I found the following BNC input output box for two channels(left side inputs, right side outputs):

enter image description here

The grounds of the BNCs are connected via the metallic case. I checked this with multi-meter continuity test.

Then I read the cap and resistor values and drew and simulated this interface for one channel as follows:

enter image description here

Could this be just a low pass filter? Anything related to common-mode? What are these 10nF caps?

  • \$\begingroup\$ You've fully defined it with your investigations - it is what it is. \$\endgroup\$
    – Andy aka
    Feb 26, 2018 at 13:28
  • \$\begingroup\$ Thanks so nothing to do with common mode? And why not just R1 C1 and what could be the purposes of 10nF caps? \$\endgroup\$
    – user1245
    Feb 26, 2018 at 13:29
  • \$\begingroup\$ The 2.2uF Works well at 1 kHz. For much higher frequencies, the 10nF will be more effective. If you were to further model the caps with their L and ESR you might see another break a few decades out. \$\endgroup\$
    – AlmostDone
    Feb 26, 2018 at 13:48
  • \$\begingroup\$ As far as common mod filtering, if the two inputs and two outputs are used as as a single differential network, then yes, it will act as a common mode filter as well. \$\endgroup\$
    – AlmostDone
    Feb 26, 2018 at 13:53
  • \$\begingroup\$ @AlmostDone Pardon me, I didnt get your last comment. What do you mean by "if the two inputs and two outputs are used as as a single differential network" ? \$\endgroup\$
    – user1245
    Feb 26, 2018 at 13:56

1 Answer 1


It is a low pass filter.

C1 does the low frequency filtering, together with R1, making a time constant of 220uS, which gives a -3dB frequency of about 720 Hz, which agrees with your graph.

At much higher frequencies, the residual inductance of C1, due to its leads and large physical size, stop the filter working, and the stopband rises again. If a filter based on C1 alone was to be used to stop RF pickup by an amplifier, it may not work too well.

The 10nF capacitors are intended to suppress this higher frequency transmission. Being a lower value and having shorter leads, they have less residual series inductance, and so filter to higher frequencies. C2 works well for this purpose.

Unfortunately C3 does not work well, as it is in shunt with C1. At some frequency where C1 has gone inductive, C3 will form a parallel tuned circuit with the inductance of C2, actually enhancing the transmission of this frequency through the filter.

Fortunately, people are rarely bitten by this mistake. In this filter, C2 may well provide enough attenuation that the C1/C3 resonance does not cause an actual problem. Or it may be used in an environment where there are no problem transmitters at the specific resonance frequency. When the large capacitor is a lossy electrolytic, it will rarely have sufficient Q to form a resonance.

All that would be needed to eliminate the C1/C3 problem is a resistor of a few ohms between C2 and C3 to 'de-Q' their resonance. Ferrite beads are often used for this purpose when the voltage drop of an extra resistor cannot be tolerated in (for instance) power supply filtering.

  • \$\begingroup\$ Very interesting details. Never can find such information in texts. Is residual inductance of a cap inductance due to its leads? How is that modeled? \$\endgroup\$
    – user1245
    Feb 26, 2018 at 14:39
  • \$\begingroup\$ Due to leads and the internal construction of the cap. It's modelled as an inductor in series. Some capacitors have this information implicitly in their data sheets as SRF (series resonant frequency), or as a graph of impedance versus frequency, which dips to a minimum at the SRF and then rises again like an inductor. \$\endgroup\$
    – Neil_UK
    Feb 26, 2018 at 14:47
  • \$\begingroup\$ Thanks a lot. One last question if you dont mind. Im wondering the grounds are connected through the metal case/box(metal case conducting all BNC grounds). Can that be on purpose? I would for instance use a plastic box and directly solder the grounds of the BNC input ground and BNC output ground, not through the metal case. \$\endgroup\$
    – user1245
    Feb 26, 2018 at 14:55

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