# Should Passive LPF/HPF Component Selection Depend on Ability of Input to Source Current?

When choosing R and C for a passive LPF or HPF, should you take into account the input's ability to source current? I'm currently working on a passive HPF for an input that can supply up to 10 mA @ 3.3 V. The input will be PWM. Should this be a factor in choosing R, for example?

Side note: The output of the HPF will go through an op amp buffer.

Not really. It's the load impedance as a whole that matters. For example:

Network 1 has a DC input impedance of 10.1k and network2 has a DC input impedance of 11k. For your 3.3v input that makes a difference of $$\i=\frac{3.3v}{10.1k} = 326uA\$$ for network 1 and $$\i=\frac{3.3v}{11k} = 300uA\$$ for network 2.

So if your load impedance is sized correctly, then the LPF impedance will have minimal impact on current draw.

• The load will be high impedance as the output of the filter will go to an op amp buffer. – HumbleOne Nov 27 '19 at 17:04
• @HumbleOne That's even better, as an opamp input impedance can be set pretty high. – Aaron Nov 27 '19 at 17:20

Within the pass-band, the filter will not have much effect on how much current the source has to provide. It will essentially just connect the source to the load, and the load will determine the current.

In the stop-band(s) it might matter. If your high-pass filter provides a DC short to ground, for example, that could alter the operating point of the source, even if it doesn't affect the current required at the operating point. Generally it's not hard to design your filter to avoid this problem, but you must be aware of it and design for it.

• Why would there be a DC short to ground? – HumbleOne Nov 27 '19 at 17:07
• For example if you connected an inductor from the signal line to ground. – The Photon Nov 27 '19 at 17:26