I have a PCB design that I need to add a low pass filter. However I can only add components in parallel. It is going to be very difficult to cut the tracks or add components in series. Is there any way to create the low pass filter or similar filter ? The frequency of noise I have to reduce is between approx. 5KHz to 45KHz

I have to add a filter to the INA826A outputs as shown below

enter image description here

  • 1
    \$\begingroup\$ Show us some details about your circuit inputs. If it simply is a trace going to some IC input, it will be pretty hard implementing a filter without cutting the trace. If there already is some sort of input resistance as a dedicated part like a resistor, then that might be something to work with. \$\endgroup\$ – jusaca Mar 17 at 14:06
  • \$\begingroup\$ Capacitor in parallel? \$\endgroup\$ – user253751 Mar 17 at 14:12
  • \$\begingroup\$ What exactly is driving those inputs? If it's low impedance, or active circuitry that becomes unstable under a capacitive load, you will need to insert series elements. \$\endgroup\$ – nanofarad Mar 17 at 14:43
  • \$\begingroup\$ It's not going to happen. \$\endgroup\$ – Andy aka Mar 17 at 14:47
  • \$\begingroup\$ you would need a balanced filter to avoid corrupting the signal to the amplifiers (one cap between the input and one cap toward ground) but to be stable it needs also some kind of resistive load. Need to know what's attached to IN23 and so on. Otherwise you'll waste the money for the precision amplifiers \$\endgroup\$ – Lorenzo Marcantonio Mar 17 at 14:52

That really depends on your impedance to ground at this spot. If it's mostly resistive, you can just add a capacitor to ground. Depending on your circuit & signals you may have to limit the current through this capacitor for very high frequencies.

  • \$\begingroup\$ My frequencies are currently less than 45KHz - is this low enough to the current limit you suggested ? How do I figure out the value of the capacitor ? \$\endgroup\$ – shmueld Mar 17 at 14:43
  • \$\begingroup\$ His impedence at the ina output is that of the driving stage, he can't put a capacitor without overloading it. It needs a series resistor. Also usually noise need to be removed before the inamp \$\endgroup\$ – Lorenzo Marcantonio Mar 17 at 14:55
  • \$\begingroup\$ So its there's no way to get this low pass filter without adding a series resistor ? \$\endgroup\$ – shmueld Mar 17 at 16:09

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.