# A question on sizing a blocking capacitor

When I was trying to find an answer for an optimum capacitor value for entire DC removal of a signal I encountered the following:

In general, DC blocking capacitor shall behave like a short at working frequency. Calculate the reactance in ohms of the DC blocking capacitor for a minimum value at your working frequency: Usually Xc(ohms) = 1/(2×3.14×f×C) shall be less than 2 ohms at working frequency.

If this is true, and lets say I want to completely remove DC of a periodic waveform which has 1kHz fundamental freq.

So if I follow that logic (setting Xc=2 Ohm), C = 1/(2×pi×f×|Xc|) and at 1kHz:

C = 1/(2×pi×1000Hz×2ohm) = 79uF

So I guess I should choose 100 uF

If now my understanding is correct, my question is what should be the upper limit and what would be an example to a bad affect of choosing overlarge cap for DC removal. Say you calculate the blocking cap 100nF but in your circuit you use 100uF. What kind of consequences can that have?

Edit: One of the reason asking this is I always see that decoupling caps around opamps are 100nF. What if I use 100uF instead. I know this is not blocking cap but similar confusion.

Whenever we need a capacitor with capacitance C, what I understand using higher value is not a problem for both decoupling and blocking caps. Is that correct? What determines the upper limits?

I do not know the circuit you are working on, but if you are putting a DC blocking capacitor in the input, then the capacitor will create a highpass filter with the input impedance of the circuit. For optimizing the value of the capacitor you need to know your input impedance.

The highpass filter will have a cut off frequency. This frequency is given by: f=1/(2*piRC) Where R is your input impedance and C is your capacitors capacitance.

Normally you want to have a cutoff frequency one decade lower than the wanted frequency (for a highpass filter). In your case, this is 100 Hz.

This circuit: Will give this response: As you see it will have very close to no attenuation at 1k Hz. This, beacuse the cutoff frequency is 100 Hz.

• Can you give an example of sizing. It would help a lot. Lets say the signal is 1kHz as in my question. And I want DC removal according to the quote my question I need at least 79uF. But since I dont want too much attenuation I need to consider attenuation affect. So if the input impedance is 100 Ohm and fc=100Hz (1kHz/10 as you say) then: C= 1/(2×pi×R×f) = 16uF. So should I choose a cap value between 16u a and 79u?? – user1245 May 30 '17 at 18:32
• An example or a possible simulation ect would help a lot. – user1245 May 30 '17 at 18:34
• If you choose a larger capacitor than 16uF, you will lower the cutoff frequency. This means that if the capacitor is atleast 16uF, you will have no atenuation at 1k. So go for it. But how did you get the 16uF, i don't see any resistance, meaning i can't calculate the capacitor value. – keffe May 30 '17 at 18:36
• I wrote in the comment to your answer: "So if the input impedance is 100 Ohm " meaning the resistor after the cap – user1245 May 30 '17 at 18:37
• Im not designing anything now. This a conceptual question. I want to learn the common practice. – user1245 May 30 '17 at 18:38

If now my understanding is correct, my question is what should be the upper limit and what would be an example to a bad affect of choosing overlarge cap for DC removal. Say you calculate the blocking cap 100nF but in your circuit you use 100uF. What kind of consequences can that have?

A higher value capacitor will typically be larger and/or more expensive.

A higher value capacitor might also require using a different dielectric material which will be less stable in value over temperature and bias voltage, although this is generally not a major issue in a simple low-voltage DC-blocking application.

It will also typically have higher ESR and ESL, which means it's ability to pass high frequencies is compromised. Exactly what "high" frequencies means here depends on exactly what the parasitic characteristics of the capacitor are and on details of the rest of the circuit.

Theoretically you can tune it to any size of capacitor.

However, to get the right cut-off frequency, smaller capacitors mean you need a smaller resistance to ground. That, in turn, means more attenuation of the input signal, especially if the source resistance is not very large.

As such, common practice is often..

• How big do we have in common stock or can we afford to add to the bill of materials,
• How physically small do I need it to be

Once you figure that out you can work out how large you can make that pull-down resistor.