpp output with the input grounded. This ripple is highly succeptible to mechanical stress on the board and even a light tough (no electrical contact) produces 200mV swings at the output. Can anyone advise me on tracking down this problem and understanding why this filter is so sensitive?

  • MikeH8

    Because of the high gain involved this circuit will be quite sensitive to leakage currents and external noise sources.

    To avoid leakage currents, make sure the pcb is thoroughly cleaned with a solvent appropriate to the solder flux used. If organic/water clean flux is used, the pcb should also be baked at 70-75 degrees C for 20 to 30 minutes to remove any residual moisture.

    External noise interference can be reduced using the setup described in section 6.1 of the Design Guide.

    Regards
    Dennis

    If this answered your question please click the green "This Resolved My Issue" button.
  • Thanks Dennis, I have cleaned as best I can and it did not made significant improvement. What really did the trick was replacing the two 1μF caps at the HPF input with new chips. This brought down the output from 45mVpp to 5mVpp and the sensitivity to stress and heat is much improved too. This corresponds to a HPF input noise of 50μV which now extends the sensitivity at 100000x down to 50nV. I'm still concerned about these capacitors so I will try replacing them with "soft termination" caps and see if it reduces the mechanical noise, I've also been testing in a faraday cage rather than a sealed box so I can probably improve the thermal stability. I have actually made some pretty good measurements consistent with datasheet specs for 0.1Hz to 10Hz noise down to 120nVpp for the OP37.